#13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification -------------------------------------+------------------------------------- Reporter: ljli | Owner: simonmar Type: bug | Status: new Priority: highest | Milestone: 8.0.3 Component: Compiler | Version: 8.1 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 rwbarton): Summary of the discussion of this ticket at today's meeting: * When we type check (in `tcApplicativeStmts`) a group of independent statements {{{#!hs do pat1 <- exp1 pat2 <- exp2 ... patN <- expN }}} stuff bound by `pat1` should ''not'' be visible in `exp2`, and so on. Here stuff includes both the (visible) values bound by `pat1`, and also (invisible) dictionaries or equality constraints bound by matching on a qualified or GADT constructor. However, ''all'' the stuff (both visible and invisible) bound by any of the patterns should be in scope after the group. * We decided how to split a `do` expression into groups of independent statements earlier, in the renamer, on syntactic grounds; that is, based only on ''visible'' stuff. But there could be invisible dependencies too, such as in {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE ApplicativeDo #-} module T12870 where data T a = Eq a => T f :: (Monad m) => a -> a -> m (T a) -> (Bool -> m b) -> m (b, b) f x y mta mb = do T <- mta r1 <- mb (x == y) r2 <- mb (x == y) return (r1, r2) }}} This program compiles today without `ApplicativeDo`, but causes the panic discussed here with `ApplicativeDo`. In the current scheme we determine the groups of independent statements in the renamer, which is too early to detect that the expression `mb (x == y)` relies on the binding of `T`. Plus Simon thinks it would be too fragile anyways. (What if there was another `Eq a` instance in scope from somewhere else? Which instance do we use? It would affect the grouping.) Simon's suggestion was to reject a program like this in the type checking stage. It would be a bit unfortunate, because the program would have compiled fine without `ApplicativeDo`. Here's another suggestion: whenever there is a pattern match that binds invisible stuff, just assume that that stuff is used in all following statements. Similar to "just disable ApplicativeDo for existential patterns", but the issue isn't existentials, but rather dictionaries or equality constraints. The original program using `data A = forall a. A a` is actually fine to treat as a single group of independent statements, since the pattern match on `A _` doesn't bind any invisible stuff. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13242#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler