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author | sheaf <sam.derbyshire@gmail.com> | 2022-03-11 17:01:33 +0100 |
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committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2022-03-14 15:08:24 -0400 |
commit | 8eadea670adb5de49ddba7e23d04ec8242ba76a3 (patch) | |
tree | 11d5284281b78446cbbe6dce54bc275b3bad3fba /compiler/GHC/Stg | |
parent | 106413f094d01485503a9b84fa4545d938ea934d (diff) | |
download | haskell-8eadea670adb5de49ddba7e23d04ec8242ba76a3.tar.gz |
Fix isLiftedType_maybe and handle fallout
As #20837 pointed out, `isLiftedType_maybe` returned `Just False` in
many situations where it should return `Nothing`, because it didn't
take into account type families or type variables.
In this patch, we fix this issue. We rename `isLiftedType_maybe` to
`typeLevity_maybe`, which now returns a `Levity` instead of a boolean.
We now return `Nothing` for types with kinds of the form
`TYPE (F a1 ... an)` for a type family `F`, as well as
`TYPE (BoxedRep l)` where `l` is a type variable.
This fix caused several other problems, as other parts of the compiler
were relying on `isLiftedType_maybe` returning a `Just` value, and were
now panicking after the above fix. There were two main situations in
which panics occurred:
1. Issues involving the let/app invariant. To uphold that invariant,
we need to know whether something is lifted or not. If we get an
answer of `Nothing` from `isLiftedType_maybe`, then we don't know
what to do. As this invariant isn't particularly invariant, we
can change the affected functions to not panic, e.g. by behaving
the same in the `Just False` case and in the `Nothing` case
(meaning: no observable change in behaviour compared to before).
2. Typechecking of data (/newtype) constructor patterns. Some programs
involving patterns with unknown representations were accepted, such
as T20363. Now that we are stricter, this caused further issues,
culminating in Core Lint errors. However, the behaviour was
incorrect the whole time; the incorrectness only being revealed by
this change, not triggered by it.
This patch fixes this by overhauling where the representation
polymorphism involving pattern matching are done. Instead of doing
it in `tcMatches`, we instead ensure that the `matchExpected`
functions such as `matchExpectedFunTys`, `matchActualFunTySigma`,
`matchActualFunTysRho` allow return argument pattern types which
have a fixed RuntimeRep (as defined in Note [Fixed RuntimeRep]).
This ensures that the pattern matching code only ever handles types
with a known runtime representation. One exception was that
patterns with an unknown representation type could sneak in via
`tcConPat`, which points to a missing representation-polymorphism
check, which this patch now adds.
This means that we now reject the program in #20363, at least until
we implement PHASE 2 of FixedRuntimeRep (allowing type families in
RuntimeRep positions). The aforementioned refactoring, in which
checks have been moved to `matchExpected` functions, is a first
step in implementing PHASE 2 for patterns.
Fixes #20837
Diffstat (limited to 'compiler/GHC/Stg')
-rw-r--r-- | compiler/GHC/Stg/BcPrep.hs | 3 |
1 files changed, 1 insertions, 2 deletions
diff --git a/compiler/GHC/Stg/BcPrep.hs b/compiler/GHC/Stg/BcPrep.hs index c1fd67d484..d167a8c791 100644 --- a/compiler/GHC/Stg/BcPrep.hs +++ b/compiler/GHC/Stg/BcPrep.hs @@ -126,8 +126,7 @@ bcPrep us bnds = evalState (mapM bcPrepTopLvl bnds) (BcPrepM_State us) -- Is this Id a not-necessarily-lifted join point? -- See Note [Not-necessarily-lifted join points], step 1 isNNLJoinPoint :: Id -> Bool -isNNLJoinPoint x = isJoinId x && - Just True /= isLiftedType_maybe (idType x) +isNNLJoinPoint x = isJoinId x && mightBeUnliftedType (idType x) -- Update an Id's type to take a Void# argument. -- Precondition: the Id is a not-necessarily-lifted join point. |