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<h3>
Simon Peyton Jones pushed to branch wip/T18223
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/731c8d3bc5a84515793e5dadb26adf52f9280e13">731c8d3b</a></strong>
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<span>by nineonine</span>
<i>at 2020-08-19T18:47:39-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">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.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/eb9bdaef6024558696e1e50b12d7fefb70483a9f">eb9bdaef</a></strong>
<div>
<span>by Simon Peyton Jones</span>
<i>at 2020-08-19T18:48:14-04:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">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
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/7137e3da1b91d46882881435b5cab9bc8eb66fa5">7137e3da</a></strong>
<div>
<span>by Simon Peyton Jones</span>
<i>at 2020-08-20T16:21:33+01:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">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.
</pre>
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compiler/GHC/Core/Coercion.hs
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compiler/GHC/Core/Opt/Arity.hs
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compiler/GHC/Core/Opt/Simplify.hs
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compiler/GHC/Core/Opt/Simplify/Env.hs
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compiler/GHC/Core/SimpleOpt.hs
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compiler/GHC/Driver/Flags.hs
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compiler/GHC/Hs/Binds.hs
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