#7828: RebindableSyntax and Arrow ----------------------------------------------+---------------------------- Reporter: AlessandroVermeulen | Owner: Type: bug | jstolarek Priority: normal | Status: new Component: Compiler (Type checker) | Milestone: 7.10.1 Resolution: | Version: 7.6.2 Operating System: Unknown/Multiple | Keywords: Type of failure: GHC rejects valid program | Architecture: Test Case: | Unknown/Multiple Blocking: | Difficulty: Unknown | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by jstolarek):
I think the idea would be that each construct becomes decorated with the overloaded operations necessary to desugar it.
Technically this is simple but I feel the resulting design will be inelegant. We use `StmtLR` datatype to represent both the monadic and arrow do-notation. This means that desugaring `BindStmt` inside a monadic `do` requires calls to `>>=` and `fail` ([https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsExpr.lhs#L765 here]), whereas desugaring the same `BindStmt` inside the arrow `do` requires a call to `first`, two different calls to `>>>` and two different calls to `arr` ([https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsArrows.lhs#L838 here]). Adding five extra fields to `BindStmt` feels like a lot of unnecessary noise. Perhaps creating helper datatypes to store these extra identifiers would be better? But then every (or almost every) constructor of `StmtLR` would require such extra datatype. I don't yet see a clean solution. Augmenting `HsCmd` with this extra information seems relatively simple, with `HsCmdCase` being the only tricky case. (It reuses a call to `(|||)` in every branch of a case ([https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsArrows.lhs#L555 here]), which means we need to generate a whole list of new identifiers.) What do you think about the design I describe above? Is it acceptable? Janek -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler