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Problems in the specs for 'gdelete' and 'gindex' Mark H Weaver (21 Jul 2019 01:46 UTC)
Problems in the specs for 'gdelete' and 'gindex'
Re: Problems in the specs for 'gdelete' and 'gindex' Mark H Weaver (21 Jul 2019 09:25 UTC)
Re: Problems in the specs for 'gdelete' and 'gindex' Arthur A. Gleckler (02 Mar 2020 23:09 UTC)
Re: Problems in the specs for 'gdelete' and 'gindex' John Cowan (02 Mar 2020 23:10 UTC)

Problems in the specs for 'gdelete' and 'gindex' Mark H Weaver 21 Jul 2019 01:46 UTC

Hi,

While working on an implementation of SRFI-121, I found three problems
in in the text.  SRFI-121 has since been withdrawn, but the same issues
are also present in SRFI-158:

(1) The spec for 'gdelete' states:

      The = predicate is passed exactly two arguments, of which the
      first was generated by gen before the second.

    However, in the case of 'gdelete', '=' will never be applied to two
    arguments generated by GEN.  Instead, '=' will be used to compare
    ITEM (an argument to 'gdelete') with each element generated by GEN.

    This leaves me unsure of whether to call (= ITEM PRODUCED-ELEMENT)
    or (= PRODUCED-ELEMENT ITEM), and yet the text seems to suggest that
    the order might be important.

    What would you suggest?

(2) In the spec for 'gindex', I believe that this text:

      It is an error if the indices are not strictly increasing, or if
      any index exceeds the number of elements generated by value-gen.

    should be changed to something equivalent to:

      It is an error if the indices are not strictly increasing, or if any
      index exceeds or equals the number of elements generated by value-gen.

    In other words, each index should be in the range 0..N-1, where N is the
    number of elements generated by VALUE-GEN.  Is that right?

(3) The spec for 'gindex' makes apparently conflicting assertions about
    what should happen if 'value-gen' is exhausted before 'index-gen' is
    exhausted.  First, it states:

      It is an error if the indices are not strictly increasing, or if
      any index exceeds the number of elements generated by value-gen.

    It then goes on to state:

      The result generator is exhausted when either generator is
      exhausted

    I think there's a condradiction here.  If 'value-gen' is exhausted
    first, that would seem to imply that there was an index that that
    exceeds the number of elements generated by value-gen.

    If you disagree, can you please give an example where the following
    rule is applicable:

      It is an error [...] if any index exceeds the number of elements
      generated by value-gen.

    and where the other error conditions are not applicable?

       Regards,
         Mark