streams.lua

function numbered_keys(step, context, position)
  local k = position
  local f = function(s, i)
    local v
    k,v = step(s, k)
    if k ~= nil then
      return (i+1), v
    end
  end
  return f, context, 0
end

function filter(predicate, step, context, position)
  local f = function (s, k)
    local v
    repeat k,v = step(s,k); until (k == nil) or predicate(k,v)
    return k,v
  end
  return f, context, position
end

function map(transformer, step, context, position)
  local f = function (s, k)
    local v
    k, v = step(s, k)
    if k ~= nil then
      return transformer(k,v)
    end
  end
  return f, context, position
end

function build_array(f, s, k)
  local v
  local t = {}
  while true do
    k, v = f(s, k)
    if k == nil then break end
    table.insert(t, v)
  end
  return t
end

function build_table(f, s, k)
  local v
  local t = {}
  while true do
    k, v = f(s, k)
    if k == nil then break end
    t[k] = v
  end
  return t
end

local t = {
  name = 'names',
  description = 'Names of People',
  'john',
  'paul',
  'george',
  'ringo',
  'bert',
}

print('pairs:')
for k,v in pairs(t) do print(k,v); end

print('\nnumbered:')
for k,v in numbered_keys(pairs(t)) do print(k,v); end

print('\nfiltered:')
for k,v in filter(function(k,v) return type(k) == 'number' and k < 3; end, pairs(t)) do print(k,v); end

print('\nmapped:')
for k,v in map(function(k,v) return k,v .. '!'; end, pairs(t)) do print(k,v); end

print('\nrebuilt:')
local t2 = build_array(filter(function(k,v) return k < 3; end, ipairs(t)))
for i = 1, #t2 do
  print(i, t2[i])
end