Can antlr do type-dependent parsing? - antlr

Question

Let me ask whether antlr3 accepts the following example grammar.
for an input , x + y * z ,
it is parsed as x+(y*z) if each in {x,y,z} is a number;
it is parsed as (x+y)*z if each in {x,y,z} is an object of a particular type T;
And let me ask whether such grammars are used sometimes or very rarely for computer languages.
Thank you very much.

Answer 1

In general, parsers (produced by parser generators) only check syntax.
A parser (produced by any means) that can explore multiple parses (I believe ANTLR does this by backtracking; other parsing engines [GLR, Earley] do it by parallel exploration of possible parses), if augmented with semantic checking information, could reject parses that didn't meet semantic constraints.
People tend not to build such parsers in my experience, partly because it is hard to explain to users. If they don't get it, your parser isn't successful; your example is especially bad IMHO in terms of explainability. They also tend not to do this because they need that type information, and that's not always convenient to collect as you parse. The GCC parsers famously do just this this to parse statements such as
X*T;
and the parser is a bit of a mess because of the need to parse and collect this type information as it goes.
I suspect ANTLR can check semantic predicates. How easy it is to get type information of the kind you discuss to those semantic checks is another question; I have no experience here.
The GLR parsing engine used by our DMS Software Reengineering Toolkit does have "semantic" predicates. It isn't particularly easy to get real semantic type information to those predicates by architectural design; we wanted such predicates to be driven off of "syntax". But then, everything (including type inference) is driven off syntax. So we stick information purely local to the reduction being proposed. This is particulary handy in (not) recognizing as separate types of parses, the following peculiar FORTRAN construct for nested-do-termination vs. shared-do-termination:
DO 10 I=1,10,1
DO 10 J=1,10,1
A(I,J)=0
10 CONTINUE
20 CONTINUE
vs.
DO 20 I=1,10,1
DO 10 J=1,10,1
A(I,J)=0
10 CONTINUE
20 CONTINUE
To the parser, at the pure syntax level, both of these look like:
DO <INT> <VAR>=...
DO <INT> <VAR>=...
<STMTS>
<INT> CONTINUE
<INT> CONTINUE
How can one determine which CONTINUE statement belongs to which DO consrtuct with only this information? You can't.
The DMS FORTRAN parser does exactly this by having two sets of rules for DO loops, one for unshared continues, an one for shared continues. They differentiate using semantic predicates that check that the CONTINUE statement label matches the DO loop designated label. And thus the DMS FORTRAN parser gets the loop nesting right as it parses. AFAIK, all the other FORTRAN compilers parse the statements individually, and then stitch the DO loop nests together in a post pass.
And yes, while FORTRAN has this (confusing) construct, no other modern language that I know copied it.