# DefUnit

DefUnit provides a pair of macros that allow you to quickly create a module that
defines a set of core units for its users to work with. The macros
create documentation attributes, conversion operators and type specs that allow
Dialyzer to detect most incompatible assignments at compile time.

DefUnit adds almost no run-time overhead to your code. The values you work with
are everyday floats.

#### Example

Create the file _units.ex_:

defmodule Unit do

  use DefUnit
  @doc_from_operator """
  Convert from other units to core units used for calculations.

  @doc_to_operator """
  Convert from core units used for calculations to other units.
  # Units we do our calculations in
  DefUnit.core  "m",        :m,     "SI length"
  DefUnit.core  "kg",       :kg,    "SI mass"
  DefUnit.core  "s",        :s,     "Time"
  DefUnit.core  "m^2",      :m2,    "SI area"
  DefUnit.core  "ms^{-1}",  :ms,    "SI velocity"
  DefUnit.core  "ms^{-2}",  :ms2,   "SI acceleration"
  DefUnit.core  "kgm^{-3}", :kgm3,  "SI density"
  # Units we convert to and from core units
  DefUnit.other "feet",     :feet,    0.3048,   :m,   "FPS length and altitude"
  DefUnit.other "kmh^{-1}", :kmh,     0.27777,  :ms,  "Kilometres per hour"
  DefUnit.other "mph",      :mph,     0.44704,  :ms,  "Imperial velocity"
  DefUnit.other "knots",    :knots,   0.514444, :ms,  "Nautical miles per hour"


The idea of core/other is that the code using this module is easier to
write and reason about if calculations are carried out in a consistent set of 'core' units.
Your core units can be whatever suit your purpose - foot/pound/seconds, currencies, or perhaps 
fully-laden-jumbo-jet/oil-rig/emperor-penguins if you're a Discovery Channel researcher.

DefUnit will create sensible `@doc` and `@typedoc` attributes. If you use 
[Pandoc]( with ex_doc on your module by installing Pandoc and adding:
  config :ex_doc, :markdown_processor, ExDoc.Markdown.Pandoc

to your project's config.exs, unit symbols like `kgm^{-3}` will support 
[LaTeX formatting]( when your documentation
is rendered.

Now in iex you can try the conversion operators `<~` and `~>`:

  iex>import Unit
  iex> 65 <~ :knots
  iex> 60 <~ :mph
  iex> 100 <~ :kmh
  iex> 100 <~ :ms

The first three values are converted to their equivalent 'core' representation, in this
example metres per second. The last value is already in metre seconds. Conversely:

  iex> 33.43886 ~> :knots
  iex> 10 ~> :mph
  iex> 10 ~> :kmh
  iex> 10 ~> :ms

takes values which are assumed to be in 'core' representation and converts them to their
corresponding 'other' unit. The operators can also be chained to convert between 'other'
types in a readable way:

  iex> 100 <~ :kmh ~> :mph

You can read the above as 'convert 100 from kmh to mph'. But what is this?

  iex> 100 <~ :kg ~> :mph

Apparently 100 kilograms is 224 miles per hour right? It is worth emphasising at this point:

*DefUnit does not provide run time type checking*

If you really want runtime type checking you should look at 
[Unit Fun]( However if you're interested in
doing static analysis at compile time using [Dialyzer](
and the type specs created by the DefUnit macros, read on.

#### Using Types

Assume we're writing a library of aerodynamics functions. Aerospace uses a mix of units
from FPS, navigation and SI unit systems, and you really don't want to mess up your units
(see [Mars Climate Orbiter](

defmodule Aero do

  @doc "pressure in standard atmosphere at `alt` feet"
  @spec p(Unit.feet) :: Unit.kgm3
  def p(alt) do
    8.0e-19 * :math.pow(alt, 4)  \
    - 4.0e-14 * :math.pow(alt, 3) \
    + 1.0e-09 * :math.pow(alt, 2) \
    - 4.0e-05 * alt \
    + 1.225
  @doc "acceleration due to gravity"
  @spec g() :: Unit.ms2
  def g() do
  @doc "stall speed given aircraft weight, wing area, max lift and altitude"
  @spec vs(, Unit.m2, float, Unit.feet) ::
  def vs(m, s, cl_max, alt \\ 0.0) do
    :math.sqrt((2.0 * m * g()) / (p(alt) * s * cl_max))

The units defined in the Unit module are available to use in specs for our
aerodynamics functions (you could also use the macros directly in the Aero module). If you were 
calling your library and wanted to be clear about the units you were using you can write:

piper_archer_stall_speed_kts = vs(1157 <~ :kg, 15.8 <~ :m2, 2.1, 0 <~ :feet) ~> :knots

If you mistakenly write:

piper_archer_stall_speed_kts = vs(1157 <~ :knots, 15.8 <~ :m2, 2.1, 0 <~ :feet) ~> :knots

or perhaps:

piper_archer_stall_speed_kts = vs(1157 <~ :m2, 15.8 <~ :m2, 2.1, 0 <~ :feet) ~> :kg

you will get a dud result. However if you run Dialyzer (the
[dialyxir]( mix plugin is easy to set up and
use) you'll get warnings similar to this:

$ mix dialyzer
Starting Dialyzer
...stuff ommitted
aero.ex:118: The call 'Elixir.Unit':'~>'(float(),'kg') will never return since
the success typing is (number(),'feet' | 'kmh' | 'knots') -> float() and the
contract is 
    ; (ms(),'knots') -> knots()
    ; (ms(),'mph') -> mph()
    ; (ms(),'kmh') -> kmh()
    ; (m(),'feet') -> feet()
 done in 0m1.36s
done (warnings were emitted)

Dialyzer can trace much more complex stuff than these examples - [LYSE](
has a good explanation of Dialyzer, its history, intent, capabilities and what the various warnings mean.

#### More About the Macros

If your 'other' unit refers to an undefined 'core' unit you will get a compile error:

== Compilation error on file test/support/def_unit_example.ex ==
** (ArgumentError) Unit 'kmh' refers to unknown core unit 'speeding_bullet'

In case you need direct access to the conversion factors you specified in `DefUnit.other`, the macro creates
a pair of module attributes:

@feet_to_m 0.3048
@m_to_feet 3.280839895

If your conversion between units is more complex (e.g. Farhrenheit to Celcius or live currency
exchange rates) you can replace the conversion ratio in the other macro with a 2-tuple of 
from/to conversion functions:

  DefUnit.core  "C", :c, "Temperature in Celcius"
  DefUnit.other "F", :f,
    &((&1 - 32.0) * (5.0 / 9.0)),
    &((&1 * (9.0 / 5.0)) + 32.0)
  :c, "Temperature in Fahrenheit"