On Thu, Nov 26, 2015 at 7:25 PM, Cev Ing <cev...@gmail.com> wrote:
> I am wondering if it is possible to define a LHS action. I mean something
> like this:
>
> <create table> ::=
> (CREATE TABLE) identifier ('(') <table element list> (')' ';')
> action => ::lhs
>
> This should mean call the action with the name of the rule. Later on it
> would be possible to define a default action for all rules:
>
> :default ::= action => ::lhs
>
> I also thought to define it myself, but I can not find a place where I can
> define what gets passed to an action. I seems to me that it is always the
> value. If I would be able to pass also the LHS to the action, I could do
> the dispatching myself.
>
> Is it possible anyhow or do I have to write the action name in every rule:
>
> <create table> ::=
> (CREATE TABLE) identifier ('(') <table element list> (')' ';')
> action => create_table
>
> The reason why I want to use the 'action' attribute instead of 'bless' is,
> that I want to build in one pass ready to use Perl objects. If I use the
> 'bless' approach, I have two passes. The first builds a bunch of blessed
> arrays, which define the AST and after that I have to traverse the AST to
> build my Perl objects. When I use the actions to define the Perl objects, I
> can avoid the AST creation and the second pass.
>
You can (1) bless each RHS alternative to specific packages, all of which
must define the same action sub, e.g. doit() as in
https://metacpan.org/pod/distribution/Marpa-R2/pod/Scanless.pod#Synopsis or
(2) use action adverb to set the action subs explicitly and set a single
semantics package for them as in
https://github.com/jeffreykegler/Marpa--R2/blob/master/cpan/t/sl_gsyn.t
In either case, actions subs will be called at the evaluation phase and
their return values will be used to build the parse value without explicit
AST traversal.
Ron Savage has recently written a good article on how to use action subs to
build parse value --
http://savage.net.au/Ron/html/Understanding.Marpa-style.action.subs.parameters.html
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