.SCM File Format

Introduction

Scheme is a small dialect of Lisp created at MIT in the 1970s by Guy Steele and Gerald Sussman. It is defined by a series of reports, the Revisedⁿ Report on the Algorithmic Language Scheme, of which R5RS (1998), R6RS (2007) and R7RS-small (2013) are the ones in common use.

Source files are usually named .scm, and .ss is common with Chez Scheme and in older PLT Scheme code. R6RS and R7RS libraries are often kept in .sls and .sld files. GIMP’s Script-Fu extensions are Scheme in .scm files, and .scm is also used for Tree-sitter query files, which are not Scheme programs but borrow its bracket syntax.

Structure

A Scheme file is plain text made of parenthesised forms, evaluated in order. Variables and functions share one namespace and are defined with define. Local names come from let and functions from lambda. Implementations must run tail calls without growing the stack, so loops are often written as recursion, and call/cc captures the rest of a computation as a value.

Comments start with ;, #| … |# marks a block and #; comments out the next form. Booleans are #t and #f. From R6RS on, symbols are case-sensitive. An R7RS program opens with import to name the libraries it needs, and an R7RS library is wrapped in define-library.

(import (scheme base) (scheme write))

; A recursive definition, with a tail call in the helper.
(define (factorial n)
  (let loop ((i n) (acc 1))
    (if (= i 0)
        acc
        (loop (- i 1) (* acc i)))))

(display (factorial 5))
(newline)

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