#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 simonpj): OK here's what I can do easily. Let's say that in a `Stmt` {{{ pat <- rhs }}} * the `Stmt` ''requires'' constraints needed to typecheck `rhs`, and to pattern-match `pat` (e.g. in a view pattern) * the `Stmt` ''binds'' the existential type variables and constraints brought into scope by the pattern `pat` So for example, given {{{ data T a where MkT :: (Eq a, Show b) => a -> b -> T a }}} Suppose that `v :: t` is in scope. Then the stmt {{{ MkT 1 x <- (...show v...) :: T t }}} * requires `Show t` (from the use of `show`) and `Num t` (from the literal `1`). * binds the existential `b` and constraint `Show b`. Now consider {{{ ...stmts... ApplicativeStmts [arg1, arg2, ... argN] ...more stmts... }}} where `argi :: ApplicativeArg`. Each `argi` itself contains one or more `Stmts`. It is easy to ensure that * Constraints required by the `argi` can be solved from constraint bound by `...stmts...` * Constraints and existentials bound by the `argi` are not available to solve constraints required either by `argj` (where i is different from j), or by `...more stmts...`. * Within the stmts of each `argi` individually, constraints bound by earlier stmts can be used to solve later ones. That is easy to implement and solves the immediate problem. I'm validating now. ------------------------ But the rule is terribly unsatisfactory. * To typecheck the program you must mentally desugar it into its applicative groups. * If you write {{{ MkT x1 _ <- rhs1 MkT x2 _ <- rhs2 }}} you really might intend that the `Eq t` bound by the first stmt is available to solve requirements of `rhs2`. And with normal monadic deguaring it would be, but not with `ApplicativeDo`. Worse, ''there is no way to fix it'' becuase `ApplicativeDo` works module-wide. My solution is: add syntax to allow the programmer to express the program in the form of Figure 2 of our paper, that is, post-rearrangement. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13242#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler