deeqSeq proposal

[Jan-Willem Maessen]

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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