status: final (2012-11-06)
keywords: Reader Syntax
Lisp-based languages, like Scheme, are almost
the only programming languages in modern use that do not support infix
notation. In addition, most languages allow infix expressions to be
combined with function call notation of the form
f(x). This SRFI provides these
capabilities, both for developers who already use Scheme and want these
conveniences, and also for other developers who may choose to use other
languages in part because they miss these conveniences. Scheme currently
reserves {...} “for possible future extensions
to the language”. We propose that {...} be used
to support “curly-infix-expression” notation as a homoiconic infix
abbreviation, as a modification of the Scheme reader. It is an
abbreviation in much the same way that 'x is an abbreviation
for (quote x).
A curly-infix list introduces a list
whose visual presentation can be in infix order instead of prefix order.
For example, {n > 5} ⇒
(> n 5), and
{a + b + c} ⇒
(+ a b c). By intent, there is no precedence,
but e.g., {x + {y * z}} maps cleanly to
(+ x (* y z)). Forms with mixed infix
operators and other complications have “$nfx$” prepended to
enable later processing, e.g.,
{4 + 5 * 6} ⇒
($nfx$ 4 + 5 * 6). Also, inside a
curly-infix list (recursively), expressions of the form
f(...) are simply an abbreviation
for (f ...).
Note that this is derived from the “readable” project. We intend to later submit at least one additional SRFI that will build on top of this SRFI, but curly-infix-expressions are useful on their own.