#5744: List layouts
---------------------------------+------------------------------------------
Reporter: nsch | Owner: nsch
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.2.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonmar):
Replying to [comment:12 ehird]:
> My special case was intentional; while the two expressions must have
equivalent *results*, right-associative use of `mappend` is more efficient
for basically all `Monoid`s. This is why, e.g. RWH suggests using a
difference-list rather than `[]` as the monoid of a `Writer`.
Then it should be a RULE, not part of desugaring. Otherwise you get
strange effects like a difference in performance between
{{{
mdo { mdo {..}, x }
}}}
and
{{{
let z = mdo {..} in mdo {z, x}
}}}
and I'm not sure I'm comfortable with a built-in bias in the associativity
of `mappend` - it might be true that most existing Monoids are naturally
right-associative, but that's not necessarily the case in general.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5744#comment:13>
GHC <http://www.haskell.org/ghc/>
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