I am a newbie trying to learn haskell, I tried to search for something similar in other forums but couldn't find a similar question.
addPoly :: (Num a)=>[[a]]->[a]
addPoly x = map sum $ transpose x
runs fine
but when I remove x at the end it errors out
addPoly :: (Num a)=>[[a]]->[a]
addPoly = map sum $ transpose
errors saying:
Couldn't match expected type `[[Integer]] -> [Integer]'
with actual type `[Integer]'
In the expression: map sum $ transpose
In an equation for `addPoly': addPoly = map sum $ transpose
Couldn't match expected type `[[Integer]]'
with actual type `[[a0]] -> [[a0]]'
In the second argument of `($)', namely `transpose'
In the expression: map sum $ transpose
In an equation for `addPoly': addPoly = map sum $ transpose
Couldn't figure out what I am missing here.
Disclaimer: this is not a homework problem
$ is defined in Haskell as
f $ x = f x
infixr 0 $
So if you expand the first snippet of your code,
map sum $ transpose x
becomes
map sum (transpose x)
which will work.
But the second snippet
map sum $ transpose
becomes
map sum transpose
and when you call that with x, you get
map sum transpose x
which actually map's sum over transpose (and calls the result with argument x, which also doesn't make sense, and causes the error message you get, as map will return a List, not a function), and not over transpose x.
You need to use the . function for this, instead of $, which is defined as
(.) f g = \x -> f (g x)
If you do that, your code
map sum . transpose
becomes
\x -> map sum (transpose x)
and when you call this from on some parameter x, it simply becomes
map sum (transpose x)
which is the (correct) code we started with.
Let me know if something isn't clear.