Table Comprehensions

The syntax for table comprehensions is very similar, only differing by using { and } and taking two values from each iteration.

This example makes a copy of the tablething:

thing = {
  color: "red"
  name: "fast"
  width: 123
}

thing_copy = {k,v for k,v in pairs thing}

Lua:

local thing = {
  color = "red",
  name = "fast",
  width = 123
}
local thing_copy
do
  local _tbl_0 = { }
  for k, v in pairs(thing) do
    _tbl_0[k] = v
  end
  thing_copy = _tbl_0
end

Table comprehensions, like list comprehensions, also support multiple for and when clauses. In this example we use a when clause to prevent the value associated with the color key from being copied.

MoonScript:

no_color = {k,v for k,v in pairs thing when k != "color"}

Lua:

local no_color
do
  local _tbl_0 = { }
  for k, v in pairs(thing) do
    if k ~= "color" then
      _tbl_0[k] = v
    end
  end
  no_color = _tbl_0
end

The * operator is also supported. Here we create a square root look up table for a few numbers.

numbers = {1,2,3,4}
sqrts = {i, math.sqrt i for i in *numbers}

Lua:

local numbers = {
  1,
  2,
  3,
  4
}
local sqrts
do
  local _tbl_0 = { }
  for _index_0 = 1, #numbers do
    local i = numbers[_index_0]
    _tbl_0[i] = math.sqrt(i)
  end
  sqrts = _tbl_0
end

The key-value tuple in a table comprehension can also come from a single expression, in which case the expression should return two values. The first is used as the key and the second is used as the value:

In this example we convert an array of pairs to a table where the first item in the pair is the key and the second is the value.

tuples = {{"hello", "world"}, {"foo", "bar"}}
tbl = {unpack tuple for tuple in *tuples}

Lua:

local tuples = {
  {
    "hello",
    "world"
  },
  {
    "foo",
    "bar"
  }
}
local tbl
do
  local _tbl_0 = { }
  for _index_0 = 1, #tuples do
    local tuple = tuples[_index_0]
    local _key_0, _val_0 = unpack(tuple)
    _tbl_0[_key_0] = _val_0
  end
  tbl = _tbl_0
end