ConstraintKinds lets you put the typechecker into a loop
Ed Kmett was playing with them the other day and Dan Doel and I reduced the issue to this:
{-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances, KindSignatures #-}
class C (p :: Constraint)
class D (p :: Constraint)
instance C (D p) => C (D (D p))
{-
-- This works
data E a = E
class F a
instance F (E a) => F (E (E a))
-}Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |