Racket also optionally supports a square bracket syntax for cons clauses. For example, in traditional Scheme one would write: ; this is in R5RS Scheme (de

Racket also optionally supports a square bracket syntax for cons clauses. For example, in traditional Scheme one would write: ; this is in R5RS Scheme (define (length elems) (cond ((null? elems) 0) (else (+ 1 (length (cdr elems)))))) But in Racket one would write: ; this is in Racket (define (length elems) (cond [(empty? elems) 0] [else (+ 1 (length (rest elems)))])) It is also used in let/let*/letrec bindings in Racket; see https://docs.racket-lang.org/guide/syntax-overview.html#%28part._.Conditionals_with_if__and__or__and_cond%29 https://docs.racket-lang.org/guide/syntax-overview.html#%28p.... I don't know the origins of when brackets started appearing in Lisps. I don't remember seeing brackets in the LISP 1.5 Programmers Manual when using S-expressions (though brackets are a core part of M-expression syntax, which LISP 1.5 still supported), and I haven't encountered brackets in Common Lisp.