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.