Map function in Haskell

Question

As far as I understand map in Haskell takes a function and a List and applies that function to every element in that list before creating a new list with the function applied to each member.

Trying it out, this works fine with really simple functions like (+5) so it works fine if I type:

map (+7) [2,8,9,3]

I get:

[9,15,16,10]

However this doesn't work with functions that I have created. Say I have made a function shiftLetter whose type declaration is:

shiftLetter :: Char -> Int -> Char

(with Int shifting how far along the letter returned is)

If I make a new function to do it with more than one letter and type:

shiftLetters :: String -> Int -> Char
shiftLetters letters shift = map shiftLetters "AKHLKHN"

I get errors, why is this?

Solution

If you check the type of the map function then you see:

map :: (a -> b) -> [a] -> [b]

So you need a function that takes one parameter, but yours takes two. You have to use

shiftLetters letters shift = map (flip shiftLetter $ shift) letters

or

shiftLetters letters shift = map (`shiftLetter` shift) letters

Pointless style:

shiftLetters = flip $ map . flip shiftLetter

Comments

[Eddie]

Because shiftLetters takes a String then an Int?

[Eddie]

Okay thanks but I've never seen flip or $ before? What are they for?

[Phil]

or you can do (`shiftLetter` shift) instead of (flip shiftLetter $ shift)

[Benjamin Hodgson]

The type system doesn't prevent you from mapping functions of more than one variable. (In fact, in the 'applicative' style it's very common.) map shiftLetters is a perfectly valid expression, with a type of String -> [(Int -> Char)].

[Eddie]

Trying this I get: Syntax error on 'shiftLetter' Perhaps you intended to use -XTemplateHaskell

[Thomas Bartscher]

shiftLetter takes a Char, not a String. Note that String is the same as [Char] which is different from Char.

[Antal Spector-Zabusky]

@Eddie: The space after $ is important.

[Eddie]

I know! Sorry the functions have very similar names. shiftLetter takes a Char but shiftLetters takes a String?

[Eddie]

I meant for Phil's function namely: ('shiftLetter' shift).

[Eddie]

Also, what do I do if say shiftLetter(s) took more than 2 arguments? Say if shiftLetter type definition was: Char -> String -> Int -> Char and shiftLetters: String -> String -> Int -> String?