Comprehensions

Comprehensions provide a convenient syntax for constructing a new table by iterating over some existing object and applying an expression to its values. There are two kinds of comprehensions: list comprehensions and table comprehensions. They both produce Lua tables; list comprehensions accumulate values into an array-like table, and table comprehensions let you set both the key and the value on each iteration. List Comprehensions

The following creates a copy of the items table but with all the values doubled.

MoonScript:

items = { 1, 2, 3, 4 }
doubled = [item * 2 for i, item in ipairs items]

Lua:

local items = {
  1,
  2,
  3,
  4
}
local doubled
do
  local _accum_0 = { }
  local _len_0 = 1
  for i, item in ipairs(items) do
    _accum_0[_len_0] = item * 2
    _len_0 = _len_0 + 1
  end
  doubled = _accum_0
end

The items included in the new table can be restricted with a when clause:

MoonScript:

iter = ipairs items
slice = [item for i, item in iter when i > 1 and i < 3]

Lua:

local iter = ipairs(items)
local slice
do
  local _accum_0 = { }
  local _len_0 = 1
  for i, item in iter do
    if i > 1 and i < 3 then
      _accum_0[_len_0] = item
      _len_0 = _len_0 + 1
    end
  end
  slice = _accum_0
end

Because it is common to iterate over the values of a numerically indexed table, an * operator is introduced. The doubled example can be rewritten as:

doubled = [item * 2 for item in *items]

Lua:

local doubled
do
  local _accum_0 = { }
  local _len_0 = 1
  local _list_0 = items
  for _index_0 = 1, #_list_0 do
    local item = _list_0[_index_0]
    _accum_0[_len_0] = item * 2
    _len_0 = _len_0 + 1
  end
  doubled = _accum_0
end

The for and when clauses can be chained as much as desired. The only requirement is that a comprehension has at least one for clause.

Using multiple for clauses is the same as using nested loops:

MoonScript:

x_coords = {4, 5, 6, 7}
y_coords = {9, 2, 3}

points = [{x,y} for x in *x_coords for y in *y_coords]

Lua:

local x_coords = {
  4,
  5,
  6,
  7
}
local y_coords = {
  9,
  2,
  3
}
local points
do
  local _accum_0 = { }
  local _len_0 = 1
  for _index_0 = 1, #x_coords do
    local x = x_coords[_index_0]
    for _index_1 = 1, #y_coords do
      local y = y_coords[_index_1]
      _accum_0[_len_0] = {
        x,
        y
      }
      _len_0 = _len_0 + 1
    end
  end
  points = _accum_0
end

Numeric for loops can also be used in comprehensions:

MoonScript:

evens = [i for i=1,100 when i % 2 == 0]

Lua:

local evens
do
  local _accum_0 = { }
  local _len_0 = 1
  for i = 1, 100 do
    if i % 2 == 0 then
      _accum_0[_len_0] = i
      _len_0 = _len_0 + 1
    end
  end
  evens = _accum_0
end