Can't call rank-2 function with type family constraint
The following code compiles with 7.10.3:
newtype Bar r = Pow r deriving (Eq)
instance (Foo r) => Foo (Bar r)
type family Ctx a where Ctx t = (Foo (Bar t), Eq (Bar t))
run :: (forall t . (Ctx t) => t -> Int) -> Int
run g = undefined
foo :: (Foo (Bar t)) => t -> Int
foo = undefined
main :: IO ()
main = print $ run foo
but fails in 8.0.1 with
• Could not deduce (Foo t) arising from a use of ‘foo’
from the context: Ctx t
bound by a type expected by the context:
Ctx t => t -> Int
at Main.hs:19:16-22
Possible fix:
add (Foo t) to the context of
a type expected by the context:
Ctx t => t -> Int
• In the first argument of ‘run’, namely ‘foo’
In the second argument of ‘($)’, namely ‘run foo’
In the expression: print $ run foo
This code seems valid to me: when I call run foo
, foo
is specialized to (Foo (Bar t), Eq (Bar t)) => t -> Int)
, which meets the requirement of the argument to run
.
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |