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