On Thu, Nov 20, 2008 at 12:57 PM, David Roundy <[EMAIL PROTECTED]> wrote:
> On Thu, Nov 20, 2008 at 02:44:33AM +0300, Dmitry Kurochkin wrote:
>> hunk ./src/Darcs/Patch/Real.hs 175
>> > a:>p':>b ->
>> > do (b',xs') <- mac b xs
>> > goneby
>> > --traceDoc
>> > (greenText "foo1" $$
>> > - --
>> > showPatch (sort_coalesceFL $
>> > - --
>> > effect $ p':<:a)) $ Just ()
>> > - NilFL <- return $
>> > sort_coalesceFL $ effect $ p':<:a
>> > + --
>> > showPatch (sort_coalesce_effects $ p':<:a)) $ Just ()
>> > + NilFL <- return $
>> > sort_coalesce_effects $ p':<:a
>> > return (reverseRL
>> > (p':<:a)+>+b', xs')
>> > `mplus` do NilFL <-
>> > return goneby
>>
>> Use sort_coalesce_effects instead of sort_coalesceFL.
>>
>> It would be easier to read if "sort_coalesce_effects $ p':<:a" is
>> bound to a name with let. Not sure if it worth changing though.
>
> But alas, that would cause ghc's brain to explode. NilFL is a GADT,
> and you can't use let bindings with GADT patterns. This relates to
> the fact that matching NilFL puts a constraint on the type witnesses,
> and because let bindings are lazy, they don't work well with that.
Actually, I bet Dmitry read it the same way I did when I first read
it. Which is to say, I thought there was a repeated subexpression:
"sort_coalesceFL $ effect $ p':<:a"
But it doesn't appear twice because one of the uses in a comment.
The suggestion would have worked though as near as I can tell. My
spacing may be off because I only have a proportional width font to
work with. But, just image the "let" has the correct indentation
level to be in the do-block
do (b',xs') <- mac
b xs goneby
--traceDoc
(greenText "foo1" $$
+ let foo =
sort_coalesce_effects $ p':<:a
+ --
showPatch foo $ Just ()
+ NilFL <- return foo
return
(reverseRL (p':<:a)+>+b', xs')
I except this to be fine because we're not actually asserting that foo
is NilFL until we do it in the monadic pattern binding (the line
"NilFL <- return foo"). But, I'm not saying anything needs to be
amended; I'm just trying to clarify what I think was the suggestion
and how it could be made to work.
Or maybe Dmitry was objecting to us gratuitously using "<- return"
with a pure value.
Well, if nothing I hope my example clarifies that we can use let here,
just not a let that requires a pattern match.
By the way, if you want to understand better why a lazy pattern match
fails to work you need to look at how Haskell is translated to System
F and the interaction between type inference and the way that System F
requires type variables to be passed explicitly. Those two things
(inference + explicit type passing) do not play well with existentials
if the pattern match is lazy. I had questions about this on
Haskell-Cafe:
http://www.mail-archive.com/[EMAIL PROTECTED]/msg47034.html
A related problem is using lazy patterns with existentials:
http://www.haskell.org/pipermail/glasgow-haskell-users/2008-November/016032.html
I hope that helps!
Jason
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