I may be missing something, but `log-debug` and friends already do that.
stamourv@westmount:2014-12-12 16:45:plt$ racket
Welcome to Racket v6.1.1.6.
-> (log-debug (begin (displayln "hello!") "log"))
->
stamourv@westmount:2014-12-12 16:45:plt$ racket -W debug
Welcome to Racket v6.1.1.6.
GC: 0:min @ 1,356K(+307K)[+160K]; free 932K(-5,028K) 4ms @ 12
GC: 0:min @ 1,813K(+3,946K)[+232K]; free 755K(-2,051K) 4ms @ 20
GC: 0:min @ 4,040K(+4,087K)[+232K]; free 1,448K(-1,448K) 4ms @ 28
GC: 0:min @ 5,015K(+3,112K)[+240K]; free 1,425K(-6,817K) 4ms @ 40
GC: 0:min @ 7,459K(+6,060K)[+272K]; free 2,162K(-3,458K) 0ms @ 56
GC: 0:min @ 9,356K(+6,211K)[+280K]; free 1,628K(-2,924K) 4ms @ 72
GC: 0:min @ 13,230K(+4,353K)[+296K]; free 2,558K(-16,142K) 12ms @ 96
GC: 0:min @ 17,224K(+13,943K)[+392K]; free 3,900K(-6,492K) 12ms @ 136
GC: 0:min @ 21,878K(+12,457K)[+400K]; free 4,741K(-6,037K) 12ms @ 172
GC: 0:min @ 27,016K(+9,255K)[+988K]; free 6,168K(-8,760K) 16ms @ 232
GC: 0:min @ 32,548K(+6,315K)[+1,276K]; free 7,665K(-26,641K) 36ms @ 312
-> (log-debug (begin (displayln "hello!") "log"))
hello!
log
->
`log-message`, however, does not.
The problem may be that, when you switch to the lower logging level,
your old listener for the higher level is still around to receive the
messages.
Vincent
At Fri, 12 Dec 2014 13:32:41 -0800,
Matthew Butterick wrote:
>
> Is there a reason not to redefine `log-debug` et al as macros that evaluate
> their arguments lazily?
>
> Sort of like this;
>
> (if ((current-logging-level) . is-equal-or-above? . 'debug)
> (log-debug arg ...)
> (void))
>
> Because I find that my use of these functions often looks like this:
>
> (log-debug "Value = ~a" (expensive-operation-for-logging-purposes))
>
> The problem is that when I change to a lower logging level, the
> (expensive-operation) is still evaluated.
>
> I asked a similar question about hash-ref! not long ago. The answer was that
> you can get lazy evaluation by wrapping the default argument in a lambda,
> like so:
>
> (hash-ref! hashtable key (λ () (expression-that-produces-value)))
>
> But it seems there's no equivalent idiom for `log-debug` et al.
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