Re: What is correct way to describe this in BNF for an LL(1) parser

  Rici Lake <rici@ricilake.net> wrote:
>mult-expr ::= pow-expr '/' mult-expr
> | pow-expr '*' mult-expr
> | pow-expr mult-expr ;; implicit multiplication
>
>term ::= '|' expr '|'
> | '(' expr ')'
> | VAR
> | NUMBER
>
>(I don't know what tool you are using or any details of your grammar;
>this is just a guess. Make appropriate substitutions, or ignore me if
>I'm guessing wrong.)
>
>If it were not for implicit multiplication, the use of |x| for abs(x)
>is unambiguous (providing you're not using | for other things as well...


It can be unambiguous even with implicit multiplication, at the cost of
a bit more grammar complexity. Consider (using RFC 2234 ABNF):


expr = term
/ expr term ; implicit multiplication


term = "|" aexpr "|"
/ aterm


aexpr = aterm
/ aexpr aterm


aterm = "(" expr ")"
/ VAR
/ NUMBER


This excludes nested absolute-value operators unless the inner ones are
enclosed in parentheses. With a full set of operators it would get
tedious, since it essentially duplicates the expression syntax with one
form excluded, but it *does* work.
--
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