SRFI 156: Syntactic combiners for binary predicates Arthur A. Gleckler 10 Jul 2017 21:06 UTC
Scheme Request for Implementation 156,
"Syntactic combiners for binary predicates,"
by Panicz Maciej Godek,
is now available for discussion.
Its draft and an archive of the ongoing discussion are
available at <https://srfi.schemers.org/srfi-156/>.
You can join the discussion of the draft by filling out the
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Here's the abstract:
The power and universality of the prefix notation employed
by Scheme and other dialects of Lisp can be intimidating.
However, we find that there are occasions when prefix
syntax can be confusing and lead to code that is not
self-documenting. We have identified that one of such
cases is the use of asymmetrical binary predicates.
Probably the most common examples are the numerical
comparison predicates <, <=, >= and >. It is non obvious,
for instance, how the expression (< a b) should be
The problem gets more serious in the case of user-defined
binary predicates. For example, in the expression
(divides? x y) the role of arguments is unclear: we don't
know whether the author of the divides? predicate intended
it to check whether x divides y or whether y divides x.
And while there seems to exist a silent convention among
Schemers to interpret predicates like (has-something-to? x
y) as "x has something to y", we believe that this
convention should be made explicit, and confirmed by the
forms available in the language.
We therefore propose a thin layer of "syntactic stevia"
that can be implemented using regular Scheme macros. We
suggest, that the code (is x < y) should be transformed to
(< x y), and (is x < y <= z) -- to (and (< x y) (<= y z)).
In addition, we suggest special meaning to the _ symbol:
(is _ < y) and (is x < _) should be transformed to (lambda
(_) (< _ y)) and (lambda (_) (< x _)), respectively. This
SRFI document also describes some other uses of the is
macro and its limitations.