[mercury-users] string__first_char

[Michael Day]

> In fact even for the current implementation I think it is just
> a property that the implementation tries hard to ensure occurs;
> AFAIK there is no code in the current implementation which actually
> requires that property.

If it is not required, could the implementation perhaps try less hard to
ensure it occurs? Define it out until/unless the property actually becomes
necessary?

> We generally run the Boehm et al conservative garbage collector in a
> mode in which it only follows pointers to bytes within the first word
> of an object; pointers into the middle of an object are ignored by the
> GC.  So although the GC doesn't require strings to be word aligned, it
> doesn't allow arbitrary pointers into the middle of strings, and so 

and so...?

> That depends on how you code it.  If you process strings using
> string__first_char, then yes, things will probably be inefficient.
> But if you make use of string__foldl and enable intermodule optimization,
> you should get C-like performance.

Well, first_char is useful when processing strings in a streamy kind of
way; in particular I've written a bunch of regular expression code which
uses first_char all over the place, where foldl is not really appropriate.

Michael

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