[Git][ghc/ghc][wip/T18223] 3 commits: Implement -Wredundant-bang-patterns (#17340)

[Simon Peyton Jones]

Simon Peyton Jones pushed to branch wip/T18223 at Glasgow Haskell Compiler / GHC


Commits:
731c8d3b by nineonine at 2020-08-19T18:47:39-04:00
Implement -Wredundant-bang-patterns (#17340)

Add new flag '-Wredundant-bang-patterns' that enables checks for "dead" bangs.
Dead bangs are the ones that under no circumstances can force a thunk that
wasn't already forced. Dead bangs are a form of redundant bangs. The new check
is performed in Pattern-Match Coverage Checker along with other checks (namely,
redundant and inaccessible RHSs). Given

    f :: Bool -> Int
    f True = 1
    f !x   = 2

we can detect dead bang patterns by checking whether @x ~ ⊥@ is satisfiable
where the PmBang appears in 'checkGrdTree'. If not, then clearly the bang is
dead. Such a dead bang is then indicated in the annotated pattern-match tree by
a 'RedundantSrcBang' wrapping. In 'redundantAndInaccessibles', we collect
all dead bangs to warn about.

Note that we don't want to warn for a dead bang that appears on a redundant
clause. That is because in that case, we recommend to delete the clause wholly,
including its leading pattern match.

Dead bang patterns are redundant. But there are bang patterns which are
redundant that aren't dead, for example

    f !() = 0

the bang still forces the match variable, before we attempt to match on (). But
it is redundant with the forcing done by the () match. We currently don't
detect redundant bangs that aren't dead.

- - - - -
eb9bdaef by Simon Peyton Jones at 2020-08-19T18:48:14-04:00
Add right-to-left rule for pattern bindings

Fix #18323 by adding a few lines of code to handle non-recursive
pattern bindings.  see GHC.Tc.Gen.Bind
Note [Special case for non-recursive pattern bindings]

Alas, this confused the pattern-match overlap checker; see #18323.

Note that this patch only affects pattern bindings like that
for (x,y) in this program

  combine :: (forall a . [a] -> a) -> [forall a. a -> a]
          -> ((forall a . [a] -> a), [forall a. a -> a])

  breaks = let (x,y) = combine head ids
           in x y True

We need ImpredicativeTypes for those [forall a. a->a] types to be
valid. And with ImpredicativeTypes the old, unprincipled "allow
unification variables to unify with a polytype" story actually
works quite well. So this test compiles fine (if delicatedly) with
old GHCs; but not with QuickLook unless we add this patch

- - - - -
7137e3da by Simon Peyton Jones at 2020-08-20T16:21:33+01:00
Work in progress on #18223

There are two main changes in here

1.  Stop Specialise specalising DFuns.  This cases a huge
    (and utterly unnecessary) blowup in program size in #18223.

    I also refactored the Specialise monad a bit... it was silly.

2.  Re-engineer eta-expansion (again).  The eta-expander was
    generating lots of intermediate stuff, which could be optimised
    away, but which choked the simplifier meanwhile.  Relatively
    easy to kill it off at source.

Needs some Notes etc, and perhaps two separate patches, but I want
to get this into CI and perf testing.

- - - - -


30 changed files:

- compiler/GHC/Core/Coercion.hs
- compiler/GHC/Core/Opt/Arity.hs
- compiler/GHC/Core/Opt/Simplify.hs
- compiler/GHC/Core/Opt/Simplify/Env.hs
- compiler/GHC/Core/Opt/Specialise.hs
- compiler/GHC/Core/Ppr.hs
- compiler/GHC/Core/SimpleOpt.hs
- compiler/GHC/Core/Subst.hs
- compiler/GHC/Driver/Flags.hs
- compiler/GHC/Driver/Session.hs
- compiler/GHC/Hs/Binds.hs
- compiler/GHC/Hs/Utils.hs
- compiler/GHC/HsToCore/Binds.hs
- compiler/GHC/HsToCore/Expr.hs
- compiler/GHC/HsToCore/PmCheck.hs
- compiler/GHC/Tc/Gen/Bind.hs
- compiler/GHC/Tc/Gen/Match.hs
- compiler/GHC/Tc/Gen/Match.hs-boot
- compiler/GHC/Tc/Utils/Zonk.hs
- + docs/users_guide/9.2.1-notes.rst
- docs/users_guide/using-warnings.rst
- + testsuite/tests/pmcheck/should_compile/T17340.hs
- + testsuite/tests/pmcheck/should_compile/T17340.stderr
- testsuite/tests/pmcheck/should_compile/all.T
- + testsuite/tests/typecheck/should_compile/T18323.hs
- testsuite/tests/typecheck/should_compile/all.T
- testsuite/tests/typecheck/should_fail/T8570.stderr
- testsuite/tests/typecheck/should_fail/tcfail004.stderr
- testsuite/tests/typecheck/should_fail/tcfail005.stderr
- testsuite/tests/typecheck/should_fail/tcfail012.stderr


The diff was not included because it is too large.


View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/940268367d30058bb27b69110705a51120f872e1...7137e3da1b91d46882881435b5cab9bc8eb66fa5

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/940268367d30058bb27b69110705a51120f872e1...7137e3da1b91d46882881435b5cab9bc8eb66fa5
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