🚀 go-pugleaf

In article <1991May2.155052.16813@hubcap.clemson.edu>, raghutp@hubcap.clemson.edu (Raghu Toppur) writes:
> In article <5525@goanna.cs.rmit.oz.au>, ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
> > You cannot parse Prolog using Yacc, because Prolog is not an LR(1)
> > language.

> Surely there is a "subset" of Prolog language for which an LR(1) grammar
> could be used?  My question is briefly as follows: What aspects of Prolog 
> syntax cause it to be non-LR(1)?

Operators.  If you restrict yourself to the operators of C Prolog,
and adopt the rule found in C Prolog 1.*.edai that
	- a token following <beginning of term> ( [ { , | cannot
	  have a left operand (so is not an infix or postfix operator)
	- a token preceding <end of term> ) ] } , | cannot have
	  a right operand (so is not an infix or postfix operator)
	- if an atom is preceded by something that _must_ be a left
	  operand of it, then it will be read as an operator even when
	  followed by (, so that 1+(2) is the same as +(1,2).
then you shouldn't have much trouble.

> Is the "op" predicate considered central to the definition of Prolog ?
> That would mean that the yacc-parser I've been using
> so far isn't fully complete (yet!) and never will be.

It all depends on what you mean by "Prolog".
Let's face it, without user-definable operators, Prolog syntax is so
_trivial_ that there just isn't any point in using Yacc.  I've got a
Prolog syntax checker built into the editor I'm using to write this
message, and the C code is shorter than any Yacc grammar I've seen for
Prolog, and was a heck of a lot easier to debug.  Good Prolog programmers
do not define many operators.  If you want to run existing code in
textbooks (such as Peter Ross's rather nice book), you had _better_ be
able to handle user-defined operators.  If you just want to explore the
really important ideas, you might as well use Lisp syntax (micro-PROLOG
did, and HP used to distribute a Prolog with a Lisp syntax option.

(Why don't people ask about Yacc grammars for Lisp?  CL syntax is at least
as complex as Prolog syntax...)
-- 
Bad things happen periodically, and they're going to happen to somebody.
Why not you?					-- John Allen Paulos.