[Chicken-hackers] Bug in SRFI 13 string-skip-right (was: fix for string-trim-right)

[John Cowan]

Alex Shinn scripsit:

> > (string-skip-right "abc   " char-whitespace? 4 4) => 2
> >
> > The correct result is #f; Riastradh confirmed that the correct semantics
> > throughout SRFI 13 for start-end is that they are conceptually applied
> > using substring/shared before anything else is done.
> >
> > A quick look at the reference implementation suggests that it needs
> > to trap the case where start = end, and always return #f then.
> 
> Shouldn't it return ""?

No, you're confusing string-skip-right (the subject of this bug) with
string-trim-right (the subject of the previous bug).  The former returns
an index or #f, whereas the latter returns a substring.  The trouble
here is that if start = end, string-skip-right should always return #f,
because there are no indexes in an empty string.  Instead, it starts
at end - 1 anyway even though that is less than start, and returns a
spurious result.  As a result, string-trim-right where start = end also
returns a spurious result.

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