#10156: Unable to coerce with flipped Coercible context -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.10.1-rc2 Component: Compiler | Operating System: Unknown/Multiple (Type checker) | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- {{{#!hs coerce' :: Coercible a b => b -> a coerce' = coerce }}} doesn't compile, forcing workarounds like: {{{#!hs 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`. https://github.com/ekmett/profunctors/blob/1c3508e5274a0ed03765c569596a8fda8... 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. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10156 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler