#7828: RebindableSyntax and Arrow ------------------------------------+--------------------------------------- Reporter: AlessandroVermeulen | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: GHC rejects valid program Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ------------------------------------+--------------------------------------- Changes (by simonpj): * difficulty: => Unknown Comment: I know exactly what is happening here, but need some arrow-aware person to do some work to fix it. Remember that GHC typechecks the user-written '''source code''' not the desugared code. What shouuld happen is that at each source-code construct that requires (say) a use of `(>>>)`, we should instantiate a fresh call of `(>>>)`, and attach that instantiated call to the syntax tree. That's what happens for monads. For example, here is the data type of `Stmt` (in `HsExpr`): {{{ data StmtLR idL idR body -- body should always be (LHs**** idR) = LastStmt -- Always the last Stmt in ListComp, MonadComp, PArrComp, -- and (after the renamer) DoExpr, MDoExpr -- Not used for GhciStmtCtxt, PatGuard, which scope over other stuff body (SyntaxExpr idR) -- The return operator, used only for MonadComp -- For ListComp, PArrComp, we use the baked-in 'return' -- For DoExpr, MDoExpr, we don't appply a 'return' at all -- See Note [Monad Comprehensions] | BindStmt (LPat idL) body (SyntaxExpr idR) -- The (>>=) operator; see Note [The type of bind] (SyntaxExpr idR) -- The fail operator -- The fail operator is noSyntaxExpr -- if the pattern match can't fail ...etc... }}} The `SyntaxExpr` on the `ReturnStmt` is for the instantiated call of `return`; the ones on `BindStmt` are for `(>>=)` and `fail`. But currently for arrow `Cmd`s we use a different (older) plan. We have a '''single''' instantiated call of `(>>>)` for the whole `Cmd`. It is held in the `CmdTop`: {{{ data HsCmdTop id = HsCmdTop (LHsCmd id) PostTcType -- Nested tuple of inputs on the command's stack PostTcType -- return type of the command (CmdSyntaxTable id) -- See Note [CmdSyntaxTable] }}} The `CmdSyntaxTable` has the calls for `(>>>)`, `first` etc. But this approach requires `(>>>)` etc to be fully polymorphic in the non- arrow argument, so that this one call can be used at every place in the command that it's needed. And yours are not. Solution: use the same approach as we use for monads: * Get rid of the `CmdSyntaxTable` on HsCmdTop` * Add `SyntaxExpr` fields to all the appropriate constructs in `HsCmd` that need them. I don't think this is really hard, but it's a bit fiddly. I'd be happy to advise if someone wants to undertake it. Simon -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7828#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler