On Wed, Dec 30, 2020 at 2:24 PM David Storrs <[email protected]> wrote: > > First off, is there a way to make ... in a pattern match non-greedily? i.e., > match as *few* elements as possible instead of as many?
As far as I know, no. However, if your first example is really illustrative of what you're trying to do, you could just use a `cons` pattern instead of a `list` pattern: (match (list '(a b c) '(d e c)) [(list (cons _ xs) (cons _ ys)) #:when (equal? xs ys) 'ok] [else 'nope]) => 'nope And that makes a nice segue into your second question: > Second, is there a way to make one pattern refer to an earlier pattern in the > same match clause? If we're limiting this to variable patterns, then yes. The documentation says: "If an id is used multiple times within a pattern, the corresponding matches must be the same according to (match-equality-test), except that instances of an id in different or and not sub-patterns are independent." So, assuming that `(match-equality-test)` is `equal?` -- which is the default -- the above could also be written as: (match (list '(a b c) '(d e c)) [(list (cons _ xs) (cons _ xs)) 'ok] [else 'nope]) Similarly, your second example could be written as: (match '(a b c c d) [(list _ ... x x _ ...) 'ok] [else 'nope]) However, I have seen some issues with this feature (using the same var pattern multiple times to get an equality restriction). Most recently: https://github.com/racket/racket/issues/3425 - Jon -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAKfDxxzm-hq8DRVF5-Ur_Q%3DdxCeQVeARERjKzxQ%2BVUU-18dsrg%40mail.gmail.com.
