I would like to know why I'm getting a particular compile time error
message.
In this program, I am specifying a function type on /combs'/ in the
where clause:
--------
module Main where
import List (delete)
combs :: Eq a => [a] -> Int -> [[a]]
combs l 1 = map (\x -> [x]) l
combs l n = foldl combs' [] l
where combs' :: Eq a => [[a]] -> a -> [[a]]
combs' acc x = let sl = delete x l in (map (\i -> x:i) $ combs
sl (n-1)) ++ acc
main = do
print $ combs ["a","b","c","d"] 3
--------
I get this error message from GHC 6.8.3:
cafe1.hs:9:43:
Couldn't match expected type `a1' against inferred type `a'
`a1' is a rigid type variable bound by
the type signature for `combs'' at cafe1.hs:8:23
`a' is a rigid type variable bound by
the type signature for `combs' at cafe1.hs:5:12
Expected type: [a1]
Inferred type: [a]
In the second argument of `delete', namely `l'
In the expression: delete x l
I don't understand why I'm seeing this. The type /a/ is the same type
in /combs/ and /combs'/. I realize I can remove the type specification
from /combs'/, and the code will compile. However, I'd like to get a
better understanding of why GHC objects to this.
Thank you,
Rob.
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