Value Types, with their methods

Integer

An integer is a whole number, i.e. one that has no fractional part. It may be represented in decimal, hexadecimal or binary:

100 (decimal), 0x64 (hexadecimal) and 0b1100100 all have the same value.

Dot methods on an Int

Notes

• A literal integer or a named value of Type Int may always be passed into a function or procedure that is expecting a Float.
• The Int type is intended to represent whole numbers in the range:
• Maximum: 2⁵³ – 1 (which is 9007199254740991)
• Minimum value: – (2⁵³ – 1)
• If you go greater than the limit given above, the number will be accurate to approximately 17 decimal digits. Larger numbers will be rounded with zeros at the end, unless they are larger than around 2⁷⁰, when they will automatically be shown in exponential (scientific) format, as shown in the example.

Example of large numbers

   ▶ This code calculates 2 raised to the power of 50 through 72, and its output is shown below:

+for n from 50 to 72 step 1 variable s set to lambda n as Int => n.asString().length() variable np set to 2^n call printTab(-s(n), "{n}") call printTab(30 - s(np), "{np}
") end for

Floating point

A floating point number is a decimal number that may have both integer and fractional parts.

It may be written using exponential (scientific) notation, e.g. 120.0 = 1.20e2, and 0.012 = 1.20e-2.

Dot methods on a Float

Notes

• The limits on floating point numbers are:
  • Maximum value: just over 1.0 × 10³⁰⁸
  • Minimum value: approximately 5.0 × 10^-324
• A variable that has been defined as being of Type Float may not be passed as an argument into a method that requires an Int, nor as an index into a List or an Array even if the variable contains no fractional part. It may, however, be converted into an Int before passing, using the function floor or ceiling.
• If you wish to define a variable to be of Type Float, but initialise it with an integer value, then add .0 on the end of the whole number, as in:
  variable hour set to 3.0
• Any variable or expression that evaluates to an Int may always be used where a Float is expected.

Boolean

A Boolean value is either true or false.

The keywords true and false must be written in lower case.

Dot method on a Boolean

String

A String represents text, i.e. a sequence of zero or more characters.

Character sets

• When typing from the keyboard into a literal string all the basic ASCII characters (0x20 to 0x7e) and' £' can be input. You cannot use Alt-numeric-keypad to enter accented letters or other special characters in the range 0x80 to 0xff.
• You can, however, copy any text from outide Elan and paste it into a string, whether ASCII, extended ASCII or UTF-8 encoded.
• To insert a character from the full range, you use its Unicode (codepoint) value, in decimal or hexadecimal, by means of function unicode: print "Spanish syntax introduces a question with a {unicode(191)}" ⟶ Spanish syntax introduces a question with a ¿
  print "This is an up arrow: {unicode(0x2191)}" ⟶ This is an up arrow: ↑

  These examples 'interpolate' the output of function unicode by using curly braces (see Interpolated fields).

Notes

• Strings can be built from other strings by concatenation using the plus operator, for example:
print "Hello " + 'world' ⟶ Hello World
• A newline may be inserted within a string with \n: print "Hello\nworld" When the field is exited, the newline will be shown in the code. The newline will also be applied when the string is printed/ ⟶ Hello
     world
• As in most languages, strings are immutable. When you apply any operation or function with the intent of modifying an existing string, the existing string is never modified. Instead, the operation or function will return a new string that is based on the original, but with the specified differences.
• There are no 'substring' methods in Elan, because you can always use an index range to get a substring, e.g. s[3..7] gives a string containing the fourth character to the seventh character inclusive of s. Note that the upper bound of the range is exclusive. See Indexed values.
• You can loop through a string picking each of its characters sequentially using the each loop.
• The dot methods that can be applied to strings are shown in the table below.

Interpolated fields in strings

Strings that are enclosed by " (double quotes) are automatically interpolated, that is you can insert the values of variables or simple expressions within the string by enclosing them in curly braces:

• If a and b are named values of, say, 3 and 17: print "{a} times {b} equals {a*b}" ⟶ 3 times 17 equals 51
• If velocity and fuel are named values of, say, 3.0 and 50.4: set message to "LANDED SAFELY AT SPEED {(velocity*100).floor()} FUEL {fuel.floor()}" print message ⟶ LANDED SAFELY AT SPEED 300 FUEL 50
• let r be 2.0 print "area = {(pi*r*r).round(2)}" ⟶ area = 12.57

Strings that are enclosed by ' (single quotes) cannot have interpolated fields in them.

Dot methods on a String

Tuple

A Tuple is a way of holding a small number of values of differing Types together as a single reference. It may be considered a lightweight alternative to defining a specific class for some purposes. Tuples are referred to as 2-tuples, 3-tuples, etc. according to the number of values they hold. Common uses include:

• Holding a pair of x,y coordinates (each of Type Float) as a single unit.
• Allowing a function to pass back a result comprised of both a message in a String and a Boolean indicating whether the operation was successful.
• Returning both an updated copy of an immutable data structure and the value of an element from it, e.g. from method pop on a Stack.

• A Tuple may hold elements of different Types
• A Tuple is immutable – you may read, but not modify, the values that it contains.

Defining a Tuple

A new Tuple is defined using this syntax:

Note that the elements may be literal or named values. An element within a Tuple may even be another Tuple.

Passing a Tuple to or from a function

You may pass a Tuple into a function, or return one from a function. The Type of a Tuple is specified in round brackets, containing the comma-separated Types for each element in order, for example:

Deconstructing a Tuple

The Tuple point defined by:

may be deconstructed into new variables or named values where the number of variables or names must match the number of elements in the Tuple, for example:

The 'discard' symbol _ (underscore) may also be used when deconstructing a Tuple when there is no need to capture some of its elements:

Where it is not convenient to use Tuple deconstruction (or not possible, e.g. within a lambda) then individual elements may be accessed using the special properties item0, item1, item2,... with dot syntax, for example:

Note that the appropriate itemNs will be listed in the Editor's drop-down menu options for the referenced Tuple.

Notes

• As in most languages, an Elan Tuple is immutable. Once defined Tuples are effectively read-only. You cannot alter any of the elements in a Tuple nor (unlike a List) can you create a new Tuple from an existing one with specified differences.
• You cannot deconstruct a Tuple into a mixture of new and existing variables.
• Tuples may be nested: you can define a Tuple within a Tuple.

Standard (mutable) data structures

Elan's four standard data structures have the Types Array, Array2D, List and Dictionary. They are all mutable, meaning that their contents (referred to as items or elements) may be changed directly by calling the various procedure dot methods defined for each structure Type.

However, since procedure methods may be called only from within the main routine or a procedure, it is also possible to make changes via function dot methods, which return a copy of the data structure with specified changes: that is why all such methods have names starting 'with'.

All four contain values (and keys in the case of a Dictionary) of only a single Type, that Type being specified either explicitly (as in <of Int>), or implicitly if the structure is created using a literal definition.

This table summarises the creation and reference to these structures. Details of their methods follow.

Array

An array is a simple data structure containing n elements of a single Type, indexed from 0 to n-1.

The array elements' Type must be one of Int, Float, String, or Boolean.

As in most languages, individual elements may be referenced by their index.

To modify an element you may either call the put procedure method, or use the withPut function dot method.

An array's size is specified when it is created, along with the single value to which each element is initialised.

It is not possible to append further elements to an array, but you can convert an array to a list using method asList and then extend or reduce the list, and possibly make a new array using method asArray on it.

Procedure methods on an Array

Function dot methods on an Array

Array2D

The Array2D Type defines a 2-dimensional array of fixed size. The two dimensions, which are specified when the array is created, may be of the same size (a square array) or different (a rectangular array). If you want to create a 'jagged' array, you should use a List of Lists.

The convention is to refer to the position of an element in such an array by two index values (x,y), where x is the column number and y is the row number of a rectangular grid.

The Type for the elements is also specified when the Array is defined and must be one of: Int, Float, String, or Boolean.

Procedure methods on an Array2D

Function dot methods on an Array2D

• Since an Array2D has a known fixed size, methods rows and columns are of use only when passing such an array into a procedure or function that needs to discover its size.

You can read individual elements with a double index, for example:

List

A List is similar to an Array in that its n elements are all of one Type and are indexed from 0 to n-1, but is more flexible because:

• While the elements must all be of one Type, that Type is not limited to the simple value Types: the element Type may also be a user-defined Type such as a class, record, another List or an Array.
• A List is created either empty or initialised from a literal definition.
• A List's size can be dynamically changed.

List Deconstruction

A List or ListImmutable may be deconstructed into new variables or named values in a similar way to deconstructing a Tuple, but in this case:

• Always use two variable names.
• Use a colon : to separate the variable names.
• The first item in the list (the head) is assigned to the first variable.
• The rest of the list (the tail) is assigned to the second variable. If the original list only contains one element, this will be an empty list.
• Like deconstructing a Tuple, you can use an underscore _ as the variable name to discard either the head or the tail.

variable x:xs set to myList
set h:t to myList
set h:myList to myList

Discarding either the head or tail:

variable _:tail set to myList
variable head:_ set to myList

Procedure methods on a List

Function dot methods on a List

Dictionary

A Dictionary works like an Array, but instead of having a numeric (integer) index, each entry has a key. Each key is associated with a value, so the Dictionary is a set of key:value pairs. The key's Type must be one of Int, Float, String or Boolean, or it may be a user-defined record. The values may be of any Type, and the Types of both key and value are fixed when the Dictionary is created.

Procedure methods on a Dictionary

Function dot methods on a Dictionary

Immutable data structures

In contrast to the standard (mutable) data structures, the contents (referred to as items or elements) of the five immutable data structures cannot be modified directly, and hence the structure Types have no procedure dot methods defined. Instead, changes are made by using function dot methods that copy the existing data structure with specified changes.

Immutable data structures are intended specifically to facilitate the Functional Programming paradigm, but some are also useful within other programming paradigms.

This table summarises the creation and reference to these immutable structures. Details of their methods follow.

ListImmutable

A ListImmutable is like a List but is immutable (like a String). You can still insert, delete or change elements in a ListImmutable, but the methods for these operations do not modify the input ListImmutable: they return a new ListImmutable based on the input ListImmutable with the specified differences.

Type name

The Type is specified in the following ways:

• ListImmutable<of String> for a ListImmutable of Type String
• ListImmutable<of Int> for a ListImmutable of Type Int
• ListImmutable<of ListImmutable<of Int>> for a ListImmutable of such lists each of Type Int

Creating a ListImmutable

A ListImmutable may be defined in 'literal' form, delimited by curly brace with all the required elements separated by commas. The elements may be literal values but must all be of the same Type, for example:

ListImmutable Deconstruction

A ListImmutable can be deconstructed just like a List, using a colon : to separate two variable names.

set h:t to myListImmutable

Function dot methods on a ListImmutable

Try these examples:

DictionaryImmutable

A DictionaryImmutable is like a Dictionary but immutable (like a String). You can still insert, delete or change elements in a DictionaryImmutable, but the methods of these operations do not modify the input DictionaryImmutable: they return a new DictionaryImmutable with the specified differences.

A DictionaryImmutable may be defined in a constant.

Type name

In the following example, the keys are of Type String, and the values associated with the keys are of Type Int:

Defining a literal DictionaryImmutable

A literal DictionaryImmutable is defined as a comma-separated list of key:value pairs surrounded by curly braces:

Using an Immutable Dictionary

Try this example:

Function dot methods on a DictionaryImmutable

Set

A set is a standard data structure that works somewhat like a ListImmutable with the important difference that in a set a given element may appear only once. If an item being added to a Set is identical to an existing item in the Set then the Set remains the same length as before.

This enables a set to work like a mathematical set so that it is possible to perform standard set operations such as union or intersection. For the same reason, a Set is an immutable data structure: there are no methods that modify the set on which they are called, but several of them (including add, remove) return a new set that is based on the original set or sets, with specified differences.

Example of use:

Notes

• When creating a set, the Type of the elements must be specified in the form Set<of String>. This applies both when creating a new, empty set and when defining the Type of a parameter to be a Set.
• You can add elements: individually with add, or multiple elements with addFromList.
• You can create a new set from an existing list or ListImmutable by calling asSet on it.

Function dot methods on a Set

Stack and Queue

• Stack and Queue are similar data structures except that Stack is ‘LIFO’ (last in, first out), while Queue is FIFO (first in, first out). The names of the methods for adding/removing are different, but there are also common methods.
• Both a Stack and a Queue are defined with the Type of the items that they can contain, similarly to how List and ListImmutable have a specified item Type, though with different syntax. The Type is specified in the form shown below e.g. Stack<of String>, Queue<of Int>, Stack<of (Float, Float)>, Queue<of Square>.
• Both Stack and Queue are dynamically extensible, like List and ListImmutable. There is no need (or means) to specify a size limit as they will continue to expand until, eventually, the computer’s memory limit is reached.
• This same syntax is used to specify the Type if you want to pass a Stack or Queue into a function, or specify it as the return Type.
• Stack and Queue have two methods in common: length and peek. peek returns the next item to be removed, without actually removing it.
• The methods for adding and removing an item are different for Stack and Queue, as shown here:

Function dot methods on a Stack

Function dot methods on a Queue

Common dot methods

Dot methods that work on more than one fundamental Type

Although all applicable methods are described in earlier sections under each Type, this table lists those which are applicable to several Types.

Click on a Type to go to the Type's dot methods, or on the ✔ tick to go to the specific method on that Type.

Notes

• The method asString is so widely applicable because it enables you to print most variables and data structures to the IDE's Display pane for debugging purposes.
• The length of a Dictionary or DictionaryImmutable can be found by applying method length to the list returned by method keys on the dictionary:

print dict.keys().length() ⟶ 15

• Checking whether an item is contained in a Dictionary or DictionaryImmutable can be done by applying method contains to the list returned by methods keys and values on the dictionary:

print dict.keys().contains(42) ⟶ true or false

print dict.values().contains("widget") ⟶ true or false

Graphics

Block graphics

Block graphics provides a simple way to create low resolution graphics, ideal for simple but engaging games for example.

The graphics are displayed on a grid that is 40 blocks wide by 30 blocks high.

Each block is be rendered as a solid colour.

An example of block graphics to produce a rapidly changing pattern of coloured blocks:

Notes

• The Array2D must be of TypeInt and of size 40 x 30.
• You may create multiple Array2Ds holding different patterns of blocks, and switch between them just by passing the required one as the argument to the displayBlocks method.
• A colour is specified as an Int, as described under Colours.

Turtle graphics

Turtle graphics are implemented with output to the IDE's Display pane, i.e. the 'paper' on which the turtle draws.

The area is 200 turtle units wide by 150 turtle units high, and both integer and floating point values of turtle units can be used. If a turtle is placed or moved outside the 200 × 150 area boundary, it will not cause an error, but the turtle and any lines drawn outside the boundary will not be visible.

The origin for turtle units (0,0) is at the centre of the area: positive x rightwards, positive y upwards. So the top left corner of the display is at (-100,75).

A new turtle's default starting position is at (0,0) facing upwards (heading is 0), from where a move will be in the y direction.

You can move and turn a turtle, causing lines to be drawn, whether or not the turtle is showing.

Procedure methods on a Turtle

Properties of a Turtle

The current location and heading of the turtle may be read using the properties x, y, and heading.

Examples using turtle graphics

   ▶ To draw a square:

let t be new Turtle() call t.placeAt(-75, 50) call t.show() +for i from 1 to 4 step 1 call t.turn(90) call t.move(80) call pause(500) end for

---

   ▶ To draw a fractal snowflake, using a procedure and recursion:

+main let t be new Turtle() call t.placeAt(-50, 30) call t.turn(90) +for i from 1 to 3 step 1 call drawSide(side, t) call t.turn(120) end for end main

+procedure drawSide(length as Float, t as Turtle) +if (length > 1) then let third be length/3 call drawSide(third, t) call t.turn(-60) call drawSide(third, t) call t.turn(120) call drawSide(third, t) call t.turn(-60) call drawSide(third, t) else call t.move(length) end if end procedure +constant side set to 100

---

Vector graphics

Vector graphics are implemented using the SVG (Scalable Vector Graphics) Html tag <svg>, and are output to the Display pane in the user interface.

