[racket-users] Capturing print?

[Kevin Forchione]

Hi guys,
Is there a wrapper or something like that that I can place around a print expression so that it captures it and returns a string value? Something like:

[capture-print [printf “hello, world!”)) => “hello, world!”

There are times when testing I’d like to capture the output and compare it with a check-equal? and suppress it from displaying in testing It could also be useful in other was possibly for delaying and manipulating printed output.

Kevin

[bruno cuconato]

there's https://docs.racket-lang.org/reference/port-lib.html#%28def._%28%28lib._racket%2Fport..rkt%29._with-output-to-string%29%29 and company.

would that work for you?

-- bruno cuconato

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[Kevin Forchione]

On Jul 10, 2019, at 1:53 PM, bruno cuconato <bcc...@gmail.com> wrote:

there's https://docs.racket-lang.org/reference/port-lib.html#%28def._%28%28lib._racket%2Fport..rkt%29._with-output-to-string%29%29 and company.

would that work for you?

-- bruno cuconato

Thanks! Yes, I think with a little tinkering I can get that to work for what I have in mind. Took me awhile to discover *how* that is supposed to work! Apparently the proc for with-output-to-string is a thunk:

(with-output-to-string (λ a

                         (printf "a=~a~%" a)

                         (define x 10)

                         (define y 20)

                         (printf "x=~a y=~a~%" x y)))

=> "a=()\nx=10 y=20\n”

Something a little macro can handle:

(define-syntax (output->string stx)

  (syntax-parse stx

    [(_ body:expr ...) #'(with-output-to-string (thunk body ...))]))

(output->string

    (define x 10)

  (define y 20)

  (printf "x=~a y=~a~%" x y))

=> "x=10 y=20\n"

Kevin