[GHC] #10249: GHCi leaky abstraction: error message mentions `ghciStepIO`

[GHC]

#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>
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