The area is 100 units wide by 75 units high, and both integer and floating point values of the units can be used.

The origin of the units (0,0) is at the top left corner of the display: positive x rightwards, positive y downwards. So for example the centre of the display is at (50, 37.5).

The names of the shapes broadly correspond to the names of SVG tags:

• CircleVG for <circle../>
• LineVG for <line../>
• RectangleVG for <rect../>
• ImageVG for <image../>

The properties of these shapes reflect the names of the attributes used in the SVG tags but some names differ. For example, SVG's stroke-width is set with property strokeWidth, to make it a valid identifier. Also stroke and fill are set by the properties strokeColour and fillColour.

Defining Vector graphic shapes

A set of SVG shapes is defined in either a List<of VectorGraphic> or a List of any specific Type of VectorGraphic (e.g. List<of CircleVG>) by using the List append method.

VectorGraphic is the abstract superclass of all ..VG shapes. You would use it if you wanted to define a List holding different types of shape.

Adding a shape returns a new instance of the list which must be assigned either to an existing variable, or to a new let.

The constructors for the VG Types require arguments defining the attributes of the relevant SVG tags, so defaults are supplied from the class properties, as shown in the table below.

To supply your own values for the properties:

• for a new instance, to set the value of property 'P..', use the with clause .
• for an existing instance, to change the value of property 'P..', call the setP.. procedure method on the VectorGraphic or
  (in a function) use the withP.. dot method on the VectorGraphic, in either case specifying new values in the arguments.

The fillColour and strokeColour properties may be specified as described under Colours. Only fillColour can be specified as transparent.

Note that the height and width properties of an image are also dependent on the dimensions of the original graphic file.

Displaying Vector graphic shapes

As with Block graphics, the assembled list of shapes is not displayed until you make a call to procedure displayVectorGraphics with the list as its argument, thus allowing you to make multiple changes before display.

And as with using SVG in Html, the shapes are drawn in the order in which they are added to the VectorGraphic instance. Later shapes are positioned over earlier ones.

Function dot methods on Vector graphic shapes

Properties of Vector graphic shapes

Combining graphic outputs

Program outputs, whether text or graphical, can be combined in the Display. In particular, Block graphics and text or Html printing can share the Display along with either Vector graphics or Turtle graphics (but not both).

If you want to share the Display in this way, remember that both text and Html print outputs appear sequentially down the Display (which can be scrolled), whereas the graphic outputs are positioned in the Display using their own absolute coordinate systems.

The order in which the outputs are displayed (and therefore overwrite) is:

1. Block graphics
2. Vector or Turtle graphics
3. Printed text or Html

So some care is needed to manage the layout in the Display.

Note that, while vector graphics are being drawn, printed text that exceeds the height of the Display does not scroll until the graphic completes.

Input/output

Reading keys ‘on the fly’

In some applications – especially in games, for example – you want the program to react to a key pressed by the user, but without holding up the program to wait for value to be input.

Whether your application makes use graphics, or just uses the Display for text, reading keystrokes ‘on the fly’ is done via one of two methods:

Notes

• When the getKey function is called, the system does not wait for a response. If a key has been pressed then that will be returned as a String e.g. "a".
• Non-printable keys will be returned in the form: "Backspace","Enter","ArrowDown", etc.
• If no key has been pressed (since the last time the method was called), it will return the empty string "".
• Pressing just the Shift, Ctrl (⌘ Command under macOS) or Alt keys will not be detected by getKey. To read those keys use…
• getKeyWithModifier which returns a 2-tuple containing the key pressed plus any ‘modifier’ key such a Shift, Ctrl, or Alt (or the empty string if no modifier key is pressed).
• Both of these getKey methods are System methods because they have a dependency on the system and so may only be used within a procedure or in main.

