Version: 5.2.1
3 Datum Pattern Matching
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.
| (datum-case datum-expr (literal-id ...) | | clause ...) |
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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: |
| '((1 "x") (2 "y") (3 "z")) |
|