#5473: sortBy is no longer a good consumer in GHC 7
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Reporter: lpsmith | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Documentation bug
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The [http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/rewrite-
rules.html#id717388 List Fusion section of the GHC User's Guide] lists
{{{sortBy}}} as a good consumer.
In past versions of GHC, {{{sortBy}}} turned its input into a list of
lists to be merged by using {{{map (:[])}}}, which is why it was a good
consumer. However this is somewhat arguable, as it's only a good
consumer in a weak sense of the word. When it comes to doing the real
work of the algorithm, sort was never a good consumer.
However, in GHC 7, {{{sortBy}}} breaks its input into monotonic runs,
producing a shorter outer list. This is not a good consumer in any sense
of the word.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5473>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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