Re: LR(1) Parsing : Error Handling & Recovery

Hello,


On 7/19/14, 4:29 PM, Hans Aberg wrote:
> A GLR parser splits the parses in face of an ambiguity, which might be
> used instead. Bison supports this, but currently the actions during a
> parse split are not executed, and so cannot set context variables for
> the lexer, until the merge.


A GLR parser splits on conflicts and thus needs two different types of
actions. I recall that DParser uses the terms speculative and final.
In my GLR Builder I use the terms semantic checks & actions.


Semantic checks are not delayed when the parser splits but they should
not have side effects. In general, it is unsafe to notify the scanner
that the parser has just reached a particular state.


In contrast, you can query a symbol table to reject an active parser or
just create a node for the AST of the current parse. BTW, its my
impression that Elkhound and Elsa parse C++ with actions that actually
are immediate; but i have not read their code to be sure.


To conclude, in a deterministic LALR parser semantic actions are both
immediate and final but in a GLR parser this principle doesn't hold.


Hope this helps,
Ev. Drikos

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