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functionhaskellparameterspointfree

Haskell function takes 2 parameters but in the definition none

I am new to Haskell and I am trying to understand a game created in Haskell (tic tac toe). I know that if a function takes n parameters then you must provide n parameters in the function definition. Example:

f :: Int -> Int -> String
f a b = "This function makes no sense"

However in this Haskell script there is a function that takes two arguments but in the definition it has none. And of course, it's working but I can't seem to figure out why.

import Data.Map qualified as M
type Board = M.Map (Int, Int) Marker
data Marker = X | O | Blank deriving Eq

getMarker :: Board -> (Int, Int) -> Marker
getMarker = flip $ M.findWithDefault Blank

Any ideas on what this function does and more importantly, why it's working (you can see that getMarker takes 0 parameters at the last line) ?

Solution

  • The misconception is here:

    ... (you can see that getMarker takes 0 parameters at the last line) ...

    and the thing that's puzzling you is partial application.

    getMarker :: Board -> (Int, Int) -> Marker
getMarker = flip $ M.findWithDefault Blank

    What that last line actually tells you is that the getMarker doesn't do anything with its arguments - but they still get passed to the function created by flip $ M.findWithDefault Blank.

    Or, more accurately, getMarker evaluates to a function of the declared type which is applied to getMarker's arguments.