in parse_query it tries to do a list_to_integer of "va" with var2 based on how it parses the args list and you can't do that since "va" doesn't produce a valid integer. Would the try need to be at the level of the list_to_integer call?
thx,
-wes
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> has_query(A) -> > try > case length(yaws_api:parse_query(A)) of > 0 -> false; > _ -> true > end > catch > _ -> false > end.
For this function to catch exceptions of all possible types (error, exit and throw) you need to use "_:_" pattern instead of just "_" (which will catch only 'throw' exceptions):
has_query(A) -> try case length(yaws_api:parse_query(A)) of 0 -> false; _ -> true end catch _:_ -> false % <----- Here "_" has been replaced by "_:_" end.
-- Sergey Samokhin
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On Tue, Jun 29, 2010 at 11:21 PM, Wes James <compte...@gmail.com> wrote: > What is the scope of try/catch. > has_query(A) -> > try > case length(yaws_api:parse_query(A)) of
[...]
> because if a user puts in an invalid character in the URL it crashes > parse_query and I want the app to continue without erroring out in the > browser.
> Is this because of the level of the error?
Yes (is my guess), see http://www.erlang.org/doc/reference_manual/expressions.html#id2276021 on the try..catch construction. Since you omit the class, it is defaulted to 'throw'. I would personally regard that as an error in the interface of parse_query. My solution would be something akin to
safe_parse_query(A) -> try Q = yaws_api:parse_query(A), {ok, Q} catch error:_ -> no_parse % You should really narrow down the class and the error Reason to the right type here. I may be wrong. end.
which transforms the function into a variant which is safer:
has_query(A) -> case yaws_api:parse_query(A) of no_parse -> false; {ok, Q} when length(Q) == 0 -> false; {ok, Q} -> true end.
alternatively, no_parse can be an empty list, if you so want.
(Caveat: I only wrote the code and did not test it).
-- J.
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On Tue, Jun 29, 2010 at 3:46 PM, Sergey Samokhin <prikru...@gmail.com> wrote: >> has_query(A) -> >> try >> case length(yaws_api:parse_query(A)) of >> 0 -> false; >> _ -> true >> end >> catch >> _ -> false >> end.
> For this function to catch exceptions of all possible types (error, > exit and throw) you need to use "_:_" pattern instead of just "_" > (which will catch only 'throw' exceptions):
> has_query(A) -> > try > case length(yaws_api:parse_query(A)) of > 0 -> false; > _ -> true > end > catch > _:_ -> false % <----- Here "_" has been replaced by "_:_" > end.
> -- > Sergey Samokhin
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No _:_ is *NOT* the same as {_,_}. It is a special pattern which can only be used in the catch part of a try. There you can give [Class1:]ExceptionPattern1 as a pattern to match which class of exception to match and also which exception. Writing _:_ here means that you match all classes and all exceptions, i.e. you catch everything.
Calling throw an exception is not really correct in my opinion as it doesn't really signify an error but is a non-local return. At least that is how it was intended to be used. Try might actually be too powerful.
Robert
On 30 June 2010 00:17, Wes James <compte...@gmail.com> wrote:
> to see how _:_ works. There is no mention of this on these pages. Is > _:_ the same as {_,_}? Is there a doc on what _:_ is?
> thx, again,
> -wes
> On Tue, Jun 29, 2010 at 3:46 PM, Sergey Samokhin <prikru...@gmail.com> wrote: >>> has_query(A) -> >>> try >>> case length(yaws_api:parse_query(A)) of >>> 0 -> false; >>> _ -> true >>> end >>> catch >>> _ -> false >>> end.
>> For this function to catch exceptions of all possible types (error, >> exit and throw) you need to use "_:_" pattern instead of just "_" >> (which will catch only 'throw' exceptions):
>> has_query(A) -> >> try >> case length(yaws_api:parse_query(A)) of >> 0 -> false; >> _ -> true >> end >> catch >> _:_ -> false % <----- Here "_" has been replaced by "_:_" >> end.
>> -- >> Sergey Samokhin
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Wes James <compte...@gmail.com> writes: > has_query(A) -> > case length(yaws_api:parse_query(A)) of > 0 -> false; > _ -> true > end.
A separate note about the code: you should avoid "case length(Foo)" when possible, since length/1 needs to traverse the entire list. Just comparing the result to the empty list is probably faster:
case yaws_api:parse_query(A) of [] -> false; _ -> true end.
And this can be simplified further:
yaws_api:parse_query(A) /= [].
(There is a difference between my two examples and yours: your code will crash with badarg if the result is not a proper list, while mine will just return false.)
<mag...@erlang-solutions.com> wrote: > Wes James <compte...@gmail.com> writes:
>> has_query(A) -> >> case length(yaws_api:parse_query(A)) of >> 0 -> false; >> _ -> true >> end.
> A separate note about the code: you should avoid "case length(Foo)" when > possible, since length/1 needs to traverse the entire list. Just > comparing the result to the empty list is probably faster:
> case yaws_api:parse_query(A) of > [] -> false; > _ -> true > end.
> And this can be simplified further:
> yaws_api:parse_query(A) /= [].
> (There is a difference between my two examples and yours: your code will > crash with badarg if the result is not a proper list, while mine will > just return false.)
This might be even clearer (separating the right and wrong cases):
has_query(A) -> try yaws_api:parse_query(A) of [] -> false; _ -> true % Were you supposed to return true for this case? catch error:_ -> false % Only cathes runtime errors, not exit/1. end.
> > A separate note about the code: you should avoid "case length(Foo)" when > > possible, since length/1 needs to traverse the entire list. Just > > comparing the result to the empty list is probably faster:
> > case yaws_api:parse_query(A) of > > [] -> false; > > _ -> true > > end.
> > And this can be simplified further:
> > yaws_api:parse_query(A) /= [].
> > (There is a difference between my two examples and yours: your code will > > crash with badarg if the result is not a proper list, while mine will > > just return false.)
> This might be even clearer (separating the right and wrong cases):
> has_query(A) -> > try yaws_api:parse_query(A) of > [] -> false; > _ -> true % Were you supposed to return true for this case? > catch > error:_ -> false % Only cathes runtime errors, not exit/1. > end.
Thanks. This works too. Bottom line, is I want to show the default search page even if the parse_query call throws an error. Maybe someone is trying to insert bad chars or such in to the URL to try to reveal something or even hack the system, etc.
thx,
-wes
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