do -- r7rs Definition

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Kind

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syntax;

Implemented by

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• do (from vonuvoli);

Syntax signature

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Syntax keywords:

• binding-variable: identifier;
• binding-initializer: expression;
• binding-updater: expression;
• binding: pattern with variants:
  • (binding-variable binding-initializer);
  • (binding-variable binding-initializer binding-updater);
• bindings: pattern with variants:
  • ();
  • (binding |...|);
• exit-test: expression;
• exit-expression: expression;
• exit-clause: pattern with variants:
  • (exit-test);
  • (exit-test exit-expression);
• iteration-expression: expression;

Syntax variants:

• (_ bindings exit-clause)
• (_ bindings exit-clause iteration-expression |...|)

Exports

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• scheme:base -- (scheme base);

Exports recursive

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• scheme -- (scheme);

Description

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(do ((<variable_1> <init_1> <step_1>)
     ...)
    (<test> <expression> ...)
  <command> ...)

Syntax: All of <init>, <step>, <test>, and <command> are expressions.

Semantics: A do expression is an iteration construct. It specifies a set of variables to be bound, how they are to be initialized at the start, and how they are to be updated on each iteration. When a termination condition is met, the loop exits after evaluating the <expression>s.

A do expression is evaluated as follows: The <init> expressions are evaluated (in some unspecified order), the <variable>s are bound to fresh locations, the results of the <init> expressions are stored in the bindings of the <variable>s, and then the iteration phase begins.

Each iteration begins by evaluating <test>; if the result is false (see section on booleans), then the <command> expressions are evaluated in order for effect, the <step> expressions are evaluated in some unspecified order, the <variable>s are bound to fresh locations, the results of the <step>s are stored in the bindings of the <variable>s, and the next iteration begins.

If <test> evaluates to a true value, then the <expression>s are evaluated from left to right and the values of the last <expression> are returned. If no <expression>s are present, then the value of the do expression is unspecified.

The region of the binding of a <variable> consists of the entire do expression except for the <init>s. It is an error for a <variable> to appear more than once in the list of do variables.

A <step> can be omitted, in which case the effect is the same as if (<variable> <init> <variable>) had been written instead of (<variable> <init>).

(do ((vec (make-vector 5))
     (i 0 (+ i 1)))
    ((= i 5) vec)
  (vector-set! vec i i))          ===>  #(0 1 2 3 4)

(let ((x '(1 3 5 7 9)))
  (do ((x x (cdr x))
       (sum 0 (+ sum (car x))))
      ((null? x) sum)))             ===>  25

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The text herein was sourced and adapted as described in the "R7RS attribution of various text snippets" appendix.