#12680: Permit type equality instances in signatures -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:4 ezyang]:
type S is much better, because users can't make any assumptions about a type declaration that they might for a data declaration.
As far as I could tell, GHC actually doesn't make any assumptions when `data S` denotes an abstract type; so it's just a syntactic wart. It's a bit bothersome to fix because hs-boot syntax only supports data.
I don't agree. In GHC 8: {{{ -- A.hs-boot module A where data T a }}} {{{ -- B.hs {-# LANGUAGE TypeFamilies #-} module B where import {-# SOURCE #-} A foo :: (T a ~ T b) => a -> b foo x = x }}} These compile fine. GHC must be assuming that `T` is injective. So we must proceed cautiously.
It would be super awesome if the user could prove the equality
manually.
Do you have an example of a case where you could do this?
Nope! :) Hence the level of awesomeness if we could do this. Admittedly well beyond the scope of Backpack. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12680#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler