8.3 Parsing syntax

► |  | Syntax: Meta-Programming Helpers

1 | Syntax Object Helpers
2 | Module-Processing Helpers
3 | Macro Transformer Helpers
4 | Reader Helpers
5 | Non-Module Compilation And Expansion
6 | Trusting Standard Recertifying Transformers
7 | Attaching Documentation to Exports
8 | Parsing and specifying syntax
| Index

▼ | 8 | Parsing and specifying syntax

8.1 | Introduction
8.2 | Examples
8.3 | Parsing syntax
8.4 | Specifying syntax with syntax classes
8.5 | Syntax patterns
8.6 | Literal sets and Conventions
8.7 | Library syntax classes and literal sets
8.8 | Debugging and inspection tools
8.9 | Experimental

On this page:

syntax-parse
syntax-parser

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#### 8.3`` Parsing syntax

This section describes syntax-parse, the syntax/parse library’s facility for parsing syntax. Both syntax-parse and the specification facility, syntax classes, use a common language of syntax patterns, which is described in detail in Syntax patterns.

Two parsing forms are provided: syntax-parse and syntax-parser.

( syntax-parse stx-expr parse-option ... clause ...+)

parse-option |  | = |  | #:context context-expr
|  | | |  | #:literals (literal ...)
|  | | |  | #:literal-sets (literal-set ...)
|  | | |  | #:conventions (convention-id ...)
|  | | |  | #:local-conventions (convention-rule ...)
literal |  | = |  | literal-id
|  | | |  | (pattern-id literal-id)
|  | | |  | (pattern-id literal-id #:phase phase-expr)
literal-set |  | = |  | literal-set-id
|  | | |  | (literal-set-id literal-set-option ...)
literal-set-option |  | = |  | #:at context-id
|  | | |  | #:phase phase-expr
clause |  | = |  | (syntax-pattern pattern-directive ... expr ...+)

> | stx-expr |  | : |  | syntax?

> | context-expr |  | : |  | syntax?

> | phase-expr |  | : |  | ( or/c exact-integer? #f)

Evaluates stx-expr, which should produce a syntax object, and matches it against the clauses in order. If some clause’s pattern matches, its attributes are bound to the corresponding subterms of the syntax object and that clause’s side conditions and expr is evaluated. The result is the result of expr.

If the syntax object fails to match any of the patterns (or all matches fail the corresponding clauses’ side conditions), a syntax error is raised.

The following options are supported:

> #:context context-expr

> | context-expr |  | : |  | syntax?

> When present, context-expr is used in reporting parse failures; otherwise stx-expr is used.

> Examples:

> > ( syntax-parse #'(a b 3)
> [(x:id ...) 'ok])

> a: expected identifier at: 3

> > ( syntax-parse #'(a b 3)
> #:context #'( lambda (a b 3) (+ a b))
> [(x:id ...) 'ok])

> lambda: expected identifier at: 3

> #:literals (literal ...)

> literal |  | = |  | literal-id
> |  | | |  | (pattern-id literal-id)
> |  | | |  | (pattern-id literal-id #:phase phase-expr)

> | phase-expr |  | : |  | ( or/c exact-integer? #f)

> Unlike syntax-case, syntax-parse requires all literals to have a binding. To match identifiers by their symbolic names, use the ~datum pattern form instead. The #:literals option specifies identifiers that should be treated as literals rather than pattern variables. An entry in the literals list has two components: the identifier used within the pattern to signify the positions to be matched (pattern-id), and the identifier expected to occur in those positions (literal-id). If the entry is a single identifier, that identifier is used for both purposes.

> If the #:phase option is given, then the literal is compared at phase phase-expr. Specifically, the binding of the literal-id at phase phase-expr must match the input’s binding at phase phase-expr.

> #:literal-sets (literal-set ...)

> literal-set |  | = |  | literal-set-id
> |  | | |  | (literal-set-id literal-set-option ...)
> literal-set-option |  | = |  | #:at context-id
> |  | | |  | #:phase phase-expr

> | phase-expr |  | : |  | ( or/c exact-integer? #f)

> Many literals can be declared at once via one or more literal sets, imported with the #:literal-sets option. See literal sets for more information.

> #:conventions (conventions-id ...)

> Imports conventions that give default syntax classes to pattern variables that do not explicitly specify a syntax class.

> #:local-conventions (convention-rule ...)

> Uses the conventions specified. The advantage of #:local-conventions over #:conventions is that local conventions can be in the scope of syntax-class parameter bindings. See the section on conventions for examples.

Each clause consists of a syntax pattern, an optional sequence of pattern directives, and a non-empty sequence of body expressions.

( syntax-parser parse-option ... clause ...+)

Like syntax-parse, but produces a matching procedure. The procedure accepts a single argument, which should be a syntax object.
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