Haskell function template problems

I'm doing my homework, and while I have some experience with SML, Haskell has some oddities. Consider this simple function:

type Pos = (Int, Int)
data Move = North | South | East | West
move :: Move -> Pos -> Pos
move North (x,y) = (x, y+1)
move South (x,y) = (x, y-1)
move East  (x,y) = (x+1, y)
move West  (x,y) = (x-1, y)

moves :: [Move] -> Pos -> Pos
moves (m:ms) (x,y) = moves ms (move m (x,y))
moves [] p = p


This code works. However, if I swap a tuple (x,y)

(which I don't use at all) with a simple one p

, it doesn't work when called (the declaration works fine, of course):

moves :: [Move] -> Pos -> Pos
moves (m:ms) p = moves ms (move m p)
moves [] p = p

*Main> let p = (1,1) :: Pos
*Main> move [North, North] p

    Couldn't match expected type `Move' against inferred type `[a]'
    In the first argument of `move', namely `[North, North]'
    In the expression: move [North, North] p
    In the definition of `it': it = move [North, North] p


Which seems strange to me, since the second parameter is already entered as Pos in the definition, since this is what happens and only when called? I am using ghci btw.


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1 answer

You forgot the "s" at the end of the call to s , didn't you?

*Main> move [North, North] p




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