Where is the mistake in this parser? [closed]

Asked by

BlueTaslem 18071

9 years ago

I have begun on a PEG parser.

The goal is to build a grammar out of trees of combinators using sequence and either. I have some mistake in my sequence implementation, because I get false as the output from this instead of an empty stream.

01-- <library>
02 
03-- Forward declare helper
04local wrap
05 
06-- e: function | string
07-- return: Parser (Stream -> Stream?)
08function expect(e)
09    local x = e
10    if type(e) == "string" then
11        x = function(n) return n == e end
12    end
13    return wrap(function(stream)
14        if x( stream:first() ) then
15            return stream:rest(), stream:first()
16        else
17            return false
18        end
19    end)
20end
21 
22-- a, b: Parser
23-- return: Parse (Stream -> Stream?)
24function seq(a, b)
25    return wrap(function(stream)
26        local u = a(stream)
27        if u then
28            return b(stream)
29        else
30            return false
31        end
32    end)
33end
34 
35-- wrappedCall(w, ...) is the same as w(...) for a value that has been
36-- `wrap`pped
37function wrappedCall(w, ...)
38    return w["function"](...)
39end
40 
41-- Allows + and * to be used as aliases for `either` and `sequence`.
42-- function -> function+*
43function wrap(f)
44    local t = {["function"] = f}
45    setmetatable(t, {
46 
47        __mul = seq,
48        __call = wrappedCall})
49    return t
50end
51 
52-- Stream<element> -> element
53function streamFirst(list)
54    return list[1]
55end
56 
57-- Stream -> Stream
58function streamRest(list)
59    local t = {unpack(list)}
60    table.remove(t, 1)
61    return Stream(t)
62end
63 
64-- string -> Stream
65function Stream(list)
66    local s = {unpack(list)}
67    s.first = streamFirst
68    s.rest = streamRest
69    return s
70end
71 
72-- </library>
 View all 72 lines...

01-- <client>
02 
03-- string -> boolean
04function isLetter(s)
05    return s:sub(1, 1):lower() ~= s:sub(1, 1):upper()
06end
07 
08local name = expect(isLetter)
09local open, close = expect("<"), expect(">")
10 
11local void = open * name * close 
12local tokens = Stream {"<", "img", ">"}
13 
14print( void(tokens) )
15-- I expect this to print `table: ABCDEF`, but I get `false`
16-- </client>

Does anyone else have an idea of where I made a mistake? I regret not testing more incrementally...

that needs a bluetaslem answer! oh, wait a minute... unmiss 337 — 9y

Try & remake it but more incrementally. Redbullusa 1580 — 9y

According to the Studio, you have not defined 'either' on line 46 (This may not be the main problem, but I thought to point it out anyway). :) TheeDeathCaster 2368 — 9y

b(stream) = false and as said before, "either" was never defined for __add = either Validark 1580 — 9y

I took out the definition of either because it wasn't used meaningfully in this script, forgot it was in the metatable -- that's not the problem. Edit the code to fix that BlueTaslem 18071 — 9y

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Where is the mistake in this parser? [closed]

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