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Lua: Lazy Map Implementation: Round One

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  1. require "std.table" -- For memoize
  2.  
  3. function map(t, f)
  4.     local nt = table.memoize(function (k) return f(t[k]) end)
  5.     local mt = getmetatable(nt)
  6.     function mt:__pairs()
  7.         local k
  8.         local function mynext()
  9.             k,v = next(t, k)
  10.             if k == nil then return nil end
  11.             return k,nt[k]
  12.         end
  13.         return mynext, self
  14.     end
  15.     return nt
  16. end
  17. require "std.base" -- For better table tostring()
  18.  
  19. local mylist = {1,3,5,7,9}
  20. local newlist = map(mylist, function(v) return 3 * v - 2 end)
  21. assert(mylist[3] == 5)
  22. assert(newlist[3] == 13)
  23.  
  24. require "std.base" -- For the pairs() that obeys the __pairs metamethod
  25. print("old:", mylist)
  26. print("new(3*v-2):", newlist)

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