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 following constraints are available:
|Constraint name||Grammar accepted||Binds pattern variable to|
|expression||expression||parsed expression fragment|
|body||bodyopt (2)||parsed expression fragment (3)|
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.
Parsing stops at an intermediate word.
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
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.
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
ifstatement. The intermediate words of a macro are identified as follows:
case-body, or names an auxiliary rule-set where some left-hand side in that rule-set ends in a body-variable. This is a least fixed point, so a recursive auxiliary rule-set does not automatically make its name into a body-variable! Note that an ellipsis that stands for a pattern variable is a body-variable when that pattern variable is one.
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