[erlang-questions] Problems With Smart Exceptions

Ulf Wiger ulf.wiger@REDACTED
Mon Dec 20 19:52:40 CET 2010

It seems as if an 80% solution could rather easily be made using
a core_transform, since the most interesting exceptions are explicit
in Core Erlang.

One question to the OTP team, though:

I made a very simple test module and inserted a 
-compile({core_transform, testmod}), in it.

Why is the core transform called twice?

…after some digging, it seems as if 


should remove core_transform directives as well, right?

clean_parse_transforms(Fs) ->
    clean_parse_transforms_1(Fs, []).

clean_parse_transforms_1([{attribute,L,compile,C0}|Fs], Acc) when is_list(C0) ->
    C = lists:filter(fun({parse_transform,_}) -> false;
			(_) -> true
		     end, C0),
    clean_parse_transforms_1(Fs, [{attribute,L,compile,C}|Acc]);
clean_parse_transforms_1([{attribute,_,compile,{parse_transform,_}}|Fs], Acc) ->
    clean_parse_transforms_1(Fs, Acc);
clean_parse_transforms_1([F|Fs], Acc) ->
    clean_parse_transforms_1(Fs, [F|Acc]);
clean_parse_transforms_1([], Acc) -> reverse(Acc).

Now, core transforms are not documented at all, so I couldn't say if there
is some secret reason why they _should_ be called twice, but I'd wager
they shouldn't. :)

Ulf W

On 20 Dec 2010, at 18:25, Olivier BOUDEVILLE wrote:

> Hi,
> I tried both versions (stable/devel) of the Smart Exceptions 
> parse-transform (as taken from https://github.com/thomasl/smart_exceptions
> ), but had not luck with either.
> With the stable version, correct user code produced then unwanted 
> warnings, like "Warning: variable 'KeyIndex' exported from 'case' (line 
> 90)" for:
> """
>  addEntry( Key, Value, HashTable ) ->
>     KeyIndex = erlang:phash2( Key, size(HashTable) ) + 1,
>     % Retrieve appropriate tuple slot:
>     PreviousList = element( KeyIndex, HashTable ),
>     NewList = replaceBucket( Key, Value, PreviousList, [] ),
>     setelement(KeyIndex,HashTable,NewList).
> """
> (line 90 is "KeyIndex = erlang:phash2( Key, size(HashTable) ) + 1"). 
> Many warnings like this one appeared, in the transformed code.
> With the devel version, I could check that the parse transform was 
> compiled and found at compile-time, but even when using the sample code in 
> http://www.erlang.org/faq/how_do_i.html#id55258 I still have the exact 
> same message that was output when this parse-transform was not used:
> ''''
> Erlang R14B01 (erts-5.8.2) [source] [64-bit] [smp:8:8] [rq:8] 
> [async-threads:0] [hipe] [kernel-poll:false]
> Eshell V5.8.2  (abort with ^G)
> 1>  c(crash, [{parse_transform, smart_exceptions}]).
> ./crash.erl:9: Warning: no clause will ever match
> {ok,crash}
> 2> crash:f().
> ** exception error: no match of right hand side value 5
>      in function  crash:g/0
> ''''
> Am I doing something wrong?
> A last question: shouldn't we expect a performance penalty when using this 
> parse-transform? I imagine that the transformed code could be (slighthly?) 
> less efficient if having to test for more matchings/wrap more exceptions?
> Thanks in advance for any information,
> Best regards,
> Olivier Boudeville.
> ---------------------------
> Olivier Boudeville
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