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
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
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participants (2)
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Alexis King -
Simon Peyton Jones