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Lua metaprogramming using custom searcher with string interpolation example
--[[
The following example illustrates string interpolation [1]
by way of a preprocessor that runs as a customer searcher
function in package.loaders.
Tested in Lua 5.2.0-beta.
Depends on
file_slurp https://gist.github.com/1325400 (for loading files simply)
luabalanced https://github.com/davidm/lua-balanced (for Lua parsing)
[1] http://lua-users.org/wiki/StringInterpolation
--DavidManura, 2011-11
--]]
-----------------------------------------------------
-- main.lua
-- main program
require 'meta'
require 'foo-m' . test()
-----------------------------------------------------
-- foo-m.lua
-- Example module that uses preprocessing.
--! code = require 'interpolate' (code)
local M = {}
local function printf(s, ...)
local vals = {...}
local i = 0
s = s:gsub('\0[^\0]*\0', function()
i = i + 1
return tostring(vals[i])
end)
print(s)
end
function M.test()
local x = 16
printf("value is $(math.sqrt(x)) ") --> This prints "value is 4"
end
return M
-------------------------------------------
-- meta.lua
-- Installs preprocessing loader.
local searcher0 = package.loaders[2]
local readfile = require 'file_slurp' . readfile
local LB = require 'luabalanced'
-- Process special comments starting with '--!'
local function extract(code)
-- Parse out special comments.
local metacode = ''
LB.gsub(code, function(u, s)
if u == 'c' then
local stats = s:match'^%-%-%!(.*)'
if stats then
metacode = metacode .. stats
end
end
end)
-- Evaluate special comments.
local E = setmetatable({code = code}, {__index = _G})
local f = assert(load(metacode, nil, nil, E))
f()
return E.code
end
-- Custom searcher function that preprocesses modules
-- with names postfixed with '-m'.
local function searcher(name)
if name:match'%-m$' then
local filename, err = package.searchpath(name, package.path)
if filename then
local code = readfile(filename)
code = extract(code)
local f = assert(load(code, filename))
return f
end
end
end
table.insert(package.loaders, 2, searcher)
-------------------------------------------------------------
--[[
interpolate.lua
Preprocessor that expands strings like
"a $(b) c $(d*(e+f))"
to
"a \0$(b)\0 c \0$(d*(e+f))\0", b, d*(e+f)
--]]
local LB = require 'luabalanced'
local function preprocess(code)
local ts = {}
LB.gsub(code, function(u, s)
if u == 's' then
local ar = {}
s = s:gsub('%$(%b())', function(e)
e = e:sub(2, -2)
ar[#ar+1] = e
return '\0' .. e .. '\0'
end)
local ars = table.concat(ar, ', ')
if ars ~= '' then s = s .. ', ' .. ars end
end
ts[#ts+1] = s
end)
local code = table.concat(ts)
return code
end
return preprocess
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