Elan Language Reference
Global Instructions
main,
procedure,
function,
test,
constant,
enum,
record,
class,
abstract,
interface
Global instructions (also referred to simply as ‘globals’) are located directly within a code file.
They are never indented from the left-hand edge, nor may they be located within other instructions.
Three of the globals – main, function, and procedure – are described as ‘methods’ and these are defined by one or more statements within them.
Four of the globals – record, class, abstract (class), and interface – define data structures and these always contain members.
The two remaining globals – constant, and enum – do not contain any further instructions.
Main
A program file must have a main method, sometimes called its main routine, if it is intended to be run as a program. You may, however, develop and test code that does not have a main method, either as a coding exercise or for subsequent use within another program.
The main defines the start point when a program is run.
The main does not have to be at the top of the file, but this is a good convention to follow.
The main may delegate work to one or more procedure or function.
There may not be more than one main in a file – and the Global prompt will not show the main option when one already exists in the file.
Example of a main method:
+main
variable liname set to [3, 6, 1, 0, 99, 4, 67]expression
call inPlaceRippleSortprocedureName(liarguments)
print liexpression
end main
Procedure
A procedure is a named piece of code that can define parameters which are given inputs via arguments in a call statement.
Unlike a function, a procedure does not return a value.
Also unlike a function, a procedure can have ‘side effects’: indeed it should have side effects, otherwise there would be no point in calling it!
For this reason the statements within a procedure can:
- Include print statements and methods.
- include input methods or other ‘system’ methods (such as a random number generation).
- call other procedures (or itself if ‘recursion’ is required).
- Assign a value to a parameter, provided that the parameter definition is preceded by the keyword out, as in this example:
+procedure inPlaceRippleSortname(out arr as List<of Int>parameter definitions)
variable changesname set to trueexpression
variable lastCompname set to arr.length() - 2expression
+repeat
set changesvariableName to falseexpression
+for ivariableName from 0expression to lastCompexpression step 1expression
+if arr[i] > arr[i + 1]condition then
let tempname be arr[i]expression
call arr.putprocedureName(i, arr[i + 1]arguments)
call arr.putprocedureName(i + 1, temparguments)
set changesvariableName to trueexpression
end if
end for
set lastCompvariableName to lastComp - 1expression
end repeat when not changescondition
end procedure
Procedures are executed by means of a call statement, for example:
variable liname set to [3, 6, 1, 0, 99, 4, 67]expression
call inPlaceRippleSortprocedureName(liarguments)
print liexpression
Function
A function is a named piece of code that can define parameters which are given inputs via arguments when reference to the function occurs in a statement or expression.
Unlike a procedure, a function returns a value. Also unlike a procedure, a function can have no ‘side effects’ and cannot depend on any System methods.
The return statement is followed by the value returned by the function.
Example of a function:
+function scorename(g as Gameparameter definitions) returns IntType
return g.body.length() - 2expression
end function
Parameters
Parameters for both procedures and functions are defined in exactly the same way: each parameter definition takes the form:
<name> as <Type>
for example:
age as Intparameter definitions
Recursion
Procedures and functions may be called or referenced recursively. For example, a simple factorial calculation:
+function factorialname(n as Intparameter definitions) returns IntType
return (if n > 1 then n*factorial(n - 1)
else 1)expression
end function
Tests
A test is a set of assertions, at the global level, about the output of functions. Example of a test, from the binary search demo program:
+test
let li1name be ["lemon", "lime", "orange"]expression
assert binarySearch(li1, "lemon")computed value is trueexpected value pass
assert binarySearch(li1, "lime")computed value is trueexpected value pass
assert binarySearch(li1, "orange")computed value is trueexpected value pass
assert binarySearch(li1, "pear")computed value is falseexpected value pass
let li2name be ["lemon", "orange"]expression
assert binarySearch(li2, "lemon")computed value is trueexpected value pass
assert binarySearch(li2, "orange")computed value is trueexpected value pass
assert binarySearch(li2, "pear")computed value is falseexpected value pass
let li3name be ["lemon"]expression
assert binarySearch(li3, "lemon")computed value is trueexpected value pass
assert binarySearch(li3, "lime")computed value is falseexpected value pass
let li4name be empty List<of String>expression
assert binarySearch(li4, "pear")computed value is falseexpected value pass
end test
Notes
- Elan tests are designed to test functions only. It is not possible to call a procedure or main routine within a test. Nor is it possible to use any System method (the same rule as for a function).
- A test may optionally be given a name or description in free-form text, just like a comment, which plays no role in the execution of the test. You might give the test the same name as a function that it is testing, or you might describe a particular scenario that is being tested.
- test methods may be written anywhere in the code, provided they are at the global level.
- A test method may contain any number of assert statements. When tests are run, the test runner (part of the Elan IDE), will attempt to run all assert statements and show each one's pass or fail outcome alongside. However, if the test hits a runtime error (as distinct from an assert failure) then execution of the test will stop and remaining asserts will be shown as ‘not run’.
- In addition to assert statements, a test may contain any other statements that may be added into a function (except return).
- All assert statements should be at the top level within the test frame; none may be put into a loop structure.
Testing Float values
When testing Float values it is recommend that you always use the round function to round the computed result to a fixed number of decimal places. This avoids rounding errors and is easier to read. For example:
+test
assert sqrt(2).round(3)computed value is 1.414expected value pass
end test
Testing for runtime errors
If the expression you are testing causes a runtime error then the error will be displayed in the red fail message:
+test
let aname be [5, 1, 7]expression
assert a[0]computed value is 5expected value pass
assert a[2]computed value is 7expected value pass
assert a[3]computed value is 0expected value actual: Out of range index: 3 size: 3
end test
If this occurs, mark the tests that you added since the last successful test run with ignore (see below), and then remove their ignore status one by one until the cause is identified and fixed.
Marking a test with ignore
It is possible to mark a test with the ignore keyword, by selecting the test frame and then hitting Ctrl-i, as in this example:
+ignore test
let g1name be newGame(new Random())expression
let g2name be newApple(g1)expression
let g3name be clockTick(g2, "s")expression
assert g3.headcomputed value is newSquare(22, 16)expected value not run
end test
When a test is marked with ignore, that test will not be executed when the tests are run, and its result will be shown as ‘not run’. The overall test status will also show in the ‘warning’ status (amber colour), even if all the tests that did run passed. This is to discourage you from leaving a test marked ignore for long.
