R-Expressions

Rex - R-Expressions

Rex is an alternative to Lisp's S-Expressions that provides a much more flexible syntax while still having a simple, uniform model.

S-Expressions have a structure that looks something like this:

name := [a-zA-Z0-9_$!#%&*+,./:<=>?@\\^`|~]+-]+

exp :=
   | name
   | ()
   | (exp)
   | (exp exp...)

For Example:

((+ 3) (+ 3 4))

(print (* 3 4))

Where R-Expression have a structure that looks something like:

rune := [$!#%&*+,./:<=>?@\\^`|~-]+
name := [a-zA-Z0-9_]+

exp :=
    | name
    | (rune)
    | (rune exp)
    | (rune exp exp...)
    | exp exp

Notice that every (nested expression) must start with a rune, and expressions may be juxtaposed:

The advantage is that this structure can be unambiguously encoded to text in a large variety of different ways. There are five different layouts, and they can be mixed freely.

  • Nested Layout:

  • Open Layout (indentation sensitive):

  • Infix Layout:

  • Closed Layouts: (nested and |prefix)

  • Or any combination:

There is also some syntactic sugar.

  • (no-rune nested expressions) default to | rune.

  • You can write multiple runes within a () form (which work the same was as open-form:

All of these different forms parse into the exact same R-expression structure.

This gives a much more expressive syntax, but still gives you all of the advantages of s-expressions:

  • Very simple parser and printer.

  • Generic parser and pretty-printer that can be shared between languages.

  • Enables traditional Lisp meta-programming (defmacro + syntax-rules).

An Example

In R-Expressions:

In S-Expressions:

The Rex Grammar

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