Hi Rodrigo,
Happy to see that you resumed your work on the pattern-match checker.
I think you are right; we could reasonably just go with your code, not
least because it is less confusing to retain as much info about `x` as
possible.
I don't think it makes a difference, because whenever we add a
constraint `x ≁ ⊥` afterwards, we call `addNotBotCt` which will
interpret this constraint as `y ≁ ⊥` via `lookupVarInfoNT`, and we have
accurate BotInfo for `y`.
Basically whenever we have seen `x ~ T y`, `T` Newtype, we will never
look at `BotInfo` of `x` again. I thought it might become relevant in
`generateInhabitingPatterns` for warning messages, but there we eagerly
instantiate through NTs anyway.
So by all means, open an MR for your change. Good work!
Sebastian
------ Originalnachricht ------
Von: "Rodrigo Mesquita" <rodrigo.m.mesqu...@gmail.com>
An: "Sebastian Graf" <sebastian.g...@kit.edu>
Cc: "GHC developers" <ghc-devs@haskell.org>
Gesendet: 27.10.2023 17:34:29
Betreff: PMC: addConCt and newtypes bottom info
Dear Sebastian and GHC devs,
Regarding this bit from the function addConCt in the
GHC.HsToCore.Pmc.Solver module,
Nothing -> do
let pos' = PACA alt tvs args : pos
let nabla_with bot' =
nabla{ nabla_tm_st = ts{ts_facts = addToUSDFM env x
(vi{vi_pos = pos', vi_bot = bot'})} }
-- Do (2) in Note [Coverage checking Newtype matches]
case (alt, args) of
(PmAltConLike (RealDataCon dc), [y]) | isNewDataCon dc ->
case bot of
MaybeBot -> pure (nabla_with MaybeBot)
IsBot -> addBotCt (nabla_with MaybeBot) y
IsNotBot -> addNotBotCt (nabla_with MaybeBot) y
_ -> assert (isPmAltConMatchStrict alt )
pure (nabla_with IsNotBot) -- strict match ==> not ⊥
My understanding is that given some x which we know e.g. cannot be
bottom, if we learn that x ~ N y, where N is a newtype (NT), we move
our knowledge of x not being bottom to the underlying NT Id y, since
forcing the newtype in a pattern is equivalent to forcing the
underlying NT Id.
Additionally, we set x’s BottomInfo to MaybeBot —
However, I don’t understand why we must reset x’s BotInfo to MaybeBot —
couldn’t we keep it as it is while setting y’s BotInfo to the same
info?
An example where resetting this info on the newtype-match is
important/necessary would be excellent.
FWIW, I built and tested the PMC of ghc devel2 with
MaybeBot -> pure (nabla_with MaybeBot)
IsBot -> addBotCt (nabla_with IsBot) y
IsNotBot -> addNotBotCt (nabla_with IsNotBot) y
And it worked without warnings or errors…
Thanks in advance!
Rodrigo
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