DatatypeContexts in data types and data families
Does datatype contexts only apply for non-phantom variables? Figured I'd ask
{-# Language DatatypeContexts, TypeFamilies #-}
-- F :: Foo a b
data (a ~ b) => Foo a b = F
and
{-# Language DatatypeContexts, KindSignatures, DatatypeContexts, DataKinds, TypeFamilies #-}
data N = O | S N
-- FO :: Fin (S n) n' O
data family Fin (n::N) (a::N) (b::N)
data instance (n ~ n') => Fin (S n) n' O = FO
It surprised me to see the first two of these (yes I know, datatype contexts):
-- F :: x -> Foo x x
data (a ~ b) => Foo a b = F a
-- F :: x -> Foo x x
data (a ~ b) => Foo a b = F b
-- F :: x -> x -> Foo x x
data (a ~ b) => Foo a b = F a b
Trac metadata
Trac field | Value |
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Version | 8.0.1 |
Type | Task |
TypeOfFailure | OtherFailure |
Priority | lowest |
Resolution | Unresolved |
Component | Compiler |
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