Question

I started playing with Racket pattern matching system recently and got into a problem i can’t understand.

If i do:

(match (list 1 2 3 4 5 6 7 8 9 10 11 12)
    [(list _ x y z ...) (list y ': x)]) 

in REPL i get

'(3 : 2)

as my desired result.

If i do:

(match (current-date)
    [(date* _ x y z ...) (list y ': x)])

or

(match (date* 5 18 13 18 11 2011 5 321 #f 3600 0 "W. Europe Standard Time")
    [(date* _ x y z ...) (list y ': x)])

i get this error:

match: wrong number for fields for structure date*: expected 12 but got 5 in: (_ x y z ...)

i suspect that the Kleene star ... does not work with struct type for some reason. Why is it so?

Answer 1

It seems that what you really want to do is match a subset of the fields in a struct, rather than actually bind the rest of the fields in the struct to (z …). In this case, you may want to try using the struct* match pattern instead.

Here’s an example:

(match (date* 5 18 13 18 11 2011 5 321 #f 3600 0 "W. Europe Standard Time")
   [(struct* date ([minute x] [hour y])) (list y ': x)])