How do I define a recursive struct in Racket? - struct

Question

I'm trying to imitate recursive types in OCaml in untyped Racket, but I can't seem to find documentation on defining recursive structs. How would I go about mapping this:
type example =
| Red
| Blue of example
| Yellow of example * example;;
into something in Racket?
I've tried the following:
(struct Red ())
(struct Blue (e1))
(struct Yellow (e1 e2))
(struct example/c
(or/c (Red)
(Blue example/c)
(Yellow example/c example/c)))
However, it doesn't work as expected when I put example/c in a contract, because it claims it's a procedure. Any help?

Answer 1

I have changed the example.
Here is an example where the variable e has the contract example/c.
#lang racket
(struct Red () #:transparent)
(struct Blue (e1) #:transparent)
(struct Yellow (e1 e2) #:transparent)
(define example/c
(flat-murec-contract ([red/c (struct/c Red)]
[blue/c (struct/c Blue example/c)]
[yellow/c (struct/c Yellow example/c example/c)]
[example/c (or/c red/c blue/c yellow/c)])
example/c))
(define r (Red))
(define b (Blue r))
(define y (Yellow r b))
(define e y)
(provide (contract-out [e example/c]))
(match e
[(Red) (list "Red")]
[(Blue e1) (list "Blue" e1)]
[(Yellow e1 e2) (list "Yellow" e1 e2)]
[else "huh"])
If you change, say, (Yellow r b) to (Yellow r 42) then you get an error.