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Syntactic constructs

Indentation

Haskell is indentation sensitive, like Python. Tabs or spaces are fine.

example input = result <> " tree" where
    result = case input of
        True -> "red"
        False -> "black"

Infixing and sections

Given a function f, one can write it infix (i.e. in between its two arguments):

repl example
-- treating a prefix function as an infix function
> let subtract x y = x - y
> subtract 6 4
2
> 6 `subtract` 4
2

Functions whose names are symbols, like +, $ and ., are written infix by default. An order of precedence is defined, to avoid the need for bracketing. For example, f a . f b means (f a) . (f b), and similarly, f a $ f b means (f a) $ (f b).

For functions like + or / that are written by default infix, Haskell has some syntactic sugar to convert functions from infix to prefix (before their arguments):

repl example
-- treating an infix function as a prefix function
> 5 / 2
2.5
> (/) 5 2 -- (1)!
2.5
> (/) 2 5
0.4
> (/ 5)  2 -- (2)!
0.4
> (5 /)  2
2.5
  1. Whether infix or not, the type of (/) is Double -> (Double -> Double).
  2. This is called a "section".

Infixing in types

Similarly, -> is a type-level infix operation: a -> b can be written (->) a b.

As this suggests, -> is like Either in that it is a function on types:

repl example
> :kind (->)
(->) :: * -> (* -> *)
> :kind (->) Bool
(->) Bool :: * -> *
> :kind (->) Bool Bool 
(->) Bool Bool :: *

Bracketing

Haskell tends to avoid brackets when possible.

Expressions like 4 * 3 + 5 take a default bracketing (given when the operators were defined).

Functions application is left-associative:

  • f x y z means ((f x) y) z
  • Either Bool Int means (Either Bool) Int

The -> type is right-associative:

  • A -> B -> C -> D means A -> (B -> (C -> D))

Dollar Sign

Code like the following is common in Haskell:

exampleWithDollar = not $ (> 3) 4

This is equivalent to:

exampleWithoutDollar = not ( (> 3) 4)

Note

$ is just a regular function, used infix, and defined so that f $ x = f x.

For more explanation, see: https://typeclasses.com/featured/dollar.

Dollars can be stacked:

exampleWithDollar = not $ (> 3) $ head [1,2,3]

means the same as:

exampleWithoutDollar = not ((> 3) (head [1, 2, 3]))

Tip

Whenever you see a $, read everything to the right as the input to what is on the left.

Case-of

Here is an example of a case _ of statement:

{-# LANGUAGE OverloadedStrings #-}
import Data.Text (Text)

data Color = Black | White
data Piece = Bishop Color | Knight Color | King Color

pieceToText :: ChessPiece -> Text
pieceToText (Piece _ color) = case color of 
    Black -> "black"
    White -> "white"

This is a convenient way to pattern match.

Guards

These are similar to case statements, but instead of pattern matching, you give a boolean condition:

example :: Int -> Bool
example x 
    | x > 0 && x < 10 = True
    | otherwise = False

Note

otherwise is not a keyword, in fact it is just the value True:

repl example
> otherwise
True
> :t otherwise
otherwise :: Bool

For this reason, otherwise will always satisfy the guard, and so is an appropriate catch-all final line.

Let-in

example :: Int
example = 
    let val1 = 5 
        val2 = val1 + 5
    in val1 + val2 + 1

Let-bindings may be recursive.

example :: Int
example = 
    let val1 = 0 : val2
        val2 = 1 : val1
    in val1

Hint

This gives an infinite list. You can sample from it as follows:

> take 20 example
[0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1]

See the section on laziness for more information.

Where

Similar to let:

example = val1 where
    val1 = 0 : val2
    val2 = 1 : val1

See here for differences .

Do-notation

Do-notation is a syntax for imperative style programming. It can be used in conjunction with the IO type:

example :: IO ()
example = do
  userInput <- getLine
  let reversed = reverse userInput
  writeFile "file/path" reversed
  print reversed

Here, the order of operations is top-down (read line, write file, print), and the <- arrow gives a name to the result of an operation (like userInput, which is the result of reading from stdIn with getLine) which can be used later.

Note

Do-notation gets converted in the following way:

do 
    x <- m
    f x

m >>= (\x -> f x)

Or for the above example:

example :: IO ()
example = do
    userInput <- getLine
    let reversed = reverse userInput
    writeFile "file/path" reversed
    print reversed

example :: IO ()
example = getLine >>= (\userInput -> 
    let reversed = reverse userInput
    in (writeFile "file/path" reversed >>= 
    (\_ -> print reversed)))

As this shows, not only IO, but any type f :: * -> * which is an instance of Monad (and thus implements >>=) can be used with do-notation. For this reason, do-notation is common in Haskell code with many different Monad instances.

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