#4148: improve new recursive do syntax
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Reporter: guest | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.12.3
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by simonpj):
I'm adding Ross's recent email to this ticket since it is relevant.
http://www.haskell.org/pipermail/glasgow-haskell-
users/2010-June/018944.html
Here it is verbatim.
There's also an underlying semantic issue, which is not yet resolved.
The GHC implementation of mdo (following Levent and John's paper) performs
a dependency analysis on the statements in the mdo to apply mfix to the
shortest possible subsegments of statements. For example,
{{{
mdo
a <- f b
b <- g b
c <- h b
d <- k d
return d
}}}
is translated to the equivalent of
{{{
do
(a,b) <- mfix $ \ (a,b) -> do
a <- f b
b <- g b
return (a,b)
c <- h b
d <- mfix $ \ d ->
d <- k d
return d
return d
}}}
As the User's Guide notes, the choice of segments can change the semantics
with some monads.
When rec was added to GHC, the implementation used the same code and thus
also did automatic segmentation. The original idea of rec (in arrow
notation) was that it would give the programmer direct control over these
segments: the loop/mfix combinators would go wherever the programmer put a
rec, but I didn't realize Simon had done it the other way until he
mentioned it last October. So:
* if rec is to continue to do automatic segmentation, it might as well
be a modifier on the do keyword (though this would break backward
compatibility of arrow notation),
* but I want to argue against automatic segmentation, because I don't
think the compiler should be making subtle changes to the program's
semantics on its own. It would simplify the compiler a bit too.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4148#comment:4>
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