This chapter describes the (rnrs control (6))library, which provides useful control structures.
Syntax: <Test> must be an expression.
Semantics: A when expression is evaluated by evaluating the <test> expression. If <test> evaluates to a true value, the remaining <expression>s are evaluated in order, and the results of the last <expression> are returned as the results of the entire when expression. Otherwise, the when expression returns unspecified values. An unless expression is evaluated by evaluating the <test> expression. If <test> evaluates to #f, the remaining <expression>s are evaluated in order, and the results of the last <expression> are returned as the results of the entire unless expression. Otherwise, the unless expression returns unspecified values.
The final <expression> is in tail context if the when or unless form is itself in tail context.
(when (> 3 2) ’greater) ⇒ greater
The when and unless expressions are derived forms. They could be defined by the following macros:
Syntax: The <init>s, <step>s, <test>s, and <command>s must be expressions. The <variable>s must be pairwise distinct variables.
Semantics: The 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.
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 #f, then the <command>s are evaluated in order for effect, the <step> expressions are evaluated in some unspecified order, the <variable>s are bound to fresh locations holding the results, and the next iteration begins.
If <test> evaluates to a true value, 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 do expression returns unspecified values.
The regionof the binding of a <variable> consists of the entire do expression except for the <init>s.
A <step> may be omitted, in which case the effect is the same as if (<variable> <init> <variable>) had been written instead of (<variable> <init>).
If a do expression appears in a tail context, the <expression>s are a <tail sequence> in the sense of report section on “Tail calls and tail contexts”, i.e., the last <expression> is also in a tail context.
(do ((vec (make-vector 5))
The following definition of do uses a trick to expand the variable clauses.
Syntax: Each <case-lambda clause> must be of the form(<formals> <body>)
<Formals> must be as in a lambda form (report section on “Procedures”), and <body> is as described in report section on “Bodies and sequences”.
Semantics: A case-lambda expression evaluates to a procedure. This procedure, when applied, tries to match its arguments to the <case-lambda clause>s in order. The arguments match a clause if one of the following conditions is fulfilled:
<Formals> has the form (<variable> ...) and the number of arguments is the same as the number of formal parameters in <formals>.
<Formals> has the form
(<variable1> ...<variablen> . <variablen+1)>
and the number of arguments is at least n.
<Formals> has the form <variable>.
For the first clause matched by the arguments, the variables of the <formals> are bound to fresh locations containing the argument values in the same arrangement as with lambda.
The last expression of a <body> in a case-lambda expression is in tail context.
If the arguments match none of the clauses, an exception with condition type &assertion is raised.
The case-lambda keyword can be defined in terms of lambda by the following macros:(define-syntax case-lambda