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2 | Automata: Compiling State Machines
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8 | Errors
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On this page:

with-logging-to-port
with-intercepted-logging
start-recording
stop-recording

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### 15`` Logging

Vincent St-Amour <stamourv@racket-lang.org>

This library is unstable; compatibility will not be maintained. See Unstable: May Change Without Warning for more information.

(require unstable/logging) | package: typed-racket-lib

This module provides tools for logging.

> procedure (with-logging-to-port port proc log-spec ...) → any
> port : output-port?
> proc : (-> any)
> log-spec : (or/c 'fatal 'error 'warning 'info 'debug symbol? #f)

Runs proc, outputting any logging that would be received by (make-log-receiver (current-logger) log-spec ...) to port. Returns whatever proc returns.

Example:
> > (let ([my-log (open-output-string)])
> (with-logging-to-port my-log
> (lambda ()
> (log-warning "Warning World!")
> (+ 2 2))
> 'warning)
> (get-output-string my-log))
> "Warning World!\n"

> > procedure
> > (with-intercepted-logging |  | interceptor |  |  |  |
> > |  | proc |  |  |  |
> > |  | log-spec ...) |  | → |  | any
> | interceptor |  | : |  |
> (-> (vector/c
> (or/c 'fatal 'error 'warning 'info 'debug)
> string?
> any/c
> (or/c symbol? #f))
> any)
> proc : (-> any)
> log-spec : (or/c 'fatal 'error 'warning 'info 'debug symbol? #f)

Runs proc, calling interceptor on any log message that would be received by (make-log-receiver (current-logger) log-spec ...). interceptor receives the entire log vectors (see Receiving Logged Events) as arguments. Returns whatever proc returns.

Example:
> > (let ([warning-counter 0])
> (with-intercepted-logging
> (lambda (l)
> (when (eq? (vector-ref l 0)
> 'warning)
> (set! warning-counter (add1 warning-counter))))
> (lambda ()
> (log-warning "Warning!")
> (log-warning "Warning again!")
> (+ 2 2))
> 'warning)
> warning-counter)
> 2

A lower-level interface to logging is also available.

> procedure (start-recording log-spec ...) → listener?
> log-spec : (or/c 'fatal 'error 'warning 'info 'debug symbol? #f)
> procedure (stop-recording listener)
> | → |  |
> (listof (vector/c (or/c 'fatal 'error 'warning 'info 'debug)
> string?
> any/c
> (or/c symbol? #f)))
> listener : listener?

start-recording starts recording log messages matching the given log-spec (see make-log-receiver for how log-spec is interpreted). Messages will be recorded until stopped by passing the returned listener object to stop-recording. stop-recording will then return a list of the log messages that have been reported.

Examples:
> (define l (start-recording 'warning))
> > (log-warning "1")
> > (log-warning "2")
> > (stop-recording l)
> '(#(warning "1" #<continuation-mark-set> #f)
> #(warning "2" #<continuation-mark-set> #f))

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