Some good-hearted criticism
Juan Carlos Arevalo Baeza
Wed, 27 Jun 2001 19:56:03 -0700
At 03:26 PM 6/27/2001 -0600, Alastair David Reid wrote:
>Other useful options that Windows (and Unix?) people may not know about are:
> --enable-timer enable evaluation timing
> --enable-stack-dumps enable stack dump on stack overflow
That might have helped...
> --enable-only98 make Hugs Haskell 98 only
Can't you already toggle this in a normal build? -98 and +98.
> --with-readline support fancy command line editing
> --with-preprocessor allow use of a preprocessor
Thanx. But all those are only available if you recompile, right? Why
are they disabled by default? Because they affect performance?
>The useful ones are documented in the file hugs98/Install - but you
>usually only read that if you install from source. Now that most
>people get their Hugs in binary form for Windows or Linux, it might be
>worth mentioning these features more prominently on the web page or in
>the Hugs documentation.
They would still be useless without recompiling, but yes, it would be
good to "let people know" :)
>Many people would code that more trickily by consing x onto the front
>of r and then returning the reverse of r:
> > manynot :: Parser p inp => p inp v -> p inp a -> p inp ([v],Maybe a)
> > manynot s p = mn  p where mn r p = force $ do a <- p
> > return (reverse r,Just a)
> > +++
> > do x <- s
> > mn (x:r) p
> > +++
> > return (reverse r,Nothing)
>This avoids quadratic cost from the repeated appends (N appends with
>time of each one proportional to length of list). Both approaches are
>equally strict. I think both require stack space proportional to the
>list length though I could be wrong about the repeated appends (too
>hot to figure it out).
Hmmm... It seems to me that this would cause the same amount of stack
hit, yes. At least, accessing the "head" of the list would mean recursively
running through the whole list.
Seems to me like Haskell could do with a better list primitive, where
you can pattern-match on either side (with two constructors, so to speak).
Making it a primitive would allow the underlying implementation to actually
use looping if appropriate when pattern-matching. It would, at least,
lessen the possibility of running out of stack space when dealing with lists.
Anyway, I just did a little test. My example had gotten fixed because
the multi-line comment is discarded. It still blew up if I tried to print
it out. With your modifications, though, it does run till the end in Hugs,
so there's some stack savings there. I'm not sure I understand why. In
fact, I just tried making the whole comment longer and longer, and it ends
up running out of heap space, not stack space, of all things. Seems like
"reverse" doesn't do its work recursively after all. Could it be a primitive?
>Less trickily, you could use one of Chris Okasaki's data structures
>that guarantees fast append to the tail of the list.
Is that like an add-on primitive or is it just normal Haskell? In
either case, maybe it's done in a way similar as I was describing above...
And thanks to you and all the others. This is great :)
Juan Carlos "JCAB" Arevalo Baeza | http://www.roningames.com
Senior Technology Engineer | mailto:firstname.lastname@example.org
Ronin Entertainment | ICQ: 101728263
(my opinions are only mine)
JCAB's Rumblings is so off-line O:-(