Choose-out: Conditional Provide🔗 i

provide-spec

( choose-out proc-expr[true-idfalse-id]...)

> (module struct-mathracket

;`num` is an internal representation

(struct num[value])

;set this to `#t` in modules that use `num` values

(define-for-syntax num-client?(box #false))

;addition for `num` values

(define (num-+n0n1)

(num(+ (num-valuen0)(num-valuen1))))

;wrapper: convert `num` addition to Racket addition

(define (racket-+v0v1)

(num-value(num-+(numv0)(numv1))))

;export `num-+` or `racket-+` depending on the importing module

(require choose-out )

(provide

num

(for-syntax num-client?)

(choose-out (λ ()(unbox num-client?))

[num-+racket-+])))

> (module struct-clientracket

;familiar client, uses the `num` representation

(require 'struct-math)

(begin-for-syntax (set-box! num-client?#true))

(displayln (num-+(num2)(num2))))

> (require 'struct-client)

#<num>

> (module racket-clientracket

;foreign client: uses Racket numbers

(require 'struct-math)

(displayln (num-+22)))

> (require 'racket-client)

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