> > Patterns and
> > expressions can look very much alike. Could one possibly expand "exp" to
> > "if exp" in Haskell 1.3 list comprehensions? Only to make deterministic
> > parsing easier...
>
> One should not make the parsing method too much influence the language
> design, PASCAL is a bad example for this.
True.
> I once had the same problem when writing a Miranda-ish compiler.
> The simple solution is to use "exp" instead of "pat" in the parsing
> grammar (in qualifiers), and when it later (e.g. encountering <- )
> becomes clear that the beast has got to be a pattern,
> you check it semantically. This works, because patterns
> are syntactically a subclass of expressions.
False. Patterns (in Haskell) also have "_", "~", and "@" which are
not allowed in expressions. Using exp instead of pat (and adding
"_", "~", and "@" to expressions) is a hack, not a proper solution.
I don't like hacks. So, I either have to massage the grammar into
deterministic LR parsable form (difficult) or use a nondeterministic
LR parser (not readily available).
> This extra effort in parser hacking is a small, one-time effort,
> compared to the really hard stuff in the compiler!
True. I'm not going to insist on a change.
/kent k
