Ken Dickey wrote:
On Sunday 19 October 2008 12:30:27 Thomas Lord wrote:
For the numeric comparison functions we have a clear
account of what the code is supposed to model, e.g.,
monotonic sequences of numbers.
Ah. I can see where you can use lists of numbers to model sequences.
I have always used numeric comparison predicates were to compare two or more
numbers.
Would it not be more clear to use 'sequence, 'sequence-append, ...
to 'list, 'append, ... and (sequence-ordered? < ...) to (apply < ...) ?
That's an interesting perspective.
You have persuaded me that:
1) all sequence predicates whose definition is (normally) founded
(in the usual pair-wise way) in the definition of a binary predicate
should accept 0 or more arguments
2) either:
2a) sequence predicates should not be primitive in Scheme
2b) every predicate that is trivially convertible to a sequence
predicate (accepting 0 or more arguments) should be specified
in the first place as a sequence predicate
Rationale for 2a: they can be constructed in a generic way
from binary predicates as in (sequence-pairwise? < ...)
Rational for 2b: the sequence predicate can always be used
as the binary predicate.
Rationale against 2a: that's inconvenient.
Rationale against 2b: weak error checking when a predicate
is used as a binary predicate.
I like 2b.
So:
(< 1 2 3) => error
(<) => error
(< 1) => error
(< 1 2) => #t
(number-seq-pairwise? <) => #t
(number-seq-pairwise? < 1) => #t
(number-seq-pairwise? < 1 2) => #t
(number-seq-pairwise? < 1 2 3) => #t
That's fine with me, too. Scheme standards might describe
some syntax, for convenience so that:
(number-seq-pairwise? <
is equivalent to
(<...
(The three dots there are part of the identifier name in the CAR position)
(< 1 2 3) => error
(<... 1 2 3) => #t
etc.
I think you have convinced me that the conflation of the
sequence operator and the "founding" binary relation is a
cute hack but one that trades away safety, needlessly.
Do we really need "number-seq-pairwise?" or is there a good way
to define just a generic "seq-pairwise?" yet somehow know
how to type-check a single argument?
-t
Cheers,
-KenD
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