With Statement

A common pattern involving the creation of an object is calling a series of functions and setting a series of properties immediately after creating it.

This results in repeating the name of the object multiple times in code, adding unnecessary noise. A common solution to this is to pass a table in as an argument which contains a collection of keys and values to overwrite. The downside to this is that the constructor of this object must support this form.

The with block helps to alleviate this. Within a with block we can use a special statements that begin with either . or \ which represent those operations applied to the object we are using with on.

For example, we work with a newly created object:

MoonScript:

with Person!
  .name = "Oswald"
  \add_relative my_dad
  \save!
  print .name

Lua:

do
  local _with_0 = Person()
  _with_0.name = "Oswald"
  _with_0:add_relative(my_dad)
  _with_0:save()
  print(_with_0.name)
end

The with statement can also be used as an expression which returns the value it has been giving access to.

MoonScript:

file = with File "favorite_foods.txt"
  \set_encoding "utf8"

Lua:

local file
do
  local _with_0 = File("favorite_foods.txt")
  _with_0:set_encoding("utf8")
  file = _with_0
end

Or…

MoonScript:

create_person = (name,  relatives) ->
  with Person!
    .name = name
    \add_relative relative for relative in *relatives

me = create_person "Leaf", {dad, mother, sister}

Lua:

local create_person
create_person = function(name, relatives)
  do
    local _with_0 = Person()
    _with_0.name = name
    for _index_0 = 1, #relatives do
      local relative = relatives[_index_0]
      _with_0:add_relative(relative)
    end
    return _with_0
  end
end
local me = create_person("Leaf", {
  dad,
  mother,
  sister
})

In this usage, with can be seen as a special form of the K combinator.

The expression in the with statement can also be an assignment, if you want to give a name to the expression.

MoonScript:

with str = "Hello"
  print "original:", str
  print "upper:", \upper!

Lua:

do
  local str = "Hello"
  print("original:", str)
  print("upper:", str:upper())
end