Hi John & the SRFI 152 list,

While transcribing the SRFI for R7RS-large, I noticed that it added
string-character polymorphism to ‘string-unfold’ and (by analogy)
‘string-unfold-right’.  That is, the ‘mapper’ and ‘make-final’ procedures
used by these unfolds are allowed to return a string or a character, and
the ‘base’ parameter may be of either type.  The SRFI 13 versions insist
on characters for string elements, and strings for string portions.

There is no rationale for this extension and no example of its
application, and it’s probable no one has ever used it: the
sample implementation doesn’t implement it.  (The string-unfold
implementations are identical to those of the SRFI 13 forms.)  The
text is also incompletely adapted.  The SRFI says that “‘base’ is the
optional initial/leftmost portion of the constructed string” and that
“‘make-final’ … produce[s] the final/rightmost portion …”.  This is
the SRFI 13 language, used in a situation where the “portions” are
always strings.  We (or maybe just I) generally think of characters
as *elements* of strings.

So either the sample implementation needs to be fixed (replaced, really,
since it’s one of Olin’s “highly tuned” jobs) or the unfolds need to
be reverted to their SRFI 13 versions.  (A very big erratum.)  If we
do change the implementation to match the SRFI document, implementers
need to know about it.  The SRFI 152 sample implementation, rather
than the specification, has propagated, so that not one ‘string-unfold’
I tried supported the given semantics.

Wolfgang

--
Wolfgang Corcoran-Mathe  <xxxxxx@sigwinch.xyz>