3Datum Pattern Matching🔗 i

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.

Using datum-case and datum is similar 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 do not create intermediate syntax objects, and they do not destroy existing syntax objects within the S-expression structure of datum-expr.

The datum form also cooperates with syntax pattern variables such as those bound by syntax-case and attributes bound by syntax-parse (see Pattern Variables and Attributes for more information about attributes). As one consequence, datum provides a convenient way of getting the list of syntax objects bound to a syntax pattern variable of depth 1. For example, the following expressions are equivalent, except that the datum version avoids creating and eliminating a superfluous syntax object wrapper:

> (with-syntax ([(x... )#'(abc)])

(syntax->list #'(x... )))

'(#<syntax:eval:3:0 a> #<syntax:eval:3:0 b> #<syntax:eval:3:0 c>)

> (with-syntax ([(x... )#'(abc)])

(datum (x... )))

'(#<syntax:eval:4:0 a> #<syntax:eval:4:0 b> #<syntax:eval:4:0 c>)

A template can also use multiple syntax or datum pattern variables and datum constants, and it can use the ~@ and ~? template forms:

> (with-syntax ([(x... )#'(abc)])

(with-datum ([(y... )(list 123)])

(datum ([x-> y]... ))))

'((#<syntax:eval:5:0 a> -> 1)

(#<syntax:eval:5:0 b> -> 2)

(#<syntax:eval:5:0 c> -> 3))

> (with-syntax ([(x... )#'(abc)])

(with-datum ([(y... )(list 123)])

(datum ((~@ xy)... ))))

'(#<syntax:eval:6:0 a> 1 #<syntax:eval:6:0 b> 2 #<syntax:eval:6:0 c> 3)

See Attributes and datum for examples of ~? with datum .

If a datum variable is used in a syntax template, a compile-time error is raised.

Changed in version 7.8.0.9 of package base: Changed datum to cooperate with syntax-case , syntax-parse , etc.

Added in version 8.2.0.8 of package base.