From Scala to Haskell

Defining functions

Scala

def triple(x: Int): Int = 3 * x

Haskell

triple :: Int -> Int
triple x = 3 * x

Haskell (GHCi)

triple :: Int -> Int; triple x = 3 * x

Many tutorials use let triple :: .... There's no difference between those two notations in GHCi so it's up to you to decide .

Haskell (GHCi) - alternative

:{
triple :: Int -> Int
triple x = 3 * x
:}

Note the :{ and :} in the first and last line. Besides of that it's the same as one would define the function in a source file.

???

Scala

def compute: Int = ???

Haskell

compute :: Int
compute = undefined

Function composition (andThen style)

Scala

// Given:
val toInt: String => Int = Integer.parseInt
val triple: Int => Int = x => 3 * x

// You can compose them:
(toInt andThen triple)("5")
// res: 15

Haskell

-- Given:
toInt  :: String -> Int; toInt  x = read x
triple :: Int    -> Int; triple x = 3 * x

-- You can compose them:
import Control.Arrow
(toInt >>> triple) "5"
-- res: 15

Function composition (compose style)

Scala

> (triple compose toInt)("5")
// res: 15

Haskell

> (triple . toInt) "5"
-- res: 15