Zelphir Kaltstahl <zelphirkaltst...@gmail.com> writes:
>>> I decided to take a look at how one can parse command line arguments in
>>> Guile and was looking for something like argparse in Python. It seems
>>> that (use-modules (ice-9 getopt-long)) does the job, except that I hit
>>> one problem and don't know what the mistake I am making is. It seems to
>>> be connected to the usage of `predicate` in my code.
>>>
>>>;; ===== EXAMPLE START =====
>>>(define (string-exact-integer? str)
>>> (exact-integer? (string->number str)))
>>>
>>>(define option-spec
>>> '((version ... (predicate string-exact-integer?))))
>>>
>>> I am providing a function that takes the
>>> option's value as string and returns #t or #f.
>> From: Matt Wette <matt.we...@gmail.com>
>>
>> You probably want to use quasi-quote + unquote:
>> ? `((version ... (predicate ,string-exact-integer?))))
Zelphir Kaltstahl <zelphirkaltst...@gmail.com> writes:
>
> That solved the problem, thank you Matt!
> I was so far quite fond of the way one specifies options with
> getopt-long, but the quasi-quote unquote was not mentioned in the Guile
> docs and feels unnatural.
Actually my copy of the manual says:
(predicate func)
...
then getopt-long will apply func to the value,
and throw an exception if it returns #f.
func should be a procedure which accepts a string and
returns a boolean value; you may need to use
quasiquotes to get it intoa grammar.
So a warning about quasiquote _is_ mentioned in the Guile docs,
although it wouldn't hurt to put in an example to clarify just _how_
to "use quasiquote to get it into the grammmar."
> There seems to be no reason for it to force me
> to do that, except that it does not work otherwise.
When I first read that, I was exasperated. What more reason
do you need to do it right? It works that way and doesn't
work if you do it wrong.
Upon further reflection I realized that you are probably not
asking why right is better than wrong, but confused about
what quasiquote does.
The manual says:
func should be a procedure
You said:
>>> I am providing a function that takes the
>>> option's value as string and returns #t or #f.
but that is _not_ what you were doing. If the whole
list is quoted, then the occurrence of "string-exact-integer?"
in the list is a _symbol_. It must be unquoted so
that it will be evaluated to a function (i.e. procedure).
It's like the difference between (+ 2 2) and ('+ 2 2).
The manual could use more explanation, but I believe
it is correct.
Hope this helps.
-- Keith
