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Haskel type declaration, function and tuple as arguments

I'm doing a haskell assignment for school. I want to make a function called MapTuple, wich maps a function with a tuple as its arguments for an array of tuple. Im declaring it on the following way:

MapTuple :: [(a,b)] -> (a -> b) -> [b]

the way i want to use the function is as follows.

MapTuple :: [(Int, String)] -> (Int -> String) -> [String]

problem however is that I get the following error when compiling:

Invalid type signature: MapTuple :: ([(a, b)]) -> (a -> b) -> [b] Should be of form ::

What am I doing wrong?

Thanks a lot for helping me in advance!

Solution

  • Haskell has a naming convention that's actually part of the language

    • Functions start with lower case
    • Constructors start with upper case

    So you need to change MapTuple to mapTuple

    As for what a constructor is,

     data Foo = ThisIsAConstructor

    It's basically a function that returns a Foo.