Chapter 12

etc.ss: Useful Procedures and Syntax

(boolean=? bool1 bool2)      PROCEDURE

Returns #t if bool1 and bool2 are both #t or both #f, and returns #f otherwise. If either bool1 or bool2 is not a Boolean, the exn:application:type exception is raised.

(build-list n f)      PROCEDURE

Creates a list of n elements by applying f to the integers from 0 to n - 1 in order, where n is a non-negative integer. The ith element of the resulting list is (f (- i 1)).

(build-string n f)      PROCEDURE

Creates a string of length n by applying f to the integers from 0 to n - 1 in order, where n is a non-negative integer and f returns a character for the n invocations. The ith character of the resulting string is (f (- i 1)).

(build-vector n f)      PROCEDURE

Creates a vector of n elements by applying f to the integers from 0 to n - 1 in order, where n is a non-negative integer. The ith element of the resulting vector is (f (- i 1)).

(compose f g)      PROCEDURE

Returns a procedure that takes x and returns (call-with-values (lambda () (g x)) f).

(define-syntax-set (identifier ···) defn ···)      SYNTAX

This form is similar to define-syntaxes, but instead of a single body expression, a sequence of definitions follows the sequence of defined identifiers. For each identifier, the defns should include a definition for identifier/proc. The value for identifier/proc is used as the (expansion-time) value for identifier.

The define-syntax-set form is especially useful for defining a set of syntax transformers that share helper functions.

Example:

(define-syntax-set (let-current-continuation let-current-escape-continuation)
  (define (mk call-id)
     (lambda (stx)
       (syntax-case stx ()
         [(_ id body1 body ...) 
          (with-syntax ([call call-id])
            (syntax (call (lambda (id) body1 body ...))))])))
  (define let-current-continuation/proc (mk (quote-syntax call/cc)))
  (define let-current-escape-continuation/proc (mk (quote-syntax call/ec))))

(evcase key-expr (value-expr body-expr ···) ···1)      SYNTAX

The evcase form is similar to case, except that expressions are provided in each clause instead of a sequence of data. After key-expr is evaluated, each value-expr is evaluated until a value is found that is eqv? to the key value; when a matching value is found, the corresponding body-exprs are evaluated and the value(s) for the last is the result of the entire evcase expression.

A value-expr can be the special identifier else. This identifier is recognized as in case (see section 2.3 in PLT MzScheme: Language Manual).

false      BOOLEAN

Boolean false.

(identity v)      PROCEDURE

Returns v.

(let+ clause body-expr ···1)      SYNTAX

A new binding construct that specifies scoping on a per-binding basis instead of a per-expression basis. It helps eliminate rightward-drift in programs. It looks similar to let, except each clause has an additional keyword tag before the binding variables.

Each clause has one of the following forms:

• (val target expr) binds target non-recursively to expr.

• (rec target expr) binds target recursively to expr.

• (vals (target expr) ···) the targets are bound to the exprs. The environment of the exprs is the environment active before this clause.

• (recs (variable expr) ···) the targetss are bound to the exprs. The environment of the exprs includes all of the targetss.

• (_ expr ···) evaluates the exprs without binding any variables.

The clauses bind left-to-right. Each target above can either be an identifier or (values variable ···). In the latter case, multiple values returned by the corresponding expression are bound to the multiple variables.

Examples:

(let+ ([val (values x y) (values 1 2)])
   (list x y)) ; => '(1 2)

(let ([x 1])
   (let+ ([val x 3]
          [val y x])
      y)) ; => 3

(local (definition ···) body-expr ···1)      SYNTAX

This is a binding form similar to letrec, except that each definition is a define-values expression (after partial macro expansion). The body-exprs are evaluated in the lexical scope of these definitions.

(loop-until start done? next f)      PROCEDURE

Repeatedly invokes the f procedure until the done? procedure returns #t. The procedure is best described by its implementation:

(define loop-until
  (lambda (start done? next f)
    (let loop ([i start])
      (unless (done? i)
        (f i)
        (loop (next i))))))

(namespace-defined? symbol)      PROCEDURE

Returns #t if namespace-variable-binding would return a value for symbol, #f otherwise. See section 8.2 in PLT MzScheme: Language Manual for further information.

(nand expr ···)      SYNTAX

Returns (not (and expr ···)).

(nor expr ···)      SYNTAX

Returns (not (or expr ···)).

(opt-lambda formals body-expr ···1)      SYNTAX

The opt-lambda form is like lambda, except that default values are assigned to arguments (C++-style). Default values are defined in the formals list by replacing each variable by [variable default-value-expression]. If an variable has a default value expression, then all (non-aggregate) variables after it must have default value expressions. A final aggregate variable can be used as in lambda, but it cannot be given a default value. Each default value expression is evaluated only if it is needed. The environment of each default value expression includes the preceding arguments.

For example:

(define f
  (opt-lambda (a [b (add1 a)] . c)
     ...))

In the example, f is a procedure which takes at least one argument. If a second argument is specified, it is the value of b, otherwise b is (add1 a). If more than two arguments are specified, then the extra arguments are placed in a new list that is the value of c.

(recur name bindings body-expr ···1)      SYNTAX

This is equivalent to a named let: (let name bindings body-expr ···1).

(rec name value-expr)      SYNTAX

This is equivalent to a letrec expression that returns its binding: (letrec ((name value-expr)) name).

(symbol=? symbol1 symbol2)      PROCEDURE

Returns #t if symbol1 and symbol2 are equivalent (as determined by eq?), #f otherwise. If either symbol1 or symbol2 is not a symbol, the exn:application:type exception is raised.

(this-expression-source-directory)      SYNTAX

Expands to a string that names the directory of the file containing the source expression. The source expression's file is detemermined through source location information associated with the syntax if it is present. Otherwise, current-load-relative-directory is used if it is not #f, and current-directory is used if all else fails.

true      BOOLEAN

Boolean true.