#7436: Derived Foldable and Traversable instances become extremely inefficient
due
to eta-expansion
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Reporter: shachaf | Owner:
Type: bug | Status: patch
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Runtime performance bug
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by simonpj):
* cc: twanvl, dreixel (added)
Comment:
OK, let's wrap this one up.
* Thank you for the patch. Are you confident that it's good to go?
'''Pedro''' I believe that you were responsible for at least some of the
`Traversable` deriving code; can you give an opinion?
* I wonder if you could add some comments to explain the construction?
As you say, it may generate slightly strange code. Something like `Note
[Avoid eta-expanded code]` with a compact explanation, an example, and a
pointer to this ticket, would be useful.
* Are there are there any library changes?
* I looked at the eta-expansion thing. Yes, GHC is doing the right thing
here, as you show. You might wonder why GHC doesn't eta-reduce `(\e -> s
e)` to `s`. The reason is that doing so is unsound if `s` is bottom; then
eta-reduction might turn a terminating program into a non-terminating one.
Then I'll commit it.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7436#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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