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Miscellaneous Differences from Standard Scheme

Square Brackets

People who know some Scheme are often surprised to see square brackets in PLT Scheme programs. They have the same meaning as regular parentheses in Scheme. They provide a bit of visual distinction and are often used by PLT Scheme programmers to differentiate clauses in conditional constructs. For example, you'll often see cond clauses written as below.

(define (display-type x)
  (cond
   [(number? x) (display "It's a number!")]
   [(string? x) (display "It's a string!")]
   [else (display "It's something else!")]))

Local

The local form is used in some textbooks, most notably "How To Design Programs", see HTDP on the local form.

local is standard in the Pretty Big languages as well as the teaching languages. It's also provided as a standard library module in (lib "etc.ss"). Here's an example:

(module example-with-local mzscheme
  (require (lib "etc.ss"))

  ;; make-adder: number -> (number -> number)
  (define (make-adder n)
    (local ((define (add-n x)
              (+ n x)))
      add-n)))

Double dot infix notation

A parenthesized sequence with two periods triggers infix parsing.

> (1 . < . 2) 
#t

> (1 2 . + . 3 4 5) 
15

See the MzScheme language manual, chapter 14 "Support facilities".