[erlang-questions] Bug, feature or limitation of erl_parse?

[Hynek Vychodil]

What is problem?

1> L = "test".
"test"
2> Match = L ++ '_'.
[116,101,115,116|'_']


On Wed, Aug 25, 2010 at 3:22 PM, Musumeci, Antonio S
<Antonio.Musumeci@REDACTED> wrote:
> http://www.erlang.org/doc/apps/erts/match_spec.html
>
> It shows that [$t|'_'] is a proper match pattern... And it does work. Since otherwise "t" ++ Else is a shortcut for [$t|Else] I was trying to find out why I appear unable to parse a string of the former to use in a matchspec as the latter one does work.
>
> -----Original Message-----
> From: erlang-questions@REDACTED [mailto:erlang-questions@REDACTED] On Behalf Of Robert Raschke
> Sent: Wednesday, August 25, 2010 8:52 AM
> To: erlang-questions@REDACTED
> Subject: Re: [erlang-questions] Bug, feature or limitation of erl_parse?
>
> Hmm, [$t,$e,$s,$t|'_'] is not a proper list.
> Do you mean [$t,$e,$s,$t,'_'] (or the equivalent [$t,$e,$s,$t|['_']])?
>
> If yes, then "test"++['_'] will do the match:
>
> Eshell V5.6.5  (abort with ^G)
> 1> L=[$t,$e,$s,$t|['_']].
> [116,101,115,116,'_']
> 2> "test"++['_']=L.
> [116,101,115,116,'_']
>
> But I'm pretty sure I am not understanding your question exactly.
>
> Robby
>
> PS Interesting, why does this work:
> 3> "test"++'_'.
> [116,101,115,116|'_']
> ???
>
> On Wed, Aug 25, 2010 at 1:17 PM, Musumeci, Antonio S < Antonio.Musumeci@REDACTED> wrote:
>
>> Understood. Is there no way to turn "test" ++ '_' into
>> [$t,$e,$s,$t|'_'] from a string for use in a matchspec?
>>
>> -----Original Message-----
>> From: Robert Virding [mailto:rvirding@REDACTED]
>> Sent: Tuesday, August 24, 2010 5:01 PM
>> To: Musumeci, Antonio S (Enterprise Infrastructure)
>> Cc: erlang-questions@REDACTED
>> Subject: Re: [erlang-questions] Bug, feature or limitation of erl_parse?
>>
>> Erl_parse does recognise '"test" ++ Else', but it is not a term it is
>> an expression. It is later inside the compiler that the expression is
>> converted into it's shortcut form. The parser just parses the
>> expression. When you did it by hand, so to speak, you got a term which
>> erl_parse could parse as a term. So:
>>
>> 17> f(S), {ok,S,_} = erl_scan:string("\"test\" ++ Else. ").
>> {ok,[{string,1,"test"},{'++',1},{var,1,'Else'},{dot,1}],1}
>> 18> erl_parse:parse_exprs(S).
>> {ok,[{op,1,'++',{string,1,"test"},{var,1,'Else'}}]}
>>
>> N.B. it it not the string "term" which causes it to be an expression
>> but the operator '++'.
>>
>> Robert
>>
>> On 24 August 2010 22:08, Musumeci, Antonio S <
>> Antonio.Musumeci@REDACTED> wrote:
>> > It doesn't appear to understand the "test" ++ Else shortcut for
>> [$t,$e,$s,$t|Else]. Reason I noticed was I'm reading in terms as
>> strings and passing them to mnesia:select. erl_parse works fine with
>> [$t|'_'] but not "t" ++ '_'.
>> >
>> >> S = "[$t,$e,$s,$t|'_']".
>> > "[$t,$e,$s,$t|'_']"
>> >> T = "\"test\" ++ '_'".
>> > "\"test\" ++ '_'"
>> >> [$t,$e,$s,$t|'_'] = "test" ++ '_'.
>> > [116,101,115,116|'_']
>> >> {ok,S2,_} = erl_scan:string(S).
>> > {ok,[{'[',1},
>> >     {char,1,116},
>> >     {',',1},
>> >     {char,1,101},
>> >     {',',1},
>> >     {char,1,115},
>> >     {',',1},
>> >     {char,1,116},
>> >     {'|',1},
>> >     {atom,1,'_'},
>> >     {']',1}],
>> >    1}
>> >> {ok,T2,_} = erl_scan:string(T).
>> > {ok,[{string,1,"test"},{'++',1},{atom,1,'_'}],1}
>> >> erl_parse:parse_term(S2++[{dot,1}]).
>> > {ok,[116,101,115,116|'_']}
>> >> erl_parse:parse_term(T2++[{dot,1}]).
>> > {error,{1,erl_parse,"bad term"}}
>> >
>>
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-- 
--Hynek (Pichi) Vychodil

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