Elm A Functional Reactive Programming Language Elm in

Elm A Functional Reactive Programming Language

Elm in general ■ ■ ■ Elm is a purely functional, single-assignment language ■ Elm syntax is strongly influenced by Haskell ■ Concepts are more like ML ■ Some ideas have been borrowed from # The new feature of Elm is signals ■ Signals are “variables” whose value changes in response to external events ■ This makes Elm a functional reactive language Elm compiles into Java. Script ■ Thus, Elm is designed to create interactive web pages 2

import Statements ■ ■ ■ > List. is. Empty [] -- List is automatically imported True : Bool import List -- Explicit qualified import ■ > List. is. Empty [] True : Bool import List as L -- Renaming ■ > L. is. Empty [] True : Bool import List exposing (is. Empty, map) ■ > is. Empty [] True : Bool import List exposing (. . ) ■ > is. Empty [] True : Bool 3

Data types ■ ■ Elm has the usual data types ■ Integers, Int ■ Floats, Float ■ Booleans, Bool, True and False ■ Strings, String, in double quotes ■ Characters, Char, in single quotes ■ Lists, List, [value, . . . , value] ■ All elements of a list must be of the same type ■ Tuples, (value, . . . , value) ■ Records (like maps) ■ { name = value, . . . , name = value } ■ The name is part of the data type Comments are -- (single line) or {-. . . -} (multiline, nestable) 4

Polymorphism ■ Elm is polymorphic -- that is, it figures out the most general type that will work ■ A number without a decimal point (for example, 5) is just a number, and may be used as either an integer or a float ■ > concat x y = x ++ y function> : appendable -> appendable 5

Operators ■ ■ ■ Logical: &&, ||, and not Arithmetic: +, -, *, /, // (integer division), % (mod), ^ (exponentiation) ■ Mixed mode arithmetic (mixing integers and floats in an expression) is not allowed ■ There are functions to. Float (for integers) and round and truncate (for floats) Strings and lists: Append with ++ 6

Lists ■ Lists are values of the same type, enclosed in brackets ■ Example: [1, 2, 3] ■ Syntax inversion: Elm and Haskell both use : and : : but reverse their meanings ■ Haskell: ■ Prelude> : t head : : [a] -> a ■ Prelude> 1 : [2, 3] [1, 2, 3] ■ Elm: ■ > 1 : : [2, 3] [1, 2, 3] : List number 7

Tuples ■ ■ Tuples are comma-separated values enclosed in parentheses ■ Example: ("Joe", 55) There are functions for creating tuples from values; these functions have the names (, ), (, , , ), and so on ■ Example: (, ) "Joe" 55 8

Functions ■ Functions are defined much as they are in Haskell, although fewer variations are possible ■ Function literals: square = n -> n ^ 2 ■ Simple functions: add x y = x + y ■ Pattern matching: first (head: : tail) = head 9

Records I ■ ■ ■ ■ point = { x = 3, y = 4 } -- create a record point. x map. x [point, {x=0, y=0}] -- access field -- field access function { { ----- point | - x z x x } = 12 } | z = point. x } | x = 6 } remove field add field rename field update field { point | x <- 6 } -- nicer way to update a field { point | x <- point. x + 1 , y <- point. y + 1 } -- batch update fields 10

Records II ■ ■ -- pattern matching on fields ■ dist {x, y} = sqrt (x^2 + y^2) ■ This shows a polymorphic function -- dict will accept any record with x and y fields ■ {x, y} -> (x, y) -- polymorphic fields ■ lib = { id x = x, flip f x y = f y x } ■ Here the fields are polymorphic functions ■ lib. id 42 gives 42 ■ lib. flip (++) "ab" "cd" gives "cdab" ■ http: //elm-lang. org/docs/syntax#functions 11

Records III ■ The type of a record includes the field names as well as their types ■ > { x = 0. 0, y = 0. 0 } { x = 0, y = 0 } : { x : Float, y : Float } 12

Types and type aliases ■ ■ The type statement defines a new type ■ type Direction = Left | Right ■ type Shape = Point Float | Line Point | Circle Point Float A type alias just gives a name to an existing type ■ type alias Location = { row: Int, column: Int } 13

if expressions ■ ■ Elm is expression-oriented, not statement-oriented, so if expressions must have both an else and a then part ■ Example: if 2+2 == 4 then "yes" else "no" Elm has multi-way if expressions ■ Example: if | n < 0 -> "negative" | n == 0 -> "zero" | otherwise -> "positive" 14

Multi-line expressions in the REPL ■ ■ In the REPL, hitting Return/Enter ends the expression To enter multiple lines, put after each line ■ Do not put a space after the ! Elm will respond with a | prompt ■ This is not the | you need in a multiline if expression! Example: > if | n < 0 -> "negative" | | n == 0 -> "zero" | | otherwise -> "positive" : String 15

case expressions ■ ■ Example: case xs of hd : : tl -> Just hd _ -> Nothing Notes: case expressions use pattern matching ■ Indentation is relevant, and each case after the first must begin in the same column as the first ■ The _ acts as a “wild card”--it will match anything ■ The result is of type Maybe, that is, class Maybe from the Maybe module 16

let expressions ■ ■ A let expression defines some values to be used in a expression ■ Example: let a = 5 b = 12 in sqrt(a ^ 2 + b ^ 2) As with case expressions, each value after the first must begin in the same column as the first 17

Reducing the number of parentheses ■ ■ ■ The function f can be applied to the value x in any of three ways ■ f x ■ f <| x ■ x |> f The x is put as the last argument of f Expressions with parentheses have to be evaluated “from the inside out” The above operators allow such expressions to be linearized The following are equivalent: ■ scale 2 (move (20, 20) (filled blue (circle 10))) ■ circle 10 |> filled blue |> move (20, 20) |> scale 2 18

Callbacks ■ ■ A callback is a construction in languages such as Java, C++, and Java. Script ■ A callback attaches listeners to events, such that when an event occurs, the listener is invoked (called) Callbacks require the programmer to write listeners for each expected kind of callback ■ The callback then manipulates data in some other part of the program 19

Signals ■ A signal is a variable that varies over time ■ Example: Mouse position ■ import Graphics. Element exposing (. . ) ■ import Mouse ■ main = Signal. map show Mouse. position ■ Mouse. position is an (x, y) pair ■ show is a function to turn a value into a string; it has type a -> Graphics. Element ■ map applies show to the elements of the pair ■ Signal. map has type (a -> b) -> Signal a -> Signal b ■ (Signal. map show) therefore has the (unqualified) type Signal a -> Signal Element ■ If you run this example, the result will be a continuous display of the mouse position 20

Algebraic Data Types ■ You can build your own data types with the type declaration ■ Examples: type Color = Black | White ■ This is a union type ■ type Piece = Pawn | Knight | Bishop | Rook | Queen | King ■ type Chess. Piece = CP Color Piece ■ piece = CP Black Queen ■ ■ Excerpt From: Bruce A. Tate, Fred Daoud, Ian Dees, Jack Moffitt. “Seven More Languages in Seven Weeks” You can use type variables in data type definitions ■ Example: type Tree a = Empty | Node a (Tree a) 21

References ■ ■ ■ ■ Elm web site: http: //elm-lang. org Introductory video: http: //www. infoq. com/presentations/elm-reactive-programming More extensive video tutorial (2½ hours): https: //www. youtube. com/playlist? list=PLtd. CJGSpculb. DT_p 4 ED 9 o. LTJQ rzo. M 1 QEL Syntax summary: http: //elm-lang. org/learn/Syntax. elm First chapter of an excellent tutorial series: http: //elm-by-example. org/Chapter 1 Hello. World. html Other learning resources: http: //elm-lang. org/Learn. elm Sublime Text 2 (not 3) plugin: https: //github. com/deadfoxygrandpa/Elm. tm. Language ■ ■ I have not gotten this to work; if you do, post what you did to Piazza The Haskell mode works well if you don’t use Markup 22

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