id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
2855,Surprising type (family) type derived,guest,,"Consider the following module
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug5 where

class C a where
    type T a
    f :: T a -> T a -> T a

data D a = D { t :: T a }

g r = f (t r) (t r)
}}}
Now ask for the type of g
{{{
*Bug5> :t g
g :: (T a ~ T a1, C a1) => D a -> T a1
}}}
Why isn't the type
{{{
g :: (C a) => D a -> T a
}}}
?

The strange type is a minor nuisance, but it gets worse
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug6 where

class C a where
    type T a
    type U a
    f :: T a -> T a -> T a
    x :: U a -> T a

data D a = D { t :: T a, u :: U a }

g r = f (t r) (x (u r))
}}}
This doesn't type check at all.

An even simpler example that fails:
{{{
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Bug7 where

class C a where
    type T a
    type U a
    x :: U a -> T a

data D a = D (U a)

g :: (C a) => D a -> T a
g (D u) = x u
}}}
",feature request,closed,normal,,Compiler,6.10.1,duplicate,,lennart@…,Unknown/Multiple,Unknown/Multiple,,Unknown,,,,