Use the procedure method clearKeyBuffer() if you want to enforce that the user cannot get too far ahead of the program by hitting keys in very rapid succession.

waitForKey waits for a key to be pressed, and returns it. pressAnyKeyToContinue gives an optional prompt and is used when you don't need to know which key was pressed.

Reading text files

The TextFileReader class is used to read textual data from a file.

An instance is created by the standalone system method openFileForReading.

The available procedure methods are:

These methods may be used to read a whole file in one go:

or to read a file line by line:

Notes

• openFileForReading will present the user with a dialog to select the file.
• readWholeFile returns a String containing every character in the file, without any trimming. It automatically closes the file after the read.
• readLine reads as far as the next newline character (\n) and then automatically trims the line to remove any spaces and/or carriage-returns (which some file systems insert after the newline automatically) from the resulting line returned as a String. If this behaviour is not desired, you can use readWholeFile, which does no trimming, and then parse the resulting String into separate lines.
• Calling file.close after reading line by line is strongly recommended to avoid any risk of leaving the file locked. It is not necessary to call it after using readWholeFile because that method automatically closes the file.
• Calling any method on a file that is already closed will result in a runtime error.

Writing text files

The TextFileWriter class is used to write textual data to a file.

An instance is created by the standalone system method createFileForWriting.

The available procedure methods are:

These methods may be used to write a whole file in one go:

or to write a file line by line:

Notes

• writeLine adds the string it is passed onto the end of any data previously written, with a newline character (\n) automatically appended.
• When execution reaches saveAndClose you will be presented with a dialog to confirm (or edit) the given filename and location where it is to be saved. It is not therefore strictly necessary to specify a filename when creating the file, since it can be specified by the user in the dialog so, in that case, you might put the empty string "" into the parameter of createFileForWriting.
• writeWholeFile puts the string it is given into the file and then automatically saves the file, so the user will be presented with the same dialog as if saveAndClose had been called.
• Calling any method on a file that has already been closed (by calling either saveAndClose or by writeWholeFile) will result in a runtime error.
• If the user were to hit Cancel on the save dialog, then the program will exit with an error. If you want to guard against this possibility (if, for example, it might mean the loss of important data) then you should perform the save and close within a try..catch like this:
+try call file.saveAndCloseprocedureName(arguments) +catch exception in evariableName print "File save cancelled"expression end try

or you could make the code offer the user options: to save again, or to continue without saving.

Rendering Html in the Display

If you embed Html code in a string and then attempt to print it, you will see the string displayed literally: the Html tags will not be recognised. This is for security reasons. However, it is possible to display formatted Html in the Display. The following code:

will produce:

This Html forms another 'layer' in the IDE's Display pane. Any plain text that you print (using print or any of the print.. procedures) will be overlaid on top of this.

You can clear just the Html layer in the display using the procedure clearHtml.

For specifying style or other attributes within Html tags, the attribute values should be enclosed in single quotation marks ' as shown above. Html will recognise single or double quotation marks, but entering double quotation marks would terminate the Elan string. Alternatively, you could use the constant quotes within braces as an interpolated field.

Using an embedded CSS stylesheet

You can also specify a <style> tag at the start of your Html string, to apply to the whole Html being displayed. In this case you would put the style definition into a named value as a literal but enclosed in single quotes, for example:

will produce:

The <style> string is displayed for editing like this:

But by inserting \n newlines thus:

the string can be shown folded and indented when not being edited:

Displaying images from URLs

An image that can be retrieved with an http or https URL may be sent to the Display, though note that other URI schemes, such as in file://host/path to access an image from your local file system, are not supported. Here are three techniques:

• put the Html <img> tag into a string and then call the displayHtml method on it, thus: let pic be "<img src='https://upload.wikimedia.org/wikipedia/commons/0/08/Corl0207_%2828225976491%29.jpg'
  width='400' height='200'
  title='shark' alt='shark'>" call displayHtml(pic)

  Notes

  • The usual <img> tag attributes are respected, though alt does not give the expected tooltip when hovering over the image, and title is only a descriptive comment.
  • The attributes can be shown listed vertically by inserting \n to signify a newline, as is done here before width and before title.
• put the URL into a Vector Graphics ImageVG definition: and then call the displayVectorGraphics method on it, thus: let pic be new ImageVG('https://upload.wikimedia.org/wikipedia/commons/0/08/Corl0207_(28225976491).jpg') with
  width set to 400,
  height set to 200,
  alt set to "Shark" call displayVectorGraphics([pic])

  Notes

  • Vector graphic image properties include height, width and alt.
  • Other vector graphic image properties such as x,y position can be added into the with clause.
• put the URL into an image definition: let pic be

  On moving the editing focus away from the image field, the instruction shows a thumbnail of the image in the code.

  Then convert it with method asHtml so that you can display it with the displayHtml method thus:

  variable vg set to new List<of VectorGraphic>() let pic be with
  width set to 304,
  height set to 139 call vg.append(pic) call displayHtml(pic.asHtml())

  Notes

  • Vector graphic image properties include height, width and alt that match the similar Html attributes and and have the same effect.
  • Other vector graphic image properties such as x,y position can be added into the with clause.
  • In this example, pic will be of type ImageVG which means that instead of displaying it as Html it may be displayed as vector graphics (like in the earlier example).

Sound

The tone procedure allows the generation of a simple tone. It requires three arguments to be provided:

• the duration of the generated tone specified in milliseconds (Int)
• the frequency of the generated tone in Hertz (Int)
• the volume of the tone (Float)

Note that the volume parameter is only relative in that the actual volume will be modified by the various sound settings on the output device, and may even be muted.

Other Types

Random

Generating random numbers within a function

It is not possible to use the system methods random() or randomInt() within a function because they create unseen side effects. You may use those system methods outside the function and pass the resulting random number (as an Int or a Float) as an argument into a function.

It is, however, possible to create and use random numbers within a function, but it requires a different approach and is a little more complex. You use the special Type Random. (Note the upper case R required for a Type name).

You start by getting a seed random number in your main or in a procedure using method initialiseFromClock:

Then, in a function, you apply the dot method next or nextInt to the Random passed in, which returns a 2-Tuple containing both a new random value and a new instance of Type Random. If you need another random value, you use the returned Random on which to apply the dot method again.

Avoid applying next or nextInt more than once on the same Random because they will return the same values.

If the initialiseFromClock call is absent, the program will still generate a sequence of random values, but the sequence will be exactly the same each time you run the program. Initialising from the clock ensures that you get a different sequence each run. Using Random without so initialising, however, can be extremely useful for testing purposes since the results are repeatable.

The initialising procedure method is:

The function dot methods on a Random are:

Func

A function may be passed as an argument into another function (or a procedure), or returned as the result of calling another function. This pattern is known as Higher-order Function (HoF), and is a key idea in the Functional Programming.

To define a function that takes in another function as a parameter, or returns a function, you need to specify the Type of the function, just as you would specify the Type of every parameter and the return Type for the function.

Type name

The Type of any function starts with the word Func followed by angle brackets defining the Type of each parameter, and the return Type for that function, following this syntax:

This example defines the Type for a function that defines three parameters of Type String, String, and Int respectively, and returns a Boolean value. This Type would match that of a function definition that started:

Constants

String constants

These constants are of use when the characters { } and " are required in a string delimited by double quotes. Within a string delimited by single quotes, those literal characters can also be used.

Boolean constants

Maths constant

Colours

A colour is specified as an Int value using one of these methods:

• the limited colours defined as library constants as in the above table.
• an integer in the decimal range 0 (black) to 2^24-1 (white).
• a six digit hexadecimal value in the range 0x000000 – 0xffffff using the same 'RGB' format as used in Html style, for example 0xff0000 for red.