The principal reason for marking a test ignore is when either the test code, or code in any function being called, does not terminate. This typically means that there is a loop (or a recursive call) with no exit condition, or where the exit condition is never met.
If you do create such code without realising it, then when the tests are executed the test runner will ‘time out’ after a few seconds (most tests will pass in milliseconds), and an error message will be shown in the System info pane. The test that caused the timeout will automatically then be marked ignore. Your priority should then be to identify the cause of the timeout and attempt to fix it before then restoring the test by selecting its frame and hitting Ctrl-i (which is a toggle for setting and unsetting an ignore status).
Constant
A constant defines a named value that cannot change, is always defined at global level (directly within a file), and is global in scope.
A constant is created at compile time, so cannot be defined with reference to any function, nor use any operators.
A constant can be defined as a literal Int, Float, Boolean,
String, ListImmutable or DictionaryImmutable.
A constant may not be defined within any method, but see the description of the let statement.
The name of a constant follows the rules for an Identifier.
Some examples:
+constant phiname set to 1.618literal value or data structure
+constant maxHitsname set to 10literal value or data structure
+constant warningMsgname set to "Limit reached"literal value or data structure
+constant fruitname set to {"apple", "orange", "banana"}literal value or data structure
+constant blackname set to 0x000000literal value or data structure
+constant redname set to 0xff0000literal value or data structure
+constant coloursname set to {Suit.spades:black, Suit.hearts:red, Suit.diamonds:red, Suit.clubs:black}literal value or data structure
+constant scrabbleValuesname set to {"A":1, "B":3, "C":3, "D":2, "E":1, "F":4, "G":2, "H":4, "I":1, "J":8, "K":5, "L":1, "M":3, "N":1, "O":1, "P":3, "Q":10, "R":1, "S":1, "T":1, "U":1, "V":4, "W":4, "X":8, "Y":4, "Z":10}literal value or data structure
In the colours example, Suit is an Enum.
Enum
An enum – short for ‘enumeration’ – is the simplest form of ‘user-defined Type’.
It specifies a set of values, each of which is defined as a name, such that a named value of Type enum necessarily always holds one of those values.
enums are read-only: once they have been defined it is not possible to add, remove, or update their values.
Some examples:
+enum SuitName spades, hearts, diamonds, clubsvalues
+enum ActionName stand, drawvalues
+enum OutcomeName undecided, win, lose, draw, winDoublevalues
+enum StatusName pending, playing, standing, blackjack, bustvalues
Type name
The name given to an enum (see below), which must begin with an upper case letter, is used as the Type name when passing a value to or from a procedure or function.
Using an enum
The value is specified by the Type name for the specified enum, followed by a dot and the value name, for example:
variable xname set to Status.pendingexpression
Record
A record is a user-defined data structure that is given a Type name (which must begin with an upper case letter).
The record defines one or more properties, each of which has a name (starting with a lower case letter) and a Type.
The Type of a property may be any simple value Type, or a ListImmutable, or another Type of record ( or even the same Type of record).
Note that the Type record has some similarity to a class in that:
- Both are user-defined data structures
- Both are given a ‘Type name’
- Both may define one or more properties, each with a name and Type
- Both may be created or copied using a with clause
- Both may define encapsulated methods
However a record differs from a class in that:
- A record is immutable (like a ListImmutable or a String). You can create a copy with specified differences but you cannot modify a property on a given instance.
- A record does not define a constructor
- A record may define only function methods, since procedure methods would imply the ability to mutate the record.
Examples:
+record SquareName
property xname as IntType
property yname as IntType
end record
+record GameName
property headname as SquareType
property bodyname as ListImmutable<of Square>Type
property priorTailname as SquareType
property applename as SquareType
property isOnname as BooleanType
property rndname as RandomType
property graphicsname as BlockGraphicsType
property keyname as StringType
end record
Having defined a record Type, such as Game above, you can create as many instances as you wish using the following syntax to specify the values:
let g1name be new Game() with
head set to newSquare(22, 15),
key set to "d",
isOn set to trueexpression
Note that you are not required to provide a value for each property because, where a property is not specified in the ‘with clause’ (as above), that property will be given the empty (default) value of the correct Type.
You can then read the values from the properties using ‘dot syntax’ for example:
print sq.sizeexpression
record Types are immutable: the properties on an instance may not be changed, directly.
However, you can easily create another instance that is a copy of the original,
with all the same property values except for any specific changes made in another with clause.
The newly-minted copy (with changes) must be assigned to a new named value. For example:
let sq1name be new Square() with
x set to 3.5,
y set to 4.0,
size set to 1.0expression
let sq2name be copy sq1 with
size set to 2.0,
colour set to redexpression
Or even to the same name if that name is a variable:
variable aname set to new Square() with
x set to 3.5,
y set to 4.0expression
set avariableName to copy a with
x set to 3.7expression
This last example shows how you enter the comma-separated with clauses. The earlier examples show how the Editor displays a set of with clauses.
If you want to use one or more existing property values in order to determine a new value, the property names must be prefixed with the name of the instance being copied, for example:
let sq2name be copy sq1 with
size set to sq1.size + 3expression
Record deconstruction
A record may be ‘deconstructed’, meaning that its properties are read into separate variables using the same syntax as for deconstructing a Tuple. For example, assuming that Square is a record defined as in the example above, then this code:
let x, y, size, colourname be mySquareexpression
will read the properties into the four names defined.
When deconstructing, the names of the values must match the names of the properties of the record. However, the ordering of the names does not have to match the order in which the properties are defined in the record.
Class
See also: Inheritance
A class is a user-defined Type offering far richer capability than an enum.
Note that a record is in some ways similar to a class but simpler: it defines properties, but has no constructor and no methods. See Working with records.
Definition
Here is an example of class definition, taken from the Snake OOP demo program:
+class AppleName inherits ClassName(s)
property locationname as SquareType
+procedure newRandomPositionname(snake as Snakeparameter definitions)
+repeat
let ranXname be randomInt(0, 39)expression
let ranYname be randomInt(0, 29)expression
set property.locationvariableName to new Square(ranX, ranY)expression
end repeat when not snake.bodyCovers(property.location)condition
end procedure
+function updateGraphicsname(gr as BlockGraphicsparameter definitions) returns BlockGraphicsType
return property.location.updateGraphics(gr, red)expression
end function
end class
A class must have a name that, like any other Type, begins with an upper case letter.
A class may define:
- One or more properties – see Property
- function methods – see Function method
- procedure methods – see Procedure method
- a constructor which may be used for setting up the values of properties. The constructor may optionally define parameters to force the calling code to provide initial values. However, it is not necessary to add a constructor if you have no need to initialise properties. Code in the constructor may make use of any functions, and follows the same constraints as a function (i.e. it may not call any procedure, whether defined on the class or outside).
Using a class
A class is instantiated using the keyword new followed by the class name and brackets, which should enclose the comma-separated arguments required to match the parameters (if any) defined on the constructor for that class. For example (also from the Snake OOP demo):
+constructor(parameter definitionsparameter definitions)
let tailname be new Square(20, 15)expression
set property.currentDirvariableName to Direction.rightexpression
set property.bodyvariableName to [tail]expression
set property.headvariableName to tail.getAdjacentSquare(property.currentDir)expression
set property.priorTailvariableName to tailexpression
end constructor
The created instance may then be used within expressions, like any other variable.
Inheritance
An ordinary class (also known as a ‘concrete class’) may optionally inherit from just one abstract class
but may additionally inherit from any number of interfaces.
The concrete class must define for itself a concrete implementation of every abstract member defined in the abstract class or any interfaces that it inherits from, directly or indirectly.
Notes
- An abstract class must be declared in the code above any class that inherits from it. This is the only case where the order of definition (of global constructs) matters.
- The abstract class (if any) and the interfaces (if any) that a concrete class inherits from may not contain duplicates of any abstract member. Any duplicated definitions in the hierarchy will result in a compile error. If such duplications arise, you should factor out the common member definitions, and move them up the hierarchy or into new interfaces inherited by the interfaces and/or classes that need them.
- Inheritance hierarchies must form a tree, that is you must avoid creating a ‘circular’ dependency where, for example, Type A inherits from Type B, which inherits from Type C, which inherits from Type A.
- The various ‘super-Types’ (abstract classes and interfaces) that a concrete class inherits from must not define conflicting members, e.g. members with the same name but having different Type signatures.
Abstract class
See also: Inheritance
An abstract class may not be instantiated (and hence may not define a constructor). It may define concrete members i.e.:
- a property
- a function
- a procedure
As with a concrete class, any of these members may be made private, after the corresponding frame has been added, by selecting that member frame and pressing Ctrl-p.
These concrete members are automatically inherited by any subclass, but they may not be overridden (re-defined) by the subclass. Therefore you should define concrete members only if they are intended to work identically on every subclass.
You may also define abstract methods on an abstract class, i.e. abstract property, abstract function, abstract procedure. Such methods define only the signature of the method, not the implementation (body), therefore they have no end statement. For example:
abstract function calculateDiscount() as Float
If you wish to have several subclasses of an abstract class that share a common implementation for a method, but require that some of the subclasses can define a different implementation, then you should:
- Define the method as abstract on the superclass.
- Define a concrete implementation on the superclass with a similar, but slightly different, name e.g. by adding a prefix such as: default.
- Each subclass must then define its implementation of the abstract method, but the ones needing a common implementation can be just one line, delegating responsibility up to the ‘default’ method on the superclass.
Interface
See also: Inheritance
An interface is similar to an abstract class, with the difference that it may define only abstract members. The advantage of using an interface instead of an abstract class is that a concrete class can inherit from multiple interfaces.
An interface may inherit only from other interfaces.
Important: An interface must not re-declare abstract interfaces that are defined in any interface it inherits from, directly or indirectly.
Member instructions
constructor,
property,
procedure,
function,
abstract property,
abstract procedure,
abstract function,
private property,
private procedure,
private function
Member instructions (also sometimes referred to simply as ‘members’)
are located within a class, abstract class, interface, or record.
A different subset of the total list above will be offered in the four different contexts.
Constructor
A concrete class may define a single constructor, which may:
- initialise any properties with fixed values
- define one or more parameters, which are then used to initialise properties
If a class does define a constructor, and the constructor defines any parameters, then when the class is instantiated (using new) then values of
the correct types must be provided, for example, if the class Square defines this constructor:
+constructor(x as Int, y as Intparameter definitions)
set property.xvariableName to xexpression
set property.yvariableName to yexpression
end constructor
then it may be instantiated like this:
let tailname be new Square(20, 15)expression
Property
Examples:
property height as Int
property board as Board
property head as Square
property body as [Square]
- A property is defined on a Class and must specify a name (conforming to the rules for an Identifier) and a Type.
- A property may be marked private, in which case it is visible only to code within the class and, if defined on an abstract class, within its subclasses. This is done by selecting the property frame and pressing Ctrl-p. (Pressing these keys again will remove the private modifier).
- If not marked private, a property may be read but not be written to. Properties may only be modified from outside the class by means of a Procedure method.
- A property may be given an initial value in the constructor.
If the property is not initialised within the constructor then it will automatically be given the empty value for that Type. You may test whether a property contains this default value by writing e.g.:
if head is empty Square
- Whenever you wish to access a property from within a method (or from within the constructor) on the same class, then the name of the property must be prefixed with the ‘qualifier’: property. (‘property-dot’). This applies whether you are reading or setting the property. By this means you can have a method parameter with the same name as a property, but they are unambiguous, because the property must be prefixed. A common pattern is to use the same name in a ‘setter’ method, for example:
constructor(board as Board)
set property.board to board
end constructor
procedure setHeight(height as Int)
set property.height to height
end procedure
Procedure Method
A ‘procedure method’ follows the same syntax and rules as a global procedure. The differences are:
- A procedure method, like a function method, is always referenced (used) by code outside the class using ‘dot syntax’ on an instance.
- A procedure method may read, or write to, any property defined on the class.
- A procedure method may be marked private, in which case it is visible only to code within the class and, if defined on an abstract class, within its subclasses. This is done by selecting the property frame and then pressing Ctrl-p. (Pressing these keys again will remove the private modifier).
Function Method
A function method follows the same syntax and rules as a global function. The differences are:
- A function method is always referenced (used) by code outside the class using ‘dot syntax’ on an instance.
- A function method may directly reference (read only) any property defined on the class as though it were a variable/parameter.
- A function method may be marked private, in which case it is visible only to code within the class and, if defined on an abstract class, within its subclasses. This is done by selecting the property frame and then pressing Ctrl-p. (Pressing these keys again will remove the private modifier).
- asString() method
- asString method. This is just a regular function method with a specific name, no parameters and returning a String. If defined for a class, then if an instance of the class is printed, the asString function method will automatically be used. Typically asString will return a string made up of one or more of the property values, perhaps with additional text, or the results of function calls.
Abstract Property
An abstract property may be defined only on an abstract class. Any concrete subclass must then implement a concrete (regular) property to match.
Abstract Procedure Method
An abstract procedure method may be defined only on an abstract class. Any concrete subclass must then implement a concrete (regular) procedure to match.
Abstract Function Method
An abstract function method may be defined only on an abstract class. Any concrete subclass must then implement a concrete (regular) function to match.
Statement instructions
assert,
call,
each,
else,
for,
if,
let,
print,
repeat,
set,
throw,
try,
variable,
while
Statement instructions (also sometimes referred to simply as ‘statements’)
are located within ‘methods’. Some of these statements may contain other statements.
Assert statement
See Tests for the use of the assert statement.
Some other programming languages have a feature for making assertions
while your program is running. In Elan, you can get equivalent functionality
by throwing an exception like this:
if yp is 0condition then
throw exception "yp is zero: {xp},{yp} + {xq},{yq}"message
end if
which will stop the program and print the message in the Debug window (unless caught by a try statement).
Procedure call
A call statement is used when you want to run a procedure.
The procedure may be:
- a procedure that you have defined at the global level
call fillRandom(grid)
- a procedure method on an object of a class that you have defined
call apple.newRandomPosition(snake)
call property.hand.draw()
- a procedure method that belongs to the same class as the procedure that you are calling from
call updateNeighbours()
- a procedure provided by the standard library
call pause(2000)
- a procedure method on an object of a Type provided by the standard library
call vg.append(rect)
call property.cards.append(card)
The arguments provided must match the number and Type of the parameters specified in the definition of the procedure. If there are no parameters, leave the brackets empty.
For procedures that you define yourself, out parameters are allowed. In this case the corresponding argument must be the name of a variable whose value gets updated by the procedure.
If the parameter is not an out parameter, any expression of the correct Type can be used as an argument.
Procedures may have side-effects, for example input/output or changing a data value in an object. They can change the contents of any mutable object passed in as an argument. For this reason, procedures cannot be called from functions, which are not allowed to have side-effects. call statements are simply not allowed in functions, to enforce this.
There is a limit to the complexity of a call statement. Only one dot is allowed in the procedure name field, or two dots if the first word is property. If you need anything more complicated, use a let statement on the line above. See the error message explanation for 'procedureName' in a call statement.
Each loop
The each..in.. construct specifies looping sequentially over the elements in a List or an Array, or over the characters in a String.
The each.. loop counter variable is of the same Type as the elements in the List or Array, or is of Type String if looping over the characters in a String. The variable does not have to have been defined in a variable statement.
Example:
variable namesname set to ["Tom", "Dick", "Harriet"]expression
print names[0]expression
+each nvariableName in namessource
print nexpression
end each
For loop
The for..from..to..step.. construct specifies looping through a sequence of integer values with a given increment.
The for.. loop counter variable (which counts from 0) is of Type Int and does not have to have been defined in a variable statement.
The three defining values, from, to, and step, must all be of Type Int, positive or negative.
and may be defined by literals, variables or expressions that evaluate to integers.
Note that, if you require a negative step value, then the literal, variable, or expression must start with a negative sign.
This is needed at compile time to determine the nature of the exit condition.
So if you have a variable s that holds a negative value to be used to step in reverse order, then you would write:
variable namesname set to ["Tom", "Dick", "Harriet"]expression
variable sname set to -1expression
+for nvariableName from 2expression to 0expression step -(-s)expression
print names[n]expression
end for
If statement
See also if expression
The if..then..else.. and if..then..else if..else.. constructs specify which of several code sequences is to be executed next.
The else clause is optional.
You can add as many else if clauses as you wish, but only one unconditional else (which, if present, must be the last clause).
Example 1:
+if head is applecondition then
call setAppleToRandomPositionprocedureName(apple, bodyarguments)
else if
call body.removeAtprocedureName(0arguments)
end if
Example 2:
+if item is valuecondition then
set resultvariableName to trueexpression
else if item.isBefore(value)condition then
set resultvariableName to binarySearch(lst[..mid], item)expression
else if
set resultvariableName to binarySearch(lst[mid + 1..], item)expression
end if
Let statement
The let statement may be thought of as being between a constant and a variable.
Like a variable a let may be defined only within a routine, but unlike a variable it may not be re-assigned once defined.
It is recommended that you always use a let in preference to a variable unless you need to be able to assign a new value to it.
You can put a let in a loop, so the variable gets a new value each time it is executed, but the value of the variable cannot be changed any other way.
Print statement
The simplest way to print is with the print statement. For example:
print "Hello"expression
let aname be 3expression
let bname be 4expression
print a*bexpression
print "{a} times {b} equals {a*b}"expression
Note
- The last line in the example above uses an interpolated strings. Arguments placed within curly braces are evaluated before printing, and these may be separated by literal text and punctuation as needed. This is one recommended way to print more than one value on a line. The other way is to use print procedures.
Repeat loop
The repeat .. end repeat when condition loop is used when you want at least one execution of the enclosed statements followed by a test that chooses either to execute them again or to exit from the loop.
condition is either a Boolean variable or an expression that evaluates to a Boolean value.
If condition is true then the loop ends, if false it resumes. For example:
+repeat
call file.writeLine(names[i])
set i to i + 1
end repeat when i is names.length()
Set statement
The set statement is used to assign a new value to an existing variable.
The new value must be of the same Type as (or a Type compatible with) that of the variable.
A set statement may not assign a new value to a parameter within a procedure.
Throw statement
You can deliberately generate, or ‘throw’, an exception when a specific circumstance is identified, using a throw statement, for example:
throw exception "something has happened"
Try statement
You can test whether another piece of code might throw an exception by wrapping it in a try statement. This might arise when calling a System method that is dependent upon external conditions, for example:
+try
call fooprocedureName()
print "not caught"expression
+catch exception in evariableName
print eexpression
end try
The variable holding the exception (by default named e, but this may be changed by you) is of Type String. You can compare the exception message to one or more expected messages and, if the message does not match an expected exception, you may choose to throw the exception 'up', as in this example:
+try
call fooprocedureName()
print "not caught"expression
+catch exception in evariableName
+if e isnt "an expected message"condition then
throw exception emessage
end if
end try
Variable statement
The variable statement defines the name to be used to store mutable (i.e. changeable) data. The name defined must be a valid Identifier,
and the initial value is given by a following expression. For example:
While loop
The while condition .. end while loop is used when you want execution of the enclosed statements to begin
only if condition is true.
condition is either a Boolean variable or an expression that evaluates to a Boolean value.
When condition is true the enclosed code is executed;
when false the loop is bypassed. For example:
+while not file.endOfFile()
print file.readLine()
end while
Expressions
One of the most important constructs in programming is the ‘expression’. An expression is evaluated to return a value. An expression is made up of the following possible elements:
- Literal value
- Named value
- Operator (including brackets)
- Function call
which this chapter describes.
Literal value
A literal value is where a value is written ‘literally’ in the code, such as 3.142 – in contrast to a value that is referred to by a name.
Here is a table showing some example literal values. Follow the links for more information about each type.
For more about List and Dictionary literals see the Standard (mutable) data structures table.
For more about ListImmutable and DictionaryImmutable literals see the Immutable data structures table.
Named value
A named value is a value that is associated with a name rather than being defined literally in code. There are various kinds of named value:
Constants, let statement, variable statement, Parameter passing, enum statement.
Once a named value has been defined, it can be referred to by the name.
Identifier
For all kinds of named values, the name must follow the rules for an ‘identifier’.
It must start with a lower case letter, followed by any combination of lower case and upper case letters,
numeric digits, and the _ (underscore) symbol. It may not contain spaces or other symbols.
If you happen to choose a language keyword, method name or other reserved word, an error message will tell you that you cannot use it for an identifier.
Scoping and name qualification
With the exception of a constant (below), which is global in scope, named values are always ‘local’: their scope is confined to the method in which they are defined.
Elan allows local named values to be defined with the same name as a constant, function, or procedure defined at global level or defined in the standard library. In such cases, when the name is used within the same method, then it will refer to the local definition. If you have done this, but then need to access the constant, function, or procedure with the same name, then you can simply prefix the use of the name with a ‘qualifier’ of either global. or library. as appropriate.
Indexed values
If a variable is of an indexable Type, then an index or index range may be applied to the variable within an expression. For example:
variable a set to "Hello World!"
print a[4] → o
print a[4..] → o World!
print a[..7] → Hello W (since the upper bound of a range is exclusive)
print a[0..4] → Hell (for the same reason)
In the examples above, the result is of type String in all cases.
When using indexing on other types:
- with a single index, the result is the same type as the elements of the data structure being indexed
- with an index range, the result is the same type as the data structure itself
Indexable types are String, Array, Array2D, List, Dictionary, ListImmutable and DictionaryImmutable.
Index ranges cannot be applied to Dictionary or DictionaryImmutable.
If the index values in a range are equal, or the second is smaller than the first,
then an empty data structure of the correct type is generated.
Important: unlike in many languages, indexes in Elan (whether, single, multiple, or a range) are only ever used for reading values.
Writing a value to a specific index location is done through a method such as in these examples:
put on a List
withPut on a ListImmutable
put on a Dictionary
withPut on a DictionaryImmutable
Operators
Arithmetic operators
Arithmetic operators can be applied to Float or Int arguments. The result may be a Float or an Int depending on the arguments.
For ^ + - *, the result is a Float if either of the arguments is a Float, and an Int if both arguments are Int.
For /, the result is always a Float. It can be converted to an Int using the floor() function.
mod and div only operate on Int arguments, and the result is Int.
2^3 → 8
2/3 → 0.666...
2*3 → 6
2 + 3 → 5
2 - 3 → -1
11 mod 3 → 2 (integer remainder)
11 div 3 → 3 (integer division)
Arithmetic operators follow the conventional rules for precedence i.e. ‘BIDMAS’ (or ‘BODMAS’).
When combining div or mod with any other operators within an expression, insert brackets to avoid ambiguity e.g.:
(5 + 6) mod 3
Note that mod is more of a remainder operator than a modulus operator -- the result takes the sign of the first argument. If both arguments are positive, there is no difference.
The minus sign may also be used as a unary operator, and this takes precedence over binary operators so:
2*-3 → -6
Note that the Elan editor automatically puts spaces around the + and – binary operators, but not around ^, / or *. This is just to visually reinforce the precedence.
The div operator rounds its result down, as if with the floor function. This can give unexpected results with negative numbers, for example:
11 div -3 → -4
The + operator is also used for concatenating Strings. See String.
Logical operators
Logical operators are applied to Boolean arguments and return a Boolean result.
and and or are binary operators
not is a unary operator.
The operator precedence is not → and → or, so this
example, which implements an ‘exclusive or’, need not use brackets and can rely on the operator precedence:
+function xorname(a as Boolean, b as Booleanparameter definitions) returns BooleanType
return a and not b or b and not aexpression
end function
Equality testing
Equality testing uses the is and isnt keywords with two arguments. The arguments may be of any Type.
- a is b returns true, if a and b are both of the same Type and their values are equal. The only exception is that if one argument is of Type Float and the other is of Type Int, then is will return true if their values are the same, i.e. are the same whole number.
- isnt returns the opposite of is.
- Where a binary operator is expected, as soon as you type is the editor will automatically insert a space after it. To enter isnt you need to delete the space (using the Backspace key) and then type nt.
Note that in Elan equality testing is always ‘equality by value’; there is no such thing as ‘equality by reference’.
If the items being compared are composite types, the elements within them are compared recursively to see if the objects are equal. For example two distinct instances of the same class compare equal if the values of all their properties compare equal. And two Lists compare equal if they contain the same elements in the same order.
The compiler rejects any attempt to compare instances of different classes unless abstract classes and inheritance are involved. Two instances which are subclasses of the same abstract class compare equal only if they are of the same class (and have the same property vlaues).
Numeric comparison
The numeric comparison operators are:
> for greater than
< for less than
>= for greater than or equal to
<= for less than or equal to
Each is applied to two arguments of Type Float, but any variable or expression that evaluates to an Int may always be used where a Float is expected.
Notes:
- These operators cannot be applied to strings. Use the dot methods isBefore and isAfter to compare strings alphabetically.
See Dot methods on a String.
- Where a binary operator is expected, as soon as you type < or > the editor will automatically insert a space after it. To enter <= or >= you need to delete the space (using the Backspace key) and then type =.
Combining operators
You can combine operators of different kinds, e.g. combining numeric comparison with logical operators in a single expression. However the rules of precedence between operators of different kinds are complex. It is strongly recommend that you always use brackets to disambiguate such expressions, for example:
(a > b) and (b < c)expression
(a + b) > (c - d)expression
Function call
An expression may simply be a function call, or it may include one or more function calls within it. Examples:
print sinDeg(30)expression
variable xname set to sinDeg(30)^2 + cosDeg(30)^2expression
variable namename set to inputString("Your name")expression
print name.upperCase()expression
Notes:
- The third example above is not strictly a function call, but is a ‘system method’ call.
System methods may be used only within the main routine or a procedure, because they have external dependencies or side effects.
- In the fourth example, upperCase is a ‘dot method’ that may be applied to any instance (variable or literal) of Type String.
See Dot methods on a String.
Lambda
A lambda is a lightweight means to define a function ‘in line’. You typically define a lambda:
- If the functionality it defines is needed in only one location: typically for a particular call to a Higher-order Function (HoF).
- If you need to capture a local variable in the implementation. This is called ‘closing around a variable’.
The syntax for a lambda is as follows:
- Start with the keyword lambda.
- Parameter definitions, comma-separated, follow the same form as parameter definitions in a function or procedure, but without surrounding brackets.
- The => symbol, which is usually articulated as ‘returns’, ‘yields’ or even ‘fat arrow’.
- An expression that makes use of the parameters, and may also make use of other variables that are in scope.
Example:
+function liveNeighboursname(cells as List<of Boolean>, c as Intparameter definitions) returns IntType
let neighboursname be neighbourCells(c)expression
let livename be neighbours.filter(lambda i as Int => cells[i])expression
return live.length()expression
end function
Notes:
- Although a lambda is commonly defined ‘inline’ (as shown above) it is possible to assign a lambda to a variable and hence to re-use it within the scope of that variable.
If expression
The ‘if expression’ is in certain respects similar to an If statement, but with the following differences:
- It is written entirely within a single expression. This is possible because the if expression always returns a value.
- There is always a single then and a single else clause, and each clause contains just a single expression. The if expression returns the result of evaluating one of these two expressions, according to whether the condition evaluates to true or false.
Here are three examples:
return if c < 1160 then c + 40
else c - 1160expression
return if isGreen(attempt, target, n) then setChar(attempt, n, "*")
else attemptexpression
return if attempt[n] is "*" then attempt
else if isYellow(attempt, target, n) then setChar(attempt, n, "+")
else setChar(attempt, n, "_")expression
New instance
A 'new instance' expression is used to create a new instance of a library data structure,
or a user-defined class or record – either to assign to a named value, or as part
of a more complex expression. Example of use (taken from the `Snake - procedural` demo):
let blocksname be new Array2D<of Int>(40, 30, white)expression
Where the new instance is of a user-defined class or record the expression may optionally
be followed by a 'with clause` in order to specify any property values. Example of this use:
+function dealCardname(random as Floatparameter definitions) returns CardType
let numbername be (random*52).floor()expression
let rankname be rankValue.keys()[number div 4]expression
let suitname be number mod 4expression
return new Card() with
rank set to rank,
suit set to suitexpression
end function
Copy with
A 'copy with' expression is used to make a copy of an existing instance, but with a different value for one or more of the properties
– either to assign to a named value, or as part of a more complex expression..
It is used extensively within 'functional programming' where you are dealing with records or other immutable types.
Example of use in this manner (taken from the 'Snake - functional' demo program):
+function newApplename(g as Gameparameter definitions) returns GameType
let x, rnd2name be g.rnd.nextInt(0, 39)expression
let y, rnd3name be rnd2.nextInt(0, 29)expression
let apple2name be newSquare(x, y)expression
let g2name be copy g with
apple set to apple2,
rnd set to rnd3expression
return if bodyOverlaps(g2, apple2) then newApple(g2)
else g2expression
end function
However, copy with may also be used in object-oriented programming, with instances of regular (mutable) classes. Also,
note that a with clause (following the same syntax as in a copy with expression), may be used
within a new instance expression to set up properties for the object not specified in the constructor.
Empty of Type
An 'empty of Type' expression is used to make the default (empty) instance of any Type
– usually only created for comparing to another instance to test whether that other instance is also empty or default.
This may arise, for example, if a class or record is defined with a property that has never had a value assigned to it.
The following example is taken from the 'Pathfinder' demo program:
+procedure visitNextPointname(parameter definitionsparameter definitions)
call updateNeighboursprocedureName()
set property.currentvariableName to nextNodeToVisit()expression
+if (property.current is empty Node) or (property.current.point is property.destination)condition then
set property.runningvariableName to falseexpression
else if
call property.current.setVisitedprocedureName(truearguments)
end if
end procedure
It is also possible explicitly to set a property or a named value to an empty instance of the appropriate Type.
# comment
A comment is not an instruction: it is ignored by the compiler and does not change how the program works.
Rather, a comment contains information about the program, intended to be read by a person seeking to understand or modify the code.
Every comment starts with the symbol '#' (known as 'hash') followed by some text or a blank line. The text field in
a comment may contain any text, except that it must not start with the open square bracket symbol '['.
Comments may be inserted at any level: in the Global,
Member, or Statement instruction levels,
as well a from the new code prompt, i.e. every selector provides a # for entering a comment.
Every Elan program has a single comment at the top of the file, which is generated by the system and cannot be edited or deleted by the user.
This comment is known as the 'file header' and shows the version of Elan being run.
Compile Errors and Warnings
Q: What is the difference between a compile error and a warning.
A: A warning usually indicates that the fix may involve adding some more code,
for example adding a definition for an unknown identifier. An error usually indicates
that you will need to alter code to fix the error.
But they are similar in that you will not be able to run your program until the issues are fixed. In all programming
languages it is a good practice 'treat all compile warnings as errors' i.e. fix them as soon as you see them appear.
Messages
Expression must be ...
An expression, when evaluated, results in a value of a Type that is not compatible with its 'target',
for example: if the result of the expression is being assigned to an existing variable, or if an expression is defined 'inline' as
an argument into a method call.
Cannot use 'this' outside class context
The keyword this may only be used within an instance method on a class to refer to the current instance.
Abstract Class ... must be declared before it is used
If a class inherits from one or more abstract classes, then the latter must all have already been declared (defined) earlier in the code file.
Member ... must be of type ...
This error occurs when a class is defined as inheriting from an abstract class,
and has implemented an inherited member (method or property) with the correct name, but with different Types.
Incompatible types. Expected: ... Provided: ...
Cannot determine common type between ... and ...
... is not defined for type ...
Arises when 'dot calling' a member (method or property) that does not exist on the Type of the named value or expression before the dot.
Cannot call a function as a procedure
A function (or function method) is to be used within an expression, not via a call instruction.
Cannot use a system method in a function
A 'system method' (defined in the Standard Library) returns a value like a function does. However, because a system method
either makes changes to the system and/or depends on external inputs, it may be used only within a procedure or the main routine.
Code change required ...
Indicates that a library method or class has been changed since the version in which your Elan code was written. The link in the
message should take you directly to information in the Library Reference documentation on how to update your code to cope with the change.
Cannot call procedure ... within an expression
A procedure may be used only within a call instruction.
Cannot invoke ... as a method
The code is attempting to use a free-standing method (function or procedure) as a 'dot method' on a named value or the result of an expression.
Cannot ...index ...
An index (in square brackets) may be applied only to certain data structure Types: String, Array, Array2D,
List, ListImmutable, Dictionary, and DictionaryImmutable.
Cannot range ...
A range may be applied only to certain data structure Types: String, Array, List, and ListImmutable.
Cannot new ...
The Type specified after the call keyword cannot be instantiated. Either the Type is unknown,
or it is an abstract class or interface.
Source for 'each' must be an Array, List, or String.
Superclass ... must be inheritable class
A concrete class may inherit from an abstract class, and/or an interface, but not from another concrete class.
In Elan, all classes must be either abstract or 'final' – a final class being concrete and not-inheritable.
Superclass ... must be an interface
An interface may inherit from other interfaces, but not from any class.
Class/interface ... cannot inherit from itself
The message is self explanatory.
There must be only one abstract superclass ...
A class may inherit from only one abstract class. However, it may additionally inherit from one or more interfaces.
Cannot reference private member ...
A private member (method or property) may be accessed only by code within the class, or within subclasses of it.
It may not be accessed by any code outside the class hierarchy.
... must implement ...
If a concrete class inherits from any abstract class or interface(s) it must implement all abstract methods defined in those Types.
... must be concrete to new
You cannot create an instance of any abstract class or interface: only of a concrete class.
Cannot pass ... as an out parameter
If a parameter of a procedure is marked with out then this means that the parameter may be
reassigned within the procedure. Therefore you must pass in a variable that can be re-assigned. You cannot pass in: a
constant, a literal value, or a named value that is defined by a let instruction.
Cannot call extension method directly
A method that is defined within the Library as an 'extension method', such as asString may only be called using 'dot syntax' on a named
value or an expression.
Cannot prefix function with 'property'
The prefix property. may only be used before a property name: not a function name.
Missing argument(s) ...
The method being called expects more arguments than have been provided.
A method has been passed more arguments than it expects.
Argument types ...
One or more arguments provided to the method are of the wrong Type.
...<of Type>...
Certain data structure Types, including Array, Array2D,
List must specify the Type of their members, for example List<of Int>.
Failure to specify the '<of Type>' on these Types will give an error, as will specifying 'of Type' where it is not required.
Dictionaries require Types to be specified for both the keys and the values,
for example: Dictionary<of String, Float>.
May not re-assign the ...
Attempting to re-assign, or mutate, a named value that may not be re-assigned in the current context.
Name ... not unique in scope ...
Attempting to create an identifier with the same name as one already defined within the same scope.
May not set ... in a function
A property can be re-assigned only within a procedure method, not within a function, because re-assigning a property is a 'side effect'.
The identifier ... is already used for a ... and cannot be re-defined here.
An existing named value may not be defined again within the same scope.
Duplicate Dictionary key(s)
Attempting to define a literal Dictionary or DictionaryImmutable
with one or more duplicated keys in the definition.
To evaluate function ... add brackets ...
If you intend to evaluate a function, the function name must be followed by brackets even if the function
defines no parameters. If your intent was to define a reference to the function (a pattern used commonly in
'Functional Programming' then the name of the function must be preceded by the keyword ref and a single space.
Library or class function ... cannot be preceded by by 'ref'
The keyword ref may be applied only to functions that you have defined in your own code as a standalone (global) function.
It may not be applied to a function method defined on a class, nor to a library function.
(You may, however, define your own function that simply delegates its implementation to a library function.)
a comment may not start with [ unless it is a recognised compiler directive
Compiler directives are a planned future capability. They will look like comments, but begin with an open square bracket.
To avoid the possibility of ambiguity, you may not start your own comments with an open square bracket.
Condition of 'if' expression does not evaluate to a Boolean.
Cannot have any clause after unconditional 'else'.
... is a keyword, and may not be used as an identifier.
An Elan keyword cannot be used to as the name for a value, property, or method. Try shortening the name, lengthening it, or using a different name
... is a reserved word, and may not be used as an identifier.
In addition to Elan keywords there are certain other 'reserved words' that cannot be used to define the name for a
value, property, or method.If you encounter this error you may eliminate the error simply by adding
more valid characters to the name – for example just by changing case to case_.
Why are there any reserved words that are not Elan keywords? There are three kinds of reserved word:
- Potential keywords that might be added to Elan in future releases.
- Lower case versions of Elan Type names, such as int, float, string, boolean, array, list, dictionary.
It is not considered good practice to use Type names for identifiers: instead you should give each identifier a name that indicates
what it does (for a method), or represents (for a named value).
- Words that are keywords in JavaScript (but not in Elan). Elan compiles to JavaScript and
use of these words as identifiers could cause errors. (If you are asking, 'Why doesn't the Elan compiler
just obfuscate these?' the answer is that we evaluated that option but concluded that it added a lot of
complexity – especially in regard to debugging and interaction with the Elan editor – just to eliminate
the minor inconvenience of having to extend the word so as to make your identifier distinct.)
For reference, the complete list of reserved words is:
action, arguments, array, async, await, boolean, break, by, byte, case, catch, char, const, continue, curry, debugger, default, delete, dictionary, do, double, eval, export, extends, final, finally, float, goto, implements, import, in, instanceof, int, into, list, long, match, namespace, native, null, on, optional, otherwise, package, partial, pattern, protected, public, short, static, string, super, switch, system, synchronized, throws, todo, transient, typeof, void, volatile, var, when, yield
Index cannot be negative.
An index into an array or list cannot have a negative value. If a negative is given in literal form e.g.
a[-3] then this will generate a compile error.
If you use a named value for an index and it is negative, then this will cause a runtime error.
Cannot do equality operations on Procedures or Functions.
It is not possible to apply comparison operations to functions (with or without the ref keyword prefix)
or procedures as themselves. It is, however, possible to compare the results of two function evaluations.
It is possible that you are seeing this message because you intended to evaluate a function but forgot to
add the brackets after the name.
Property ... is not of an immutable type.
Properties on a record may only be of immutable Types.
... cannot be of mutable type ...
Element Type for a ListImmutable must itself be an immutable Type.
Similarly, for an DictionaryImmutable the Types for both the key and the value must be immutable ones.
... cannot have key of type ...
The Type of the key for any dictionary Dictionary must be an immutable Type, and not itself an indexable Type.
No such property ... on record ...
The property name given in the record deconstruction does not match a property on the given Type of record.
Cannot discard in record deconstruction ...
Wrong number of deconstructed variables.
referencing a property requires a prefix.
If you are referring to a property of a class from code defined within the class then the
property name must be preceded by property.
'out' parameters are only supported on procedures.
You cannot defined an out parameter in a function (because that would
imply the possibility of creating a side effect).
There can only be one 'main' in a program.
Unsupported operation.
You cannot chain two 'unary' operators (those that apply to a single value), such as -
or not successively within an expression.
Field help
'arguments' field in a call instruction
An argument list passed into a function or procedure call, must consist of one or more arguments separated by commas.
Each argument may in general be any of:
- A literal value
- A named value
- An expression
In certain very specific contexts, however, some options are disallowed by the compiler.
'computed value' field in an assert instruction
The 'actual' field should be kept as simple as possible, preferably
just a named value or a function evaluation. Generally, if you want to use a more
complex expression, it is better to evaluate it in a preceding let
instruction and then use the named value in the 'actual' field of the assert
instruction. Some more complex expressions are permissible, but these two restrictions apply:
- Any expression involving a binary operator such as +, isnt, etc.,
must have brackets around it.
- You may not use the is operator within the 'actual' field, because
the parser will confuse this with the is keyword that is part of the assert instruction.
'variable name' field in a set instruction
The first field in a set instruction most commonly takes the name of an existing variable.
It may, however, may also take the following forms:
- Within a class it may take the form property.name to set the property name
- A tuple deconstruction. See Tuple
- A list deconstruction. See List
literal value or data structure in a constant
The value of a constant must be a literal value of a Type that is not mutable.
This can be a simple value (e.g. a number or string), or an immutable List or Dictionary.
'values' field in an enum definition
enum values must each be a valid identifier, separated by commas.
'message' field in a throw instruction
An exception message must be either a literal string or a named value holding a string.
expression field - used within multiple instructions
This field expects an expression. For the various forms of expression see Expressions.
identifier field - used within multiple instructions
'if' field in an else clause
'inherits ClassName' field in a class
An inheritance clause, if used, must consist of the keyword inherits
followed by a space and then one or more Type names separated by commas.
'name' field in a function or procedure definition
A method name must follow the rules for an identifier.
'parameter definitions' in a function or procedure definition
Each parameter definition takes the form:
name as Type
The name must follow the rules for an identifier.
The Type must follow the rules for a Type.
If more than one parameter is defined, the definitions must be separated by commas.
'procedureName' in a call statement
Valid forms for a procedure call are
- call procedureName()
- call instanceName.procedureMethodName()
- call property.propertyName.procedureMethodName()
- call library.procedureName()
The last one is used only if there is a need to disambiguate between a library procedure and a user-defined (global)
procedure with the same name.
'Type' field in a function or property definition
For certain Types the name may be followed by an of clause, for example:
List<of Int>
Dictionary<of String, Int>
'Name' field in a class or enum definition
Type names always begin with a capital letter, optionally followed by letters of either case, numeric digits,
or underscore symbols. Nothing else.
'name' field in a let or variable instruction
The definition for a variable or for a let statement is most commonly
a simple name. Less commonly, it may take the form of:
tuple deconstruction, list deconstruction, or record deconstruction.
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