Transform (cond ? then : else) to ifthenelse(cond,then,else) with lpeg

Question

I'm trying to convert a string like 'a?(b?c:d):e' to another string 'ifthenelse(a,ifthenelse(b,c,d),e)' using the lpeg lua parser. I'm slowly learning how to use lpeg but still I can't find a suitable solution to do this with captures. Any ideas?

Here is what I did so far.

local lpeg = require("lpeg")

local S, P, R = lpeg.S, lpeg.P, lpeg.R
local C, Cc, Ct = lpeg.C, lpeg.Cc, lpeg.Ct
local Cf, Cg, Cs = lpeg.Cf, lpeg.Cg, lpeg.Cs
local V = lpeg.V

local thenop = P("?")
local elseop = P(":")
local openpar = P("(")
local closepar = P(")")
local digit = R("09")
local letter = R("az") + R("AZ")

local parser = 
   P({
    "F",
    F = V("E") * (thenop * V("E") * elseop * V("E"))^0,
    E = (letter + digit)^1 + (openpar * V("F") * closepar)
 }) -- * -1 -- Is it needed?
print(lpeg.match(parser,"a?(b?c:d):e"))
print(lpeg.match(parser,"a"))

Answer 1

Here is another solution to the problem given by William Ahern on the lua-list.

local lpeg = require("lpeg")

lpeg.locale(lpeg)

local function tr(a, b, c)
   if not b then
       return a
   else
       return string.format("ifthenelse(%s,%s,%s)", a, b, c)
   end
end

local var = lpeg.C(lpeg.alpha * (lpeg.alnum^0))

local E, G = lpeg.V"E", lpeg.V"G"

local grammar = lpeg.P{ "E",
   E = ((var + G) * (lpeg.P"?" * E * lpeg.P":" * E)^-1) / tr,
   G = lpeg.P"(" * E * lpeg.P")",
}

print(lpeg.match(grammar, "a?(b?c:d):e"))

Answer 2

I changed a bit the grammar, but still the same I think:

S = E "?" E ":" E | E
E = "(" S ")" | T
T = (L | D) +
L = [a-z] | [A-Z]
D = [0-9]

The grammar without captures:

local grammar = lpeg.P{
  "S";
  S = (lpeg.V"E" * lpeg.P"?" * lpeg.V"E" * lpeg.P":" * lpeg.V"E") + lpeg.V"E",
  E = (lpeg.P"(" * lpeg.V"S" * lpeg.P")") + lpeg.V"T",
  T = (lpeg.V"L" + lpeg.V"D")^1,
  L = lpeg.R("az") + lpeg.R("AZ"),
  D = lpeg.R("09")
}

The grammar with captures:

local grammar2 = lpeg.P{
  "S";
  S = (lpeg.Cs(lpeg.V"E") / "ifthenelse(%1") * (lpeg.P"?" / ",") * lpeg.V"E" * (lpeg.P":" / ",") * (lpeg.Cs(lpeg.V"E") / "%1)") + lpeg.V"E",
  E = (lpeg.P"(" / "") * lpeg.V"S" * (lpeg.P")" / "") + lpeg.V"T",
  T = (lpeg.V"L" + lpeg.V"D")^1,
  L = lpeg.R("az") + lpeg.R("AZ"),
  D = lpeg.R("09")
}

Captures replace:

• "(" and ")" with empty string (lpeg.P"(" / "" and lpeg.P")" / ""))
• First variable match with "ifthenelse(variable" (lpeg.Cs(lpeg.V"E") / "ifthenelse(%1"))
• "?" and ":" with "," (lpeg.P"?" / "," and lpeg.P":" / ",")
• Last variable match with "variable)" (lpeg.Cs(lpeg.V"E") / "%1)")

Some random tests (output in comments):

print( lpeg.match( lpeg.Cs(grammar2), "a") )
-- a
print( lpeg.match( lpeg.Cs(grammar2), "a?b:c") )
-- ifthenelse(a,b,c)
print( lpeg.match( lpeg.Cs(grammar2), "a?(i?j:k):c") )
-- ifthenelse(a,ifthenelse(i,j,k),c)
print( lpeg.match( lpeg.Cs(grammar2), "(a?(i?j:(x?y:z)):b)?c:(u?v:w)") )
-- ifthenelse(ifthenelse(a,ifthenelse(i,j,ifthenelse(x,y,z)),b),c,ifthenelse(u,v,w))

I hope you can continue from here.