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haskellcurrying

Writing an equivalent functions by leveraging currying

i am studying Haskell and I do not understand why the following two implementation are not equivalent. Rather why I get the error message if I try to load it in REPL.

addStuff :: Integer -> Integer -> Integer
addStuff x y = x + y + 5

addStuff_ :: Integer -> Integer -> Integer
addStuff_ x = x + (\i -> i + 5)

The erro I get is the following

   • Couldn't match expected type ‘Integer’
                  with actual type ‘a0 -> a0’
    • The lambda expression ‘\ i -> i + 5’ has one argument,
      but its type ‘Integer’ has none
      In the second argument of ‘(+)’, namely ‘(\ i -> i + 5)’
      In the expression: x + (\ i -> i + 5)
Solution

  • The second function is trying to sum Integer with another function. What you are looking for is function composition (using . operator):

    addStuff_ :: Integer -> Integer -> Integer
addStuff_ x = (x +) . (+ 5)

    Or just:

    addStuff' :: Integer -> Integer -> Integer
addStuff'  = (+) . (+ 5)