Parsing a TeX-like language with lpeg - lua

Question

I am struggling to get my head around LPEG. I have managed to produce one grammar which does what I want, but I have been beating my head against this one and not getting far. The idea is to parse a document which is a simplified form of TeX. I want to split a document into:
Environments, which are \begin{cmd} and \end{cmd} pairs.
Commands which can either take an argument like so: \foo{bar} or can be bare: \foo.
Both environments and commands can have parameters like so: \command[color=green,background=blue]{content}.
Other stuff.
I also would like to keep track of line number information for error handling purposes. Here's what I have so far:
lpeg = require("lpeg")
lpeg.locale(lpeg)
-- Assume a lot of "X = lpeg.X" here.
-- Line number handling from http://lua-users.org/lists/lua-l/2011-05/msg00607.html
-- with additional print statements to check they are working.
local newline = P"\r"^-1 * "\n" / function (a) print("New"); end
local incrementline = Cg( Cb"linenum" )/ function ( a ) print("NL"); return a + 1 end , "linenum"
local setup = Cg ( Cc ( 1) , "linenum" )
nl = newline * incrementline
space = nl + lpeg.space
-- Taken from "Name-value lists" in http://www.inf.puc-rio.br/~roberto/lpeg/
local identifier = (R("AZ") + R("az") + P("_") + R("09"))^1
local sep = lpeg.S(",;") * space^0
local value = (1-lpeg.S(",;]"))^1
local pair = lpeg.Cg(C(identifier) * space ^0 * "=" * space ^0 * C(value)) * sep^-1
local list = lpeg.Cf(lpeg.Ct("") * pair^0, rawset)
local parameters = (P("[") * list * P("]")) ^-1
-- And the rest is mine
anything = C( (space^1 + (1-lpeg.S("\\{}")) )^1) * Cb("linenum") / function (a,b) return { text = a, line = b } end
begin_environment = P("\\begin") * Ct(parameters) * P("{") * Cg(identifier, "environment") * Cb("environment") * P("}") / function (a,b) return { params = a[1], environment = b } end
end_environment = P("\\end{") * Cg(identifier) * P("}")
texlike = lpeg.P{
"document";
document = setup * V("stuff") * -1,
stuff = Cg(V"environment" + anything + V"bracketed_stuff" + V"command_with" + V"command_without")^0,
bracketed_stuff = P"{" * V"stuff" * P"}" / function (a) return a end,
command_with =((P("\\") * Cg(identifier) * Ct(parameters) * Ct(V"bracketed_stuff"))-P("\\end{")) / function (i,p,n) return { command = i, parameters = p, nodes = n } end,
command_without = (( P("\\") * Cg(identifier) * Ct(parameters) )-P("\\end{")) / function (i,p) return { command = i, parameters = p } end,
environment = Cg(begin_environment * Ct(V("stuff")) * end_environment) / function (b,stuff, e) return { b = b, stuff = stuff, e = e} end
}
It almost works!
> texlike:match("\\foo[one=two]thing\\bar")
{
command = "foo",
parameters = {
{
one = "two",
},
},
}
{
line = 1,
text = "thing",
}
{
command = "bar",
parameters = {
},
}
But! First, I can't get the line number handling part to work at all. The function within incrementline is never fired.
I also can't quite work out how nested capture information is passed to handling functions (which is why I have scattered Cg, C and Ct semirandomly over the grammar). This means that only one item is returned from within a command_with:
> texlike:match("\\foo{text \\command moretext}")
{
command = "foo",
nodes = {
{
line = 1,
text = "text ",
},
},
parameters = {
},
}
I would also love to be able to check that the environment start and ends match up but when I tried to do so, my back references from "begin" were not in scope by the time I got to "end". I don't know where to go from here.

Answer 1

Late answer but hopefully it'll offer some insight if you're still looking for a solution or wondering what the problem was.
There are a couple of issues with your grammar, some of which can be tricky to spot.
Your line increment here looks incorrect:
local incrementline = Cg( Cb"linenum" ) /
function ( a ) print("NL"); return a + 1 end,
"linenum"
It looks like you meant to create a named capture group and not an anonymous group. The backcapture linenum is essentially being used like a variable. The problem is because this is inside an anonymous capture, linenum will not update properly -- function(a) will always receive 1 when called. You need to move the closing ) to the end so "linenum" is included:
local incrementline = Cg( Cb"linenum" /
function ( a ) print("NL"); return a + 1 end,
"linenum")
Relevant LPeg documentation for Cg capture.
The second problem is with your anything non-terminal rule:
anything = C( (space^1 + (1-lpeg.S("\\{}")) )^1) * Cb("linenum") ...
There are several things to be careful here. First, a named Cg capture (from incrementline rule once it's fixed) doesn't produce anything unless it's in a table or you backref it. The second major thing is that it has an adhoc scope like a variable. More precisely, its scope ends once you close it in an outer capture -- like what you're doing here:
C( (space^1 + (...) )^1)
Which means by the time you reference its backcapture with * Cb("linenum"), that's already too late -- the linenum you really want already closed its scope.
I always found LPeg's re syntax a bit easier to grok so I've rewritten the grammar with that instead:
local grammar_cb =
{
fold = pairfold,
resetlinenum = resetlinenum,
incrementlinenum = incrementlinenum, getlinenum = getlinenum,
error = error
}
local texlike_grammar = re.compile(
[[
document <- '' -> resetlinenum {| docpiece* |} !.
docpiece <- {| envcmd |} / {| cmd |} / multiline
beginslash <- cmdslash 'begin'
endslash <- cmdslash 'end'
envcmd <- beginslash paramblock? {:beginenv: envblock :} (!endslash docpiece)*
endslash openbrace {:endenv: =beginenv :} closebrace / &beginslash {} -> error .
envblock <- openbrace key closebrace
cmd <- cmdslash {:command: identifier :} (paramblock? cmdblock)?
cmdblock <- openbrace {:nodes: {| docpiece* |} :} closebrace
paramblock <- opensq ( {:parameters: {| parampairs |} -> fold :} / whitesp) closesq
parampairs <- parampair (sep parampair)*
parampair <- key assign value
key <- whitesp { identifier }
value <- whitesp { [^],;%s]+ }
multiline <- (nl? text)+
text <- {| {:text: (!cmd !closebrace !%nl [_%w%p%s])+ :} {:line: '' -> getlinenum :} |}
identifier <- [_%w]+
cmdslash <- whitesp '\'
assign <- whitesp '='
sep <- whitesp ','
openbrace <- whitesp '{'
closebrace <- whitesp '}'
opensq <- whitesp '['
closesq <- whitesp ']'
nl <- {%nl+} -> incrementlinenum
whitesp <- (nl / %s)*
]], grammar_cb)
The callback functions are straight-forwardly defined as:
local function pairfold(...)
local t, kv = {}, ...
if #kv % 2 == 1 then return ... end
for i = #kv, 2, -2 do
t[ kv[i - 1] ] = kv[i]
end
return t
end
local incrementlinenum, getlinenum, resetlinenum do
local line = 1
function incrementlinenum(nl)
assert(not nl:match "%S")
line = line + #nl
end
function getlinenum() return line end
function resetlinenum() line = 1 end
end
Testing the grammar with a non-trivial tex-like str with multiple lines:
local test1 = [[\foo{text \bar[color = red, background = black]{
moretext \baz{
even
more text} }
this time skipping multiple
lines even, such wow!}]]
Produces the follow AST in lua-table format:
{
command = "foo",
nodes = {
{
text = "text",
line = 1
},
{
parameters = {
color = "red",
background = "black"
},
command = "bar",
nodes = {
{
text = " moretext",
line = 2
},
{
command = "baz",
nodes = {
{
text = "even ",
line = 3
},
{
text = "more text",
line = 4
}
}
}
}
},
{
text = "this time skipping multiple",
line = 7
},
{
text = "lines even, such wow!",
line = 9
}
}
}
And a second test for begin/end environments:
local test2 = [[\begin[p1
=apple,
p2=blue]{scope} scope foobar
\end{scope} global foobar]]
Which seems to give approximately what you're looking for:
{
{
{
text = " scope foobar",
line = 3
},
parameters = {
p1 = "apple",
p2 = "blue"
},
beginenv = "scope",
endenv = "scope"
},
{
text = " global foobar",
line = 4
}
}