[erlang-questions] BNF/EBNF Grammar for Erlang

Ryan Molden <>
Fri Nov 11 21:56:58 CET 2011


Great, thanks I will give the YRL a shot (ignoring 'everything to the right
of the ':' may really be all that is needed, it wasn't clear to me if that
was somehow essentially in expressing what the YRL was trying to represent).

Ryan

On Fri, Nov 11, 2011 at 12:38 PM, Joe Armstrong <> wrote:

> I don't think there are any BNF/EBNF grammars for erlang
> there might be grammars for subsets of the language but not the
> entire language.  The problem with NBF/EBNF/PEG grammars
> is that decent error reporting is very difficult. In practice
> virtual all languages use hand written recursive descent parsers
> or LALR(1) parsers for which acceptable error recovery strategies exist.
>
> The official grammar is: (official means this is the actual grammar used
> by the erlang compiler and all tools)
>
> https://github.com/erlang/otp/blob/master/lib/stdlib/src/erl_parse.yrl
>
> It you look at the productions they are very similar to yacc
> productions:
>
> For example a tuple is defined like this (line 330,331)
>
> tuple -> '{' '}'       : {tuple,?line('$1'),[]}.
> tuple -> '{' exprs '}' : {tuple,?line('$1'),'$2'}.
>
> If you forget about the stuff after the ':' this
> reads
>
>    tuple -> '{' '}'
>    tuple -> {' exprs '}'
>
> ie a tuple is either {} or { exprs }
>
> The part after the ':' define the parse tree that
> is returned if the expression is recognised.
>
> A production like:
>
> a -> b c d : {something, '$2'}
>
> means is we match an 'a' then the parse
> tree we want returned is {something, '$2'}
>
>
> '$1' is the parse tree of b, '$2' is the parse tree
> of c etc.
>
> and exprs (line 445/446) is defined
>
> exprs -> expr           : ['$1'].
> exprs -> expr ',' exprs : ['$1' | '$3'].
>
>
> ie exprs is an expr or a comma separated
> sequence of expr's
>
> In the original yacc this would be written
> something like
>
> exprs: expr {$$ = $1}
> | expr ',' exprs {$$ = [$1|$3]}
>
> The yecc manual is at http://www.erlang.org/doc/man/yecc.html
>
> the command
>
> > erlc erlang.yecc
>
> compiles the grammar into a beam file
>
> Cheers
>
> /Joe
>
>
>
>
> On Fri, Nov 11, 2011 at 5:08 PM, Ryan Molden <> wrote:
>
>> Howdy fellow Erlangeers.  I was wondering if anyone knew of a BNF/EBNF
>> grammar for Erlang?  Joe Armstrong pointed me at the YRL grammar at github;
>> unfortunately, I am YRL illiterate so it looked mostly like gibberish to me.
>>
>> Ryan
>>
>> _______________________________________________
>> erlang-questions mailing list
>> 
>> http://erlang.org/mailman/listinfo/erlang-questions
>>
>>
>
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