Alexis
We have
data MatchGroup p body
= MG { mg_ext :: XMG p body -- Post-typechecker, types of args and result
, mg_alts :: Located [LMatch p body] -- The alternatives
, mg_origin :: Origin }
explicitly parameterised over 'body'. And we use that parameterisation:
| HsLam (XLam p)
(MatchGroup p (LHsExpr p))
| HsCmdLam (XCmdLam id)
(MatchGroup id (LHsCmd id)) -- kappa
So it makes perfect sense to me that the desugarer for MatchGroup should be parameterised with a function for desugaring 'body'.
(Let's be consistent about whether we use 'rhs' or 'body' for this parmemeterisation.)
TL;DR: Yes, what you suggest sounds sensible to me.
Simon
| -----Original Message-----
| From: ghc-devs On Behalf Of Alexis King
| Sent: 26 April 2020 02:40
| To: ghc-devs
| Subject: Desugaring matches with already-desugared RHSs?
|
| Hi all,
|
| I’m currently in the process of rewriting most of the arrow desugaring
| code. One of the most awkward parts of the current implementation is
| the way case commands are desugared. Given a case command like
|
| case e1 of
| A a b -> cmd1
| B c -> cmd2
| C d e f -> cmd3
|
| the desugarer actually replaces each command on the RHS with an
| Either-wrapped tuple to get something like this:
|
| arr (\env -> case e1 of
| A a b -> Left (Left (a, b))
| B c -> Left (Right c)
| C d e f -> Right (d, e, f))
| >>> ((cmd1 ||| cmd2) ||| cmd3)
|
| This means the RHSs of the case expression are really already
| desugared, and ideally they would be CoreExprs, but matchWrapper
| expects the RHSs to be HsExprs. The current implementation
| accommodates this restriction by building fake HsExprs with no
| location information, but this means the logic for building the tuples
| in the RHSs has to be duplicated (since other places do want
| CoreExprs).
|
| I was thinking it would be nice to avoid this hack, but I’m not sure
| what the best way to do it is. One way would be to create a variant of
| matchWrapper with a type like
|
| matchWrapper'
| :: HsMatchContext GhcRn
| -> Maybe (LHsExpr GhcTc)
| -> MatchGroup GhcTc rhs
| -> (rhs -> DsM CoreExpr) -- how to desugar the RHSs
| -> DsM ([Id], CoreExpr)
|
| and update dsGRHSs to accept an extra argument as well. Then the arrow
| desugaring code could just pass `return` to matchWrapper' so it
| wouldn’t touch its RHSs. But I’m not sure if this approach makes sense
| — nothing else in the desugarer seems to work this way. Is there a
| better approach I’m not seeing?
|
| Thanks,
| Alexis
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