Standalone functions

Standalone library functions always return a value and are therefore used in contexts that expect a value, such as in the right-hand side of a variable declaration or a set assignment, either on their own or within a more complex expression. All standalone library functions require at least one argument to be passed in brackets, corresponding to the parameters defined for that function.

unicode

Function unicode converts a Unicode (codepoint) value expressed as an integer value in decimal or hexadecimal notation, or as any expression evaluating to an Int, into a string of a single character. For example:

parseAsInt and parseAsFloat

Function parseAsInt attempts to parse the input String as an Int, and returns a 2-tuple, the first value of which is Boolean, with true indicating whether or not the parse has succeeded, and the second value being the resulting Int. parseAsFloat does the equivalent for floating point. Operation is illustrated by these tests:

Notes

• Any string that parses as an Int will also parse as a Float.
• If the parse fails, the second value will become zero, so you should always check the first value to see if the second value is a correct parse or just the default.
• You can 'deconstruct' the Tuple into two variables: variable success, parsedValuename set to parseAsInt(myString)
• One use of these parsing methods is for validating user input, but note that an easier way to do this is to use the various input methods.

floor, ceiling, round, isNaN, and isInfinite

All of these functions are called as 'dot methods' on a numeric value of Type Float or Int. NaN is short for 'Not A (Real) Number' Their use is illustrated with the following tests:

Maths functions

Examples of some maths functions and constants being tested:

Regular expressions

Elan's regular expressions are modelled on those of JavaScript, including the syntax for literal regular expressions. See, for example this Guide to Regular Expressions.

More functions for using regular expressions will be added in a future release of Elan. For now…

The method matchesRegExp is applied to a String using dot syntax and requires a RegExp parameter specified as a literal or as variable. It returns a Boolean. For example:

You can convert a valid string without /../ delimiters to a RegExp using function asRegExp:

Although it is recommended that literal regular expressions are written with /../ delimiters, the ability to convert a string allows you to input a regular expression into a running program.

Bitwise functions

These functions take in an integer value, and manipulate the bit representation of that value.

Sequence and SequenceWithStep

The sequence function is used to create a List containing a sequence of integer values as defined by the two integer parameters: start and end (both being inclusive values).

Here is an example which uses Higher-order Functions to print the prime numbers between 2 and 30:

The sequenceWithStep function is similar to sequence but takes a third integer parameter specifying the step size, which may be positive or negative.

Standalone procedures

All procedures are accessed via a call statement.

Method printTab helps in the layout of information printed to the Display, in particular, when printing columns of data. For example:

Right-align numeric output (powers of 9) using a lambda:

System methods

System methods appear to work like functions, because:

• they may require one or more arguments to be provided.
• they always return a value.
• they are used in expressions.

They are not, however, pure functions because:

• They may have a dependency on data that is not provided as an argument.
• They may generate side effects, such as changing the screen display or writing to a file.

Because of these properties, system methods may be used only within the main routine or within a procedure. They may not be used inside a function that you have defined, because to do so would mean that your function would not be pure.

System methods are all defined within the standard library: you cannot write a system method yourself.

System methods are commonly associated with Input/output, but note that:

• Input/output may be performed in procedures as well as in main.
• Some system methods are not concerned with input/output, e.g. some generate random numbers.

The reason those are system methods is that they have a dependency on data that is not passed into them as arguments.

Note

• Methods random and randomInt cannot be used in a function because of their dependence on external factors, i.e. they are subject to side effects. To obtain random numbers within a function, use Type Random.

Higher-order Functions (HoFs)

A higher-order function (HoF) is one that takes in a reference to another function as a parameter, or (less commonly) that returns a reference to another function as its result.

Standard HoFs

The standard library contains several HoFs that are widely recognised and used within functional programming, namely filter, map and reduce, and also provides maxBy, minBy and sortBy.

Passing a function as a reference

To use a higher-order function (HoF), you have to pass in a reference to a function. This can be either a lambda or the keyword ref followed by the name of a function.

It is also possible to use references to functions not in the context of HoFs.

On most occasions when you write the name of an existing function elsewhere in code your intent is to evaluate the function and, to do so, you write the name of the function followed by brackets containing such arguments as are required by the function. For this reason if you forget to add the brackets, you will get an error, for example:

The second sentence in this error message is for when your intention is not to evaluate the function, but to create a reference to the function. This is a valid thing to do in functional programming but is not generally done in procedural programming. As the error message says, to create a reference to a function you need to precede it by ref and the name of the function should then not be followed by brackets (or any arguments). For example: