[plt-scheme] scribblings on PLT Scheme 4.0

At Mon, 28 May 2007 11:17:52 +0100, "Paulo J. Matos" wrote:
> On 5/28/07, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> > At Mon, 28 May 2007 09:44:38 +1000, Andrew Reilly wrote:
> > > I'm new to scheme myself, so I probably sholdn't have been
> > > making such liberal use of append! in my own code.  I will
> > > repent...
> >
> > Do you used them for performance reasons?
> >
> > I used `append!' and `reverse!' a lot in my early Scheming, too,
> > because they were faster. It bit me often enough (no surprise) that I
> > decided to stay away, though there are certainly cases where mutating
> > versions work fine.
> >
> 
> I also use them for performance reasons sometimes. I use them for
> example when I want to sort a mapped list. I use (sort! (map f lst)).
> Or when I want to append a set of lists that results from map like
> (apply append! (map f lst)). I think using ! in these cases is good,
> afaik,  because: 1. it's faster, 2. you know the list was just
> created, 3. you won't have problems.

I agree that you won't have problems, as long as you know that `f'
doesn't capture any continuations to be applied after the `sort!' or
`append!'.

How much time does `append!' save you in an actual application? My
guess is that the savings will be minor, but I'm interested in
measurements you might be able to take.

Matthew