#15470: Record projections with ambiguous types -------------------------------------+------------------------------------- Reporter: sweirich | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.10.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: 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 RyanGlScott): I'm a bit leery about changing the order of type variables just to support this one use case, especially since if we had the ability to bind type variables in lambdas, then we could implement these record projections without the need to change the original types: {{{#!hs type family F a data T a = MkT (forall a. F a) fld :: T -> (forall a. F a) fld (MkT x) = (\@a -> x @a) }}} Moreover, this type-variable-reordering hack really only works in one direction. That is to say, you can define record selectors since the position of the `forall` allows you to float those type variables to the front, but you can't define a synonym for the constructor itself, like so: {{{#!hs mkT :: (forall a. F a) -> T mkT = ??? }}} Here, we can't float the `a` to the front, so we're left without a way to implement this. With type variables in lambdas, we could, however: {{{#!hs mkT :: (forall a. F a) -> T mkT x = T (\@a -> x @a) }}} Of course, I'm speculating a bit here and assuming that the ambiguity check won't kick in on the examples above. (I have no way of knowing this, since type-variables-in-lambdas aren't yet implemented.) But it seems like it could be a viable alternative. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15470#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler