[Oberon] Oberon-07 formal context free

Michael Schierl schierlm at gmx.de
Tue Jun 16 19:41:38 CEST 2020

Hello Tomas,

Am 16.06.2020 um 09:36 schrieb Tomas Kral:

> Read a bit on `Compiler construction', formalism and context free
> allows for simple compiler coding from EBNF.
> Oberon-07 has a few exceptions to free context, if I got the idea
> right?

I believe that Oberon-07 (at least the EBNF in the book) can be parsed
with a context-free parser, and I think the actual ORP implementation is
even LL(k) [or even LL(1)?]

In general, Oberon-07 is not LL(1), since I can think of at least one
case where the parse tree cannot be constructed without a larger lookahead:

While parsing a type guard


when parser is at the "B" (which can even be M.B when qualified by a
module), the parser can not yet tell whether "a" is a procedure variable
and "B" is its argument or "a" is a record variable and "B" is a type.
With a finite lookahead the compiler can see the "." and therefore
disambiguate the situation. ORP does not have the problem as it will
check the type of "a" during parsing, and if it is a record type or
pointer type it can be parsed as a type guard:


>From my head, I cannot think of a case why Oberon is not LL(k), but
probably somebody else has a good example.



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