SRFI 105: Curly-infix-expressions

by David A. Wheeler and Alan Manuel K. Gloria

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.