<p> We already could write and compile next code:</p><p>{-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies #-}</p><p>
{-# LANGUAGE
FlexibleInstances, UndecidableInstances #-}</p><p>class Monoid (a :: k) where<br> type Pr a :: *<br> mempty :: PA a<br> mappend :: Pr a -> Pr a -> Pr a<br><br> <br>instance Monoid [b] where <br> type Pr [b] = [b]<br> mempty = [] <br> mappend = (++)<br> <br>instance (Monad m, MonadPlus m) => Monoid ( m :: * -> * ) where <br>type Pr m = m Int -- Unfortunally, we can't write now
type Pr m =
forall b. m b
</p><p> --Illegal polymorphic or qualified type: forall b. m b<br> mempty = mzero<br> mappend = mplus <br> </p><p>This code is already valid.</p><p>But we can't use any written instance at all, the ambiguity problem.</p><p><br></p><div id="editor_compose_signature"><p>Wvv</p></div><blockquote style='border-left:2px solid #CCCCCC;padding:0 1em' style="border-left:1px solid #BBBBBB;margin:1ex;padding:1ex;"><p>29 / 06 / 2013 17:39:25 user Ben Franksen [via Haskell] (<a href="/user/SendEmail.jtp?type=node&node=5732455&i=0" target="_top" rel="nofollow" link="external">[hidden email]</a>) wrote:</p>This is a kind error. The m in Monoid has kind *, while the one in Monad has <br>kind * -> *.
<br><br>I suggest you (try to) rewrite your proposal to eliminate these kind errors
<br>before any further elaboration.
<br><br>Cheers
<br>--
<br>Ben Franksen
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