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sugar/scribblings/debug.scrbl

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Racket

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#lang scribble/manual
@(require scribble/eval (for-label racket sugar))
@(define my-eval (make-base-eval))
@(my-eval `(require sugar))
@title{Debug}
@defmodule[sugar/debug]
Debugging utilities.
@defform*[((report expr) (report expr maybe-name))]
Print the name and value of @racket[_expr] to @racket[current-error-port], but also return the evaluated result of @racket[_expr] as usual. This lets you see the value of an expression or variable at runtime without disrupting any of the surrounding code. Optionally, you can use @racket[_maybe-name] to change the name shown in @racket[current-error-port].
For instance, suppose you wanted to see how @racket[first-condition?] was being evaluted in this expression:
@racketblock[
(if (and (first-condition? x) (second-condition? x))
(one-thing)
(other-thing))]
You can wrap it in @racket[report] and find out:
@racketblock[
(if (and (report (first-condition? x)) (second-condition? x))
(one-thing)
(other-thing))]
This code will run the same way as before. But when it reaches @racket[first-condition?], you willl see in @racket[current-error-port]:
@racketerror{(first-condition? x) = #t}
You can also add standalone calls to @racket[report] as a debugging aid at points where the return value will be irrelevant, for instance:
@racketblock[
(report x x-before-function)
(if (and (report (first-condition? x)) (second-condition? x))
(one-thing)
(other-thing))]
@racketerror{x-before-function = 42
@(linebreak)(first-condition? x) = #t}
But be careful  in the example below, the result of the @racket[if] expression will be skipped in favor of the last expression, which will be the value of @racket[_x]:
@racketblock[
(if (and (report (first-condition? x)) (second-condition? x))
(one-thing)
(other-thing))
(report x)]