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10 Macros

Pattern Variable Constraints

Each pattern-variable in the left-hand side of a rule in a macro definition has a constraint associated with it. This prevents the pattern from matching unless the fragment matched to the pattern-variable satisfies the constraint. In most cases it also controls how the matching fragment is parsed.

You specify a constraint in a pattern-variable by suffixing a colon and the constraint name to the pattern variable name. Intervening whitespace is not allowed. As an abbreviation, if a pattern variable has the same name as its constraint, the pattern-variable can be written ?:the-name instead of ?the-name:the-name.

The available constraints are listed in Table 10-1.

Table 10-1 Available constraints

Constraint name

Grammar accepted

Binds pattern variable to

expression

expression

parsed expression fragment(1)

variable

variable

fragment(2)

name

name

one-token fragment

token

token

one-token fragment

body

bodyopt (3)

parsed expression fragment (4)

case-body

case-bodyopt (3)

fragment(2)

macro

macro

fragment(5)

*

(wildcard)

fragment

Notes:

  1. Parsed expression fragments are described on page 146.
  2. Where expression, operand, constituents or body appears in the grammar that this constraint accepts, the bound fragment contains a parsed expression fragment, not the original elementary fragments.
  3. Parsing stops at an intermediate word.
  4. The body is wrapped in begin … end to make it an expression, using the standard binding of begin in the Dylan module. An empty body defaults to #f.
  5. A pattern-variable with a macro constraint accepts exactly one elementary fragment, which must be a macro call fragment. It binds the pattern variable to the expansion of the macro.

Some implementations and a future version of the Dylan language specification might add more constraint choices to this table.

When a pattern variable has the same name as an auxiliary rule-set, its constraint defaults to wildcard and can be omitted. Otherwise a constraint must be specified in every pattern-variable and pattern-keyword.

A constraint applies only to the specific pattern variable occurrence to which it is attached. It does not constrain other pattern variable occurrences with the same name.

Intermediate Words

When a pattern-variable has a constraint of body or case-body, its parsing of the fragment stops before any token that is an intermediate word. This allows intermediate words to delimit clauses that have separate bodies, like else and elseif in an if statement. The intermediate words of a macro are identified as follows: