#10249: GHCi leaky abstraction: error message mentions `ghciStepIO` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: GHCi | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect | Test Case: warning at compile-time | ghci/scripts/T10249 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1527, Wiki Page: | Phab:D1528 -------------------------------------+------------------------------------- Comment (by simonpj): Gah. I had a quick look, and it's fiddly. When you write {{{ a <- e }}} at the prompt, we get into `TcRnDriver.tcUserStmt`; in the second equation (i.e. not the `BodyStmt` case). It does this {{{ ; let gi_stmt | (L loc (BindStmt ty pat expr op1 op2)) <- rn_stmt = L loc $ BindStmt ty pat (nlHsApp ghciStep expr) op1 op2 | otherwise = rn_stmt }}} which transforms the Stmt to {{{ a <- (ghciStepIO :: forall a. M a -> IO a) e }}} where `M` is the currently-in-force GHCi monad. Apparently, via `TcRnMonad.getGHCiMonad` and `setGHCiMonad`, it is possible to change the monad in which GHCi bindings are understood, using the library class {{{ module GHC.Ghci where class (Monad m) => GHCiSandboxIO m where ghciStepIO :: m a -> IO a }}} to mediate. I can't see this documented in the user manual, or indeed anywhere else. Sigh. Anyway it's this `ghcStepIO` call that the typechecker is complaining about. I can see various ways to avoid this leakage * Typecheck `e` all by iself, checking that it has type `M <something>`, before attempting to typecheck `a <- ghcStepIO e`. This is a bit similar to what happens in "Plan C" of the `BodyStmt` case of `tcUserStmt` (see `This two-step story is very clunky, alas`). * Some how avoid adding the error contexts in `tcExpr` for generated code. Neither of these seems particularly easy. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10249#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler