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

Hynek Vychodil <>
Wed Aug 25 17:10:51 CEST 2010


I still don't understand.

1> T = ets:new(foo,[private]).
20496
2> ets:insert(T, [{"foo", 1}, {"foobar", 2}, {"bar", 3}, {"foobaz", 4}]).
true
3> Prefix = "foo".
"foo"
4> catch ets:match(T,{Prefix++'$1', '$2'}).
[["bar",2],[[],1],["baz",4]]
5> catch ets:select(T,[{{Prefix++'_', '_'}, [], ['$_']}]).
[{"foobar",2},{"foo",1},{"foobaz",4}]

On Wed, Aug 25, 2010 at 4:47 PM,  <> wrote:
> Try that using erl_scan and erl_parse. That's what doesn't work and
> that's what I'm trying to solve. I am reading in strings of erlang
> terms for running selects on mnesia. "prefix" ++ '_' is easier for my
> customer to understand and deal with than [$p,$r,$e,$f,$i,$x|'_']. I
> can make a preprocessor which does the substitution but I wanted to
> know if I was doing something wrong.
>
> On Wed, 25 Aug 2010 15:38:21 +0200
> Hynek Vychodil <> wrote:
>
>> 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
>> <> 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: 
>> > [mailto:] On Behalf Of Robert Raschke
>> > Sent: Wednesday, August 25, 2010 8:52 AM To:
>> >  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 <
>> > > 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:]
>> >> Sent: Tuesday, August 24, 2010 5:01 PM
>> >> To: Musumeci, Antonio S (Enterprise Infrastructure)
>> >> Cc: 
>> >> 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 <
>> >> > 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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