Re: [Haskell-cafe] Specifying a function type in a where clause.

Also, in Haskell98 you can also use this hack to document & coerce the typechecker: combs' acc x | const False (acc `asTypeOf` [l]) = undefined `asTypeOf` [l] combs' acc x | const False (x `asTypeOf` head l) = undefined combs' acc x = ... definition as before The compiler should remove the "const False" branches so they don't use up performance. -- ryan On Thu, Sep 18, 2008 at 4:09 PM, Ryan Ingram wrote:

The Haskell98 solution is to use "asTypeOf" to document types in helper functions

combs' acc x = let sl = delete (x `asTypeOf` head l) l in ...

Using ScopedTypeVariables is much nicer, though.

-- ryan

On Thu, Sep 18, 2008 at 3:55 PM, Daniel Fischer wrote:

...

Am Freitag, 19. September 2008 00:33 schrieb Rob deFriesse:

...

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'/.

Not quite. As it is, the type signature of combs' promises that for all types b which belong to Eq, combs' has type [[b]] -> b -> [[b]], because the a from the top level type signature is not in scope. Therefore, ghc complains about the use of l in the definition of combs', since l has a specific type. You can either remove the signature for combs' or bring the type variable a into scope by the pragma {-# LANGUAGE ScopedTypeVariables #-} at the top of your file or the flag -XScopedTypeVariables on the command line and changing the signature of combs to combs :: forall a. (Eq a) => [a] -> Int -> [[a]], I think then you must leave off the (Eq a) from the signature of 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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