[Scheme-reports] 6.4 control features: -map and -for-each procedures

[Andy Wingo <wingo@x>]

Hello!  Where was I? :)

Right, control features.  I already mentioned the
multiple-return-from-mapping proc issue before.  There are three options
that I am aware of:

  1) Ignore the issue.

  2) Specify that a second return from a mapped procedure cannot affect
     the data structure returned by the first return.  Effectively
     prohibits the build-it-backwards-and-reverse-in-place idiom, though
     you may reverse and form a new list.

  3) Specify that it is an error to return multiple times.

Suggestion: choose one.

Suggestion: that it be an error for a mapped procedure to mutate the
list being mapped.


Also, what is the motivation for having multi-arg `map' terminate at the
first empty list?  If it is to allow for circular lists, then the
implicit type of `map' is incorrect:

  (map PROC LIST1 LIST2 ...)

because AFAIK a circular list does not pass `list?'.

Suggestion: provide a rationale, or revert to the previous spec.
Perhaps mention the expected behavior for circular lists.


The multiple-return issue is more important for string-map and
vector-map, IMO, as one natural technique would be mutate a string or
vector in-place.

Suggestion: choose a behavior for vector-map consistent with the
previous choice, or remove vector-map and string-map from the spec.


The early-termination behavior seems to have no corresponding
justification for vector-map and string-map.


Which, now that we've come to it, I don't like very much:

  1) The obvious implementation (mutate-in-place) should be allowed, but
     it prevents multiple returns, so perhaps it's not general enough
     for this spec.

  2) Vector-fold is often really what you want.

  3) Vector-map is not common.

  4) String-map is less common.

  5) You'd need blob-map to be complete, for this report...

Basically I think they are sometimes useful combinators, but not useful
enough for WG1 scheme, and trivial to build yourself.

Suggestion: remove vector-map, string-map, and their for-each procedures
from the WG1 report.

Andy
-- 
http://wingolog.org/

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