Unable to coerce with flipped Coercible context
coerce' :: Coercible a b => b -> a
coerce' = coercedoesn't compile, forcing workarounds like:
coerce' :: forall a b. Coercible a b => b -> a
coerce' = coerce (id :: a -> a)This arises in practice in several places in the profunctors package and lens.
http://hackage.haskell.org/package/lens-4.8/docs/src/Control-Lens-Internal-Coerce.html#coerce%27
Ideally we'd be able to retire coerce' entirely and just use coerce in those contexts.
The tricky part is of course avoiding infinite loops once you add symmetry.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.10.1-rc2 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |