Pattern language

A transformer spec has one of the following forms:

(syntax-rules ({literal} ...) {syntax-rule} ... ) syntax
(syntax-rules ({ellipsis} {literal} ...) {syntax-rule} ... ) syntax
_ auxiliary syntax
... auxiliary syntax

Syntax: It is an error if any of the literals, or the ellipsis in the second form, is not an identifier. It is also an error if syntax rule is not of the form

({pattern} {template})

The pattern in a syntax rule is a list pattern whose first element is an identifier.

A pattern is either an identifier, a constant, or one of the following

({pattern} ...) ({pattern} {pattern} ... . {pattern}) ({pattern} ... {pattern} {ellipsis} {pattern} ...) ({pattern} ... {pattern} {ellipsis} {pattern} ... . {pattern}) #({pattern} ...) #({pattern} ... {pattern} {ellipsis} {pattern} ...)

and a template is either an identifier, a constant, or one of the following

({element} ...) ({element} {element} ... . {template}) ({ellipsis} {template}) #({element} ...)

where an element is a template optionally followed by an ellipsis. An ellipsis is the identifier specified in the second form of syntax-rules, or the default identifier ... (three consecutive periods) otherwise.

Semantics: An instance of syntax-rules produces a new macro transformer by specifying a sequence of hygienic rewrite rules. A use of a macro whose keyword is associated with a transformer specified by syntax-rules is matched against the patterns contained in the syntax rules, beginning with the leftmost syntax rule. When a match is found, the macro use is transcribed hygienically according to the template.

An identifier appearing within a pattern can be an underscore (_), a literal identifier listed in the list of literals, or the ellipsis. All other identifiers appearing within a pattern are pattern variables.

The keyword at the beginning of the pattern in a syntax rule is not involved in the matching and is considered neither a pattern variable nor a literal identifier.

Pattern variables match arbitrary input elements and are used to refer to elements of the input in the template. It is an error for the same pattern variable to appear more than once in a pattern.

Underscores also match arbitrary input elements but are not pattern variables and so cannot be used to refer to those elements. If an underscore appears in the literals list, then that takes precedence and underscores in the pattern match as literals. Multiple underscores can appear in a pattern.

Identifiers that appear in ({literal} ...) are interpreted as literal identifiers to be matched against corresponding elements of the input. An element in the input matches a literal identifier if and only if it is an identifier and either both its occurrence in the macro expression and its occurrence in the macro definition have the same lexical binding, or the two identifiers are the same and both have no lexical binding.

A subpattern followed by ellipsis can match zero or more elements of the input, unless ellipsis appears in the literals, in which case it is matched as a literal.

More formally, an input expression E matches a pattern P if and only if:

  • P is an underscore (_).
  • P is a non-literal identifier; or
  • P is a literal identifier and E is an identifier with the same binding; or
  • P is a list (P1 ... Pn) and E is a list of n elements that match P1 through Pn, respectively; or
  • P is an improper list (P1 P2 ... Pn . Pn+1) and E is a list or improper list of n or more elements that match P1 though Pn respectively, and whose nth tail matches Pn+1; or
  • P is of the form (P1 ... Pk Pe {ellipsis} Pm+1 ... Pn) where E is a proper list of n elements, the first k of which match P1 through Pk, respectively, whose next m..k elements each match Pe, whose remaining n .. m elements match Pm+1 through Pn; or
  • P is of the form (P1 ... Pk Pe hellipsisi Pm+1 ... Pn . Px) where E is a list or improper list of n elements, the first k of which match P1 through Pk, whose next m .. k elements each match Pe, whose remaining n..m elements match Pm+1 through Pn, and whose nth and final cdr matches Px; or
  • P is a vector of the form #(P1 ... Pn) and E is a vector of n elements that match P1 through Pn; or
  • P is of the form #(P1 ... Pk Pe hellipsisi Pm+1 ... Pn) where E is a vector of n elements the first k of which match P1 through Pk, whose next m .. k elements each match Pe, and whose remaining n .. m elements match Pm+1 through Pn; or
  • P is a constant and E is equal to P in the sense of the equal? procedure.

It is an error to use a macro keyword, within the scope of its binding, in an expression that does not match any of the patterns.

When a macro use is transcribed according to the template of the matching syntax rule, pattern variables that occur in the template are replaced by the elements they match in the input. Pattern variables that occur in subpatterns followed by one or more instances of the identifier ellipsis are allowed only in subtemplates that are followed by as many instances of ellipsis. They are replaced in the output by all of the elements they match in the input, distributed as indicated. It is an error if the output cannot be built up as specified.

Identifiers that appear in the template but are not pattern variables or the identifier ellipsis are inserted into the output as literal identifiers. If a literal identifier is inserted as a free identifier then it refers to the binding of that identifier within whose scope the instance of syntax-rules appears. If a literal identifier is inserted as a bound identifier then it is in effect renamed to prevent inadvertent captures of free identifiers.

A template of the form (ellipsis template) is identical to template, except that ellipses within the template have no special meaning. That is, any ellipses contained within template are treated as ordinary identifiers. In particular, the template (ellipsis ellipsis) produces a single ellipsis. This allows syntactic abstractions to expand into code containing ellipses.

(define-syntax be-like-begin (syntax-rules () ((be-like-begin name) (define-syntax name (syntax-rules () ((name expr (... ...)) (begin expr (... ...)))))))) (be-like-begin sequence) (sequence 1 2 3 4) ==> 4

As an example, if let and cond are defined as in section 7.3 then they are hygienic (as required) and the following is not an error.

(let ((=> #f)) (cond (#t => 'ok))) ==> ok

The macro transformer for cond recognizes => as a local variable, and hence an expression, and not as the base identifier =>, which the macro transformer treats as a syntactic keyword. Thus the example expands into

(let ((=> #f)) (if #t (begin => 'ok)))

instead of

(let ((=> #f)) (let ((temp #t)) (if temp ('ok temp))))

which would result in an invalid procedure call.

husk-scheme online documentation rev 3.19.3 (2016.07.10)