Re: yet another pattern-matching library for Lua
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- Subject: Re: yet another pattern-matching library for Lua
- From: "Eric Raible" <raible@...>
- Date: 2007年1月02日 21:24:06 -0800
Bravo, Roberto.
Lpeg is an useful and undeniably elegant addition to lua.
That said, I've got a few comments:
1) This pattern causes an infinite loop. And while I understand
why it does, it might be useful to either detect infinite loops
or to disallow exponentiation on lookahead patterns:
m.match( "asdf", (-m.P("x"))^0)
2) Documentation suggestion #0
How about a table summarizing the operators as follows:
Operator Precedence Description
-------- ---------- -----------
e^n 4 repetition
n>=0: n or more
n<0: n at most
#e 3 Positive lookahead (never consumes characters)
-e 3 Negative lookahead (never consumes characters)
e1 * e2 2 Sequence
e1 + e2 1 Prioritized Choice
3) Documentation suggestion #1
function anywhere (p)
return (1 - lpeg.P(p))^0 * lpeg.I() * p * lpeg.I()
end
should perhaps instead be:
function anywhere (p)
local I = lpeg.I()
return (1 - p)^0 * I * p * I
end
4) Documentation suggestion #2
Consider this example:
assert( m.match("foo123", m.R("a","z")^0) == 4 )
assert( string.sub("foo123", 4 ) == "123" )
The return value is the index in the string _after_ the match.
So perhaps this sentence:
"If the match succeeds, returns the index in the subject where
the match finished."
should perhaps instead be:
"If the match succeeds, returns the index in the subject of the
first character after match."
5) Documentation suggestion #3
The following note in the documentation is misleading:
"(Maybe the # should be used for optional pattern; it seems to be
more useful, and it has no symbol for it right now. We must write
patt + "".)"
But optional patterns _can_ be elegantly specified (with p^-1).
If anything, #patt can be replaced with -(-patt), which currently
works just fine.
Alternatively, it might be worth adding lpeg.pos(patt) and lpeg.neg(patt)
6) Documentation suggestion #4
Typo in "where the math occurs"
7) The "green" library re.lua has a bug in the following:
re.match("parens don't work", "(.)")
This can be fixed by changing the definition of expression() from
local function expression (t) ... end
to
expression = function(t) ... end
since it is already declared local.
8) A suggestion for test.lua:
Negative numbers can be trivially handled in the evaluator:
local Number = lpeg.C(lpeg.S("-")^-1 * lpeg.R("0", "9")^1) * Space
9) An implementation question/suggestion
Since strings are hashed in lua, I don't understand why
the optional capture name must be copied (670). Seems to me
that it would be more useful to allow any lua type
(stored simply as a pointer).
That's it for now. Thanks for a really interesting library
(and a great language as well!).
- Eric