#2102: Typeclass membership doesn't bring coercion superclass requirements into
scope
-------------------------+--------------------------------------------------
Reporter: ryani | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
-------------------------+--------------------------------------------------
Comment (by ryani):
This is related to a feature request:
http://hackage.haskell.org/trac/ghc/ticket/2101
Here's the motivating example:
{{{
type family Cat ts0 ts
type instance Cat () ts' = ts'
type instance Cat (s, ts) ts' = (s, Cat ts ts')
class (Cat ts () ~ ts) => Valid ts
instance Valid () -- compiles OK
instance Valid ts => Valid (s, ts) -- fails to compile
-- need to prove Cat (s, ts) () ~ (s, Cat ts ())
-- for the superclass of class Valid.
-- (1) From Valid ts: Cat ts () ~ ts
-- (2) Therefore: (s, Cat ts ()) ~ (s, ts)
coerce :: forall f ts. Valid ts => f (Cat ts ()) -> f ts
coerce x = x
}}}
GHC gives the following two errors:
{{{
tyfam.hs:38:0:
Couldn't match expected type `ts' against inferred type `Cat ts ()'
`ts' is a rigid type variable bound by
the instance declaration at tyfam.hs:38:15
When checking the super-classes of an instance declaration
In the instance declaration for `Valid (s, ts)'
tyfam.hs:46:11:
Couldn't match expected type `ts' against inferred type `Cat ts ()'
`ts' is a rigid type variable bound by
the type signature for `coerce' at tyfam.hs:45:19
Expected type: f ts
Inferred type: f (Cat ts ())
In the expression: x
In the definition of `coerce': coerce x = x
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2102#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
