[m-rev.] for review: string__substring in Mercury

[Ralph Becket]

> From: Ralph Becket [mailto:rbeck at microsoft.com]
> Sent: 07 September 2001 14:49

> 
> A slightly more efficient way of doing this is
> 
> string__substring(Str::in, Start::in, Count::in, SubStr::out) :-
>     End    = min(Start + Count, string__length(Str)),
>     SubStr = string__from_char_list(strchars(Start, End, Str)).
> 
> :- func strchars(int, int, string) = list(char).
> 
> strchars(I, End, Str) =
>     ( if I >= End
>       then []
>       else [string__index_det(Str, I) | strchars(I + 1, End, Str)]
>     ).
> 
> This way we avoid creating any more cons cells than strictly
> necessary.

Sorry, this isn't quite correct: strchars should be

strchars(I, End, Str) =
    ( if   ( I < 0 ; End =< I )
      then []
      else [string__index_det(Str, I) | strchars(I + 1, End, Str)]
    ).

since string__substring/3 is defined when Start is out of range.

- Ralph
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