Re: BNF to LPEG

[Dirk Laurie <dirk.laurie@...>]

2016-04-07 15:22 GMT+02:00 Roberto Ierusalimschy <roberto@inf.puc-rio.br>:
>> Is there a module that can convert BNF to LPEG?
>> The test case being the Lua grammar in §9 of in the manual.
>
> Not that I know. Such a conversion is tricky for at least two reasons:
>
> - Usually, BNF assumes a separate scanner, while LPeg works directly
> on the character level.
>
> - Although BNF and LPeg may look similar, they have different meanings.
> (If the BNF is LL(n), then a corresponding LPeg would have equal
> meaning, but the scanner problem is still there. In particular, the
> Lua grammar in the manual is not LL(n).)

I can understand that to get from BNF (or EBNF) to LPeg with equivalent
functionality is highly nontrivial.

When doing it manually, though, a very large part of the work is tedious
but straightforward: precisely the sort of thing that computers do perfectly
but humans don't.

For example, just mechanically substituting notation.

{something} becomes (something)^0
[something] becomes (something)^-1
| becomes +
juxtaposition becomes multiplication

Etc.

After converting about twenty lines of a particular EBNF (not Lua's)
in this way, and making the occasional mistake, I thought "This job
will take less total time if I write a litlle Lua program". After writing
about twenty lines of that program, I thought "someone must have
done this before".

Oh well. :-(