map/reduce function for lua

functional.lua

--
-- convenience functional utilities
--

 -- map(table, function)
 -- e.g: map({1,2,3}, function(a) return a*2 end) -> {2,4,6}
 function map(tbl, func)
     local newtbl = {}
     for i,v in pairs(tbl) do
         newtbl[i] = func(v)
     end
     return newtbl
 end

 -- filter(table, function)
 -- e.g: filter({1,2,3,4}, function(a) return a%2==0 end) -> {2,4}
 function filter(tbl, func)
     local newtbl = {}
     for i,v in pairs(tbl) do
         if func(v) then
        newtbl[i] = v
         end
     end
     return newtbl
 end

 -- head(table)
 -- e.g: head({1,2,3}) -> 1
 local function head(tbl)
     return tbl[1]
 end

 -- tail(table)
 -- e.g: tail({1,2,3}) -> {2,3}
 local function tail(tbl)
  local size = #tbl
     if size > 0 then
      return {unpack(tbl, 2)}
     end
 end

 -- foldr(table, default_value, function)
 -- e.g: foldr({1,2,3,4,5}, 1, function(a, b) return a*b end) -> 120
 local function foldr(tbl, val, func)
     for i, v in pairs(tbl) do
         val = func(val, v)
     end
     return val
 end

 -- reduce(table, function)
 -- e.g: reduce({1,2,3,4}, function(a, b) return a+b end) -> 10
 function reduce(tbl, func)
     return foldr(tail(tbl), head(tbl), func)
 end