Coercible constraint solver misses one
When I say
import Data.Type.Coercion
import Data.Coerce
eta :: Coercible f g => Coercion (f a) (g a)
eta = Coercion
I get
Could not coerce from ‘f a’ to ‘g a’
because ‘f a’ and ‘g a’ are different types.
arising from a use of ‘Coercion’
from the context (Coercible f g)
bound by the type signature for
eta :: Coercible f g => Coercion (f a) (g a)
at /Users/rae/temp/Roles.hs:6:8-44
In the expression: Coercion
In an equation for ‘eta’: eta = Coercion
But, this coercion is easily expressible in Core. If (co :: f ~R# g)
then (co <a> :: f a ~R# g a)
, where <a>
is the notation for a reflexivity coercion for the type a
. The constraint solver should be able to do this.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |