6.9
3 Datum Pattern Matching
| (require syntax/datum) | package: base |
The syntax/datum library
provides forms that implement the pattern and template language of
syntax-case, but for matching and constructing datum values
instead of syntax.
For most pattern-matching purposes, racket/match is a better choice than syntax/datum. The syntax/datum library is useful mainly for its template support (i.e., datum) and, to a lesser extent, its direct correspondence to syntax-case patterns.
syntax
(datum-case datum-expr (literal-id ...) clause ...)
syntax
(datum template)
Like syntax-case and syntax, but datum-expr
in datum-case should produce a datum
(i.e., plain S-expression) instead of a syntax
object to be matched in clauses, and datum
similarly produces a datum. Pattern variables bound in each
clause of datum-case are accessible via
datum instead of syntax. When a literal-id
appears in a clause’s pattern, it matches the corresponding
symbol (using eq?).
Using datum-case and datum is essentially equivalent to converting the input to syntax-case using datum->syntax and then wrapping each use of syntax with syntax->datum, but datum-case and datum to not create intermediate syntax objects.
Example:
syntax
(with-datum ([pattern datum-expr] ...) body ...+)
Example:
> (with-datum ([(a ...) '(1 2 3)] [(b ...) '("x" "y" "z")]) (datum ((a b) ...))) '((1 "x") (2 "y") (3 "z"))
syntax
(quasidatum template)
syntax
(undatum expr)
syntax
(undatum-splicing expr)
Example:
> (with-datum ([(a ...) '(1 2 3)]) (quasidatum ((undatum (- 1 1)) a ... (undatum (+ 2 2))))) '(0 1 2 3 4)