[racket-dev] racket/match is broken

Sure you COULD do that, but app is so cool, I just like the way it looks.

Also, Sam, I've thought more about this and I've definitely made the
assumption that the "and" pattern evaluates left to right. For
example, in the bidirectional matchers that are underneath the URL
dispatch library, I assume that to protect usages of 'app' from
getting the right typed arguments.

Jay

On Wed, Oct 5, 2011 at 8:55 PM, Robby Findler
<robby at eecs.northwestern.edu> wrote:
> Why wouldn't you write that match like this:
>
> (match ..
>  [`(+ ,lhs ,rhs) (make-plus (parse lhs) (parse rhs))]
>  [`(fun (,(? symbol arg) ...) ,body) (make-fun arg (parse body))])
>
> Robby
>
> On Wed, Oct 5, 2011 at 9:48 PM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
>> Okay. I think it is strange, but feel free to do that and revert my
>> change. Apologies for the confusion.
>>
>> The use case I was actually doing was like this:
>>
>> [(list '+ (app parse lhs) (app parse rhs))
>>  ...]
>> [(list 'fun (list (? symbol? arg) ...) (app parse body))
>>  ...]
>>
>> And when you gave it a '(fun () ...) it would try to parse the '() in
>> the fun as the lhs of a + and then throw an exception and crash the
>> whole match.
>>
>> Jay
>>
>> On Wed, Oct 5, 2011 at 7:39 PM, Sam Tobin-Hochstadt <samth at ccs.neu.edu> wrote:
>>> On Wed, Oct 5, 2011 at 5:13 PM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
>>>>
>>>> Basically, it calls the app function on the second element of the list
>>>> even though the head of the list has failed to pattern match.
>>>
>>> This is not a bug.  `match' makes no guarantee about the order in
>>> which it checks elements of the pattern.  If it were to try to support
>>> this kind of use case, it would have to specify how many times it
>>> would call each function that might be embedded in a match, which
>>> requires a detailed semantics of backtracking in the matcher, and lock
>>> in a particular implementation.
>>>
>>> I will clarify the documentation to say that you shouldn't use
>>> functions that aren't safe to call 0, 1, or more times, in various
>>> orders, inside `match' patterns.
>>> --
>>> sam th
>>> samth at ccs.neu.edu
>>>
>>
>>
>>
>> --
>> Jay McCarthy <jay at cs.byu.edu>
>> Assistant Professor / Brigham Young University
>> http://faculty.cs.byu.edu/~jay
>>
>> "The glory of God is Intelligence" - D&C 93
>>
>> _________________________________________________
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>



-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93