id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7095	Kind-polymorphic typechecking requires better documentation.	schernichkin		"Hi. I faced some difficulties using kind-polymorphic instances and I think documentation should be improved. Consider following code:

{{{
{-# LANGUAGE DataKinds, TypeOperators, PolyKinds, FlexibleInstances, FlexibleContexts #-}

data Wrapped t = Wrapped 

class Len l where
    len :: l -> Int

instance Len (Wrapped '[]) where
    len = const 0

instance (Len (Wrapped xs)) => Len (Wrapped (x ': xs)) where
    len x = 1 + (len $ wrappedTail x)   

wrappedTail :: Wrapped (x ': xs) -> Wrapped xs
wrappedTail = const Wrapped  

-- | test1 == zero just as excepted. 
test1 = len (undefined  :: Wrapped '[])

-- | Since I have typeclasses defined for Wrapped (* ': [*]) and for (Wrapped '[])
-- I except to get 1 here, but this does not typecheck with following  message:
-- No instance for (Len (Wrapped [*] ([] *)))  
test2 = len (undefined  :: Wrapped '[Int]) 
}}}

It will not typecheck. Of couse it is possible to fix it by adding type signature to Wrapped:

{{{
{-# LANGUAGE DataKinds, TypeOperators, PolyKinds, FlexibleInstances, FlexibleContexts, KindSignatures #-}

data Wrapped (t :: [*]) = Wrapped 
}}}

but it is extremely hard to figure out relaying just on information provided here: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-polymorphism-and-promotion.html, especially in complex scenarios. I think this page should provide more information on type-checker messages format related to polymorphic kind, and some common pitfalls."	feature request	closed	normal		Documentation	7.4.2	fixed	Kind polymorphism		Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown	polykinds/T7095