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https://github.com/eeue56/haskell-to-elm


https://github.com/eeue56/haskell-to-elm

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# haskell-to-elm



Collection of examples on places where Elm is different to Haskell.



Used for helping beginners moving from Haskell to Elm. Non-exhaustive list, only to be used alongside the documentation on the [Elm](http://elm-lang.org) site.



* [Functional Programming](#functional-programming)

	* [Type signatures](#type-signatures)

	* [Function application](#function-application)

	* [Function composition](#function-composition)

	* [List comprehensions](#list-comprenhensions)

	* [Lenses](#lenses)

	* [Where](#where-vs-let)

	* [Purity](#purity)

* [Prelude functions](#built-in-prelude-methods)

	* [id](#id)

	* [cons](#cons)

	* [head](#head)

	* [tail](#tail)

	* [zip](#zip)

	* [show](#show)

	* [mod](#mod)

	* [unwords](#unwords)

	* [cycle](#cycle)

	* [foldl](#foldl)

* [Modules](#module-syntax)

	* [Importing](#importing-names)

	* [Exporting](#defining-exportable-names)



# Functional programming 



## Type signatures



Elm uses a [single colon `(:)`](http://elm-lang.org/docs/syntax#type-annotations) for type signatures. Double colons `(::)` is used for [cons](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/List#::).



Example



```

	add :: Int -> Int

```



becomes

```

	add : Int -> Int

```



## Function application



Instead of using the dollar symbol `$` elm uses `<|` and `|>`  for [application in different directions](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Basics#|>). 



Example:



```

    collage (round board.width) (round board.height) $ map (genRect board) board.pieces

```

becomes

```

    collage (round board.width) (round board.height) <| List.map (genRect board) board.pieces

    --

    List.map (genRect board) board.pieces |> collage (round board.width) (round board.height)

```



## Function composition



Instead of using the `(.)` symbol Elm uses `<<` and `>>` for [composition in different directions](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Basics#>>)



Example:

```

    isEvenSquareRoot = isEven . sqrt

```

becomes

```

    isEvenSquareRoot = sqrt >> isEven

    -- or

    isEvenSquareRoot = isEven << sqrt

```



## List comprenhensions



There are [no list comprehensions](https://github.com/elm-lang/elm-compiler/issues/147#issuecomment-17439271) in Elm.



## Lenses



Elm has the package [focus](http://package.elm-lang.org/packages/evancz/focus/1.0.1) for lense-like accessors. Due to a lack of template-haskell like functionality, you must always [manually create your own focus](http://package.elm-lang.org/packages/evancz/focus/1.0.1/Focus#create)



Example:

``` 

    data Patch = Patch {

		_colour :: Colour,

		_size :: Double, 

		_coord :: Coordinate

	} deriving (Show, Eq, Ord)

	

	mkLabels[''Patch]

```

becomes

```

    type alias Patch = {

        colour: Colour,

        size: Float,

        coord: Coordinate

    }

    

    colour = create .colour (\f r -> { r | colour = f r.colour })

    coord = create .coord (\f r -> { r | coord = f r.coord })

    size = create .size (\f r -> { r | size = f r.size })

```



## where vs let



Elm has no where binding - instead use [let](http://elm-lang.org/docs/syntax#let-expressions)



## Pattern matching



Elm doesn't support multiple body declarations for functions, so instead you have to use [case..of](http://elm-lang.org/docs/syntax#conditionals)



Example:

```

	head [] = error

	head (X:xs) = x

```

becomes

```

	head xs = case xs of

  		x::xs -> Just x

  		[] -> Nothing

```



## Purity



Functions in Elm as of 0.15.1 have pure type signatures. However, as they are actually implented in JS, it's possible that the underlying code you're calling isn't pure. This gives the effect of Elm the language being pure, but the things it can be used to do can be impure (eg, drawing to screen). Native functions can also produce runtime errors, though there is a drive to rid these from Elm entirely.



# Built-in (Prelude) methods



## id



Elm has renamed id to [identity](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Basics#identity)



Example:

```

	id xs

```

becomes

```

	identity xs

```



## cons



Elm uses [double colons `(::)`](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/List#::) for cons.



Example

```

	5 : 6 : [7, 8]

```

becomes

```

	5 :: 6 :: [7, 8]

```



## head



Instead of throwing errors for empty lists, Elm uses Maybe for [head](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/List#head)



Example

```

	head [4, 5] == 4

```

becomes

```

	case head [4, 5] of

		Just x -> x == 4

		Nothing -> False

```



## tail



Instead of throwing errors for empty lists, Elm uses Maybe for [tail](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/List#tail)



Example

```

	tail [4, 5] == [5]

```

becomes

```

	case tail [4, 5] of

		Just x -> x == [5]

		Nothing -> False

```



## zip



Elm has [no built-in zip](http://elm-lang.org/examples/zip) method - instead it provides a [map2](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/List#map2) function that can be used with the tuple creator `(,)` to make a list of size 2 tuples from two lists.



Example:



```

    zip xs ys

```

becomes

```

    map2 (,) xs ys

```



## show



Elm renamed [show](http://zvon.org/other/haskell/Outputprelude/show_f.html) to [toString](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Basics#toString). Confusingly, there is also a [method called show](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Graphics-Element#show) in Elm - this generates a HTML element containing a textual representation of the data.



Example:

```

    show [1, 2, 3]

```

becomes

```

    toString [1, 2, 3]

```



## mod



mod in Elm uses the [`(%)` symbol](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Basics#%).



Example:

```

    isEven x = x `mod` 2 == 0

```

becomes

```

    isEven x = x % 2 == 0

```



## unwords



unwords is replaced by the [join function](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/String#join)



Example:

```

	unwords $ words "Hello Dave and Jeremy"

```

becomes

```

	join " " <| words "Hello Dave and Jeremy"

```



## cycle



Elm has no cycle built in. 



TODO: find documentation for this



## foldl



The order of the accumalator function arguments are [swapped in Elm](http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Set#foldl).



Example:

```

	idx xs = foldl (\x y -> y : x) [] xs

```

becomes

```

	id xs = List.foldl (\x y -> x :: y) xs [] 

```



# Module syntax



## Importing names



Elm uses the [`exposing` keyword](http://elm-lang.org/docs/syntax#modules) to import names into the current namespace.



Example:

```

    import List (map, foldl)

```

becomes

```

    import List exposing (map, foldl)

```



## Defining exportable names



TODO: find documentation on elm site for this



Following the module declaration, you must have *no* identnation level.



Example:

```

    module Coords (pos) where

        pos x y = (x, y)

```

becomes

```

    module Coords exposing (pos)

    pos x y = (x, y)

```