[Haskell-cafe] Type error with Type families

[Marco Túlio Pimenta Gontijo]

Hi.

I cannot make this program type check:

    {-# LANGUAGE TypeFamilies, FlexibleContexts #-}
    import qualified Data.ListLike as LL

    class LL.ListLike full (Item full) => ReplaceOneOf full where
      type Item full :: *
      replaceOneOf :: [Item full] -> full -> full -> full
      replaceOneOf from to list
        | LL.null list = list
        | x `element` from
          = LL.concat [to, replaceOneOf from to $ LL.dropWhile
(`element` from) xs]
        | otherwise = LL.cons x $ replaceOneOf from to xs
        where
          x = LL.head list
          xs = LL.tail list
      element :: Item full -> [Item full] -> Bool

The error message is:

    Line 9: 1 error(s), 0 warning(s)

    Could not deduce (Item full0 ~ Item full)
    from the context (ReplaceOneOf full)
      bound by the class declaration for `ReplaceOneOf'
      at /home/marcot/tmp/test_flymake.hs:(4,1)-(15,45)
    NB: `Item' is a type function, and may not be injective
    Expected type: [Item full0]
      Actual type: [Item full]
    In the second argument of `element', namely `from'
    In the expression: x `element` from

I have tried using asTypeOf, but it did not work:

    {-# LANGUAGE TypeFamilies, FlexibleContexts, ScopedTypeVariables #-}
    import qualified Data.ListLike as LL

    class LL.ListLike full (Item full) => ReplaceOneOf full where
      type Item full :: *
      replaceOneOf :: Item full -> [Item full] -> full -> full -> full
      replaceOneOf xt from to list
        | LL.null list = list
        | (x `asTypeOf` xt) `element` from
          = LL.concat [to, replaceOneOf xt from to $ LL.dropWhile
(`element` from) xs]
        | otherwise = LL.cons x $ replaceOneOf xt from to xs
        where
          x = LL.head list
          xs = LL.tail list
      element :: Item full -> [Item full] -> Bool

Can someone point me to a solution?

Greetings.

-- 
marcot
http://marcot.eti.br/