Hi,
Ticket #8776 (https://ghc.haskell.org/trac/ghc/ticket/8776) is about the
output formatting of pattern synonym type signatures. With -dppr-debug,
the problematic signature looks like this:
(base:GHC.Num.Num{tc 2b} t{tv aQa} [sk],
ghc-prim:GHC.Classes.Eq{tc 23} t{tv aQb} [sk]) =>
main:Main.P{d rs2} :: main:Main.A{tc rpC} t{tv aQa} [sk] t1{tv aQb} [sk]
The problem is that the two type variables occuring in the provided
context both have "t" as their user-visible name; even though the second
one is projected as "t1" in the tau type on the right-hand side.
The code to generate this output is in HsBinds.ppr_sig, using pprPatSynSig
which just adds the "pattern" keyword and the separating "=>" symbols etc:
ppr_sig (PatSynSig name arg_tys ty prov req)
= pprPatSynSig (unLoc name) False args (ppr ty) (pprCtx prov) (pprCtx req)
where
args = fmap ppr arg_tys
pprCtx lctx = case unLoc lctx of
[] -> Nothing
ctx -> Just (pprHsContextNoArrow ctx)
My guess is that the problem is 'pprHsContextNoArrow' projects individual
constraints one-by-one and so doesn't notice the name clash on
't' between the two constraints in the example; whereas 'ppr ty' walks the
whole right-hand tau type and thus has the opportunity to maintain a set
of type variable names used.
My question is, where is that logic, and how can I use that in this
instance? My hope is to be shown a design where I can run 'ppr ty',
'pprCtx prov' and 'pprCtx req' in the same "naming environment" (I hope
such a thing exists) so that they use a consistent naming scheme. This
looks like a problem that must have popped up at a lot of places in GHC
already and must have an idiomatic solution.
Thanks,
Gergo
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