On Apr 12, 2006, at 4:25 PM, John Meacham wrote:
On Wed, Apr 12, 2006 at 09:21:10AM -0400, Jan-Willem Maessen wrote:
Though, to be fair, an awful lot of Prelude code didn't work in pH
unless it was re-written to vary slightly from the specification. So
the assumption of laziness was more deeply embedded than the spec was
willing to acknowledge.
out of curiosity what sort of things had to be rewritten? I have been
toying with the idea of relaxing sharing to help some optimizations
and
was curious what I was in for.
Well, the differences really had to do with termination under an
eager strategy.
But beyond obvious problems such as defining things in terms of take
+ iterate (numericEnumFrom[Then]To is an obvious example), we ran
into terrible performance problems with Read instances. Programs
would spend minutes running read, then a few fractions of a second
computing. We ended up doing a lot of tweaking, none of which was
ever quite correct. Ditching ReadS in terms of ReadP would do an
enormous amount of good here, I think---it would at least put all the
re-coding in one centralized place, which is what we ended up having
to do anyhow.
Finally, there are a bunch of Haskell idioms which don't work in pH.
The most obvious examples are numbering a list:
zip [0..] xs
and where-binding a value which is unused in one clause:
f x
| p x = ... r ...
| q x = ... r ...
| otherwise = ... no r ...
where r = something very expensive
I suppose you could view this as a "sharing problem": the expression
r is shared down two of the branches and not down the other. But I
don't think that's what you meant.
A lot of these can be solved by a certain amount of code motion---but
note that this code motion changes the termination properties of the
program as it was written. In pH that was naughty.
-Jan
John
--
John Meacham - ⑆repetae.net⑆john⑈
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