It shoudn't typecheck. Suppose you have instances like instance ReplaceOneOf Foo where type Item Foo = Baz element = elementFoo instance ReplaceOneOf Bar where type Item Bar = Baz element = elementBar Now if you call replaceOneOf manyBazs foo1 foo2, Haskell should consult "element :: Baz -> [Baz] -> Baz" — but which one, elementBar or elementFoo? The error message is a bit criptic, but what it really means is that Haskell sees the possibility of such confusion and has to resort to the general "element" function of type Item something -> [Item something] -> Item something, and then fails to unify this "Item something" with "Item full". It correctly notes that the type function "Item" is not injective, which means that "Item Foo ~ Item Bar" is possible even if "Foo ~ Bar". For the solution. I assume that the standard "elementOf" function doesn't suit you. If so, you can make your Items instances of another class: class Element item where element :: item -> [item] -> Bool and change ReplaceOneOf declaration to class (LL.ListLike full (Item full), Element (Item full)) => ReplaceOneOf full where ... On Sep 16, 2012, at 3:05 PM, Marco Túlio Pimenta Gontijo wrote:

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/

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