Kind inference fails in data instance definition
Consider the following shenanigans:
{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, GADTs #-}
data family SingDF (a :: (k, k2 -> *))
data Ctor :: k -> *
data instance SingDF (a :: (Bool, Bool -> *)) where
SFalse :: SingDF '(False, Ctor)HEAD reports (with -fprint-explicit-kinds)
Data constructor ‛SFalse’ returns type ‛SingDF
Bool k '('False, Ctor k)’
instead of an instance of its parent type ‛SingDF Bool Bool a’
In the definition of data constructor ‛SFalse’
In the data instance declaration for ‛SingDF’I see two problems here:
-
Kind inference should fix the problem. If I add a kind annotation to
Ctor, the code works. GHC should be able to infer this kind annotation for me. -
The error message is not particularly helpful. GHC 7.6.3 had a more verbose, but more helpful message.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.7 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |