#15589: Always promoting metavariables during type inference may be wrong -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: TypeInType 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): The "tyvars in scope" is a red herring. Or, rather, your approach of linking a unification var to its enclosing implication carries the same data, so there is no difference between our approaches. My comment:3 tersely suggested a way to allow `x`'s instantiation, even before everything has settled. It might be equivalent to your instantiation constraints -- hard to say. (Are these specified in the paper? I don't recall.) Consider this version: {{{ f (y::Proxy c) = let x :: forall a b (d :: a) z. z -> SameKind (IfK c b d) d x = undefined in (x (y :: Proxy True), blah) }}} Now, we get the key bit of information -- that `c` is `True` -- in an argument passed to `x`, which we cannot look at without instantiating `x`. (We need to instantiate `x` before looking at its arguments because perhaps `x` is higher-rank and we'll need to "push down" the polymorphic type of `x`'s argument. Indeed, I could have made my example higher-rank to demonstrate this, but didn't for simplicity.) If the instantiation constraints can handle this case, then they're likely equivalent to what I was thinking about in comment:3. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15589#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler