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π Dimension based safety in elixir

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# UnitFun

Attempt to add units to numbers in elixir to give some added type saftey when dealing with numeric quantities.

## Why?

One good example: pounds(dollars) should never be accidentally added to pence(cents) without conversion. Both are numeric. Wrapping the numeric data in a tuple with unit information seems like a good idea. This library gives a neat way of expressing this.

## Example - Basic

First define some units:

```defmodule Pounds do
use UnitFun.Unit
end

defmodule Pence do
use UnitFun.Unit
end```

Then do something with them:

```use UnitFun.MathsOperators
import UnitFun.UnitTypes

item_cost = 5 <~ Pounds # UnitFun.with_units(5, Pounds)
item_tax = 100 <~ Pence # UnitFun.with_units(100, Pence)

# The following will throw an error as the units mismatch:
item_cost + item_tax # UnitFun.add(item_cost, item_tax)```

## Example - Conversions

Conversions can be defined:

```defimpl UnitFun.Convertor, for: Pence do
def convert(_, Pounds, value), do: (value * 100)
end

defimpl UnitFun.Convertor, for: Pounds do
#Note: please don't actually do a divison for any financial maths
# You're going to lose data and have a bad time.
def convert(_, Pence, value), do: (value / 100)
end```

And now the following:

```# returns: %UnitFun.Value{value: 6, units: Pounds}
total = item_cost + item_tax # UnitFun.add(item_cost, item_tax)

# returns: %UnitFun.Value{value: 600, units: Pence}
total_in_pence = total <~ Pence # UnitFun.with_units(total, Pence)```

## Example - Assertion

Errors can be raised if units aren't what they are expected to be:

`UnitFun.assert_units(total_money, Miles)`

## Example - Composite units

New units can also be composed by multiplying existing units together:

```  use UnitFun.MathsOperators
import UnitFun.UnitTypes

km_squared = Kilometers * Kilometers # UnitFun.multiply(Kilometers, Kilometers)```

These newly defined units can then be used as with all previous examples

```  edge = 4 <~ Kilometers # UnitFun.with_units(4, Kilometers)

area = edge * edge # UnitFun.multiply(edge, edge)

expected_area = 16 <~ km_squared # UnitFun.with_units(16, km_squared)
assert area == expected_area # UnitFun.equals(area, expected_area)```

Dividing/multiplying by united types returns values with new types so correctness can be asserted on.

```  miles_per_hour = Miles / Hours # UnitFun.divide(Miles, Hours)

speed = 40 <~ miles_per_hour # UnitFun.with_units(40, miles_per_hour)
time_spent_travelling = 2 <~ Hours # UnitFun.with_units(2, hours)

#the distance will be in Miles as the hours cancel out
distance_travelled_in_two_hours = time_spent_travelling * speed  # UnitFun.multiply(time_spent_travelling, speed)

assert distance_travelled_in_two_hours == 80 <~ Miles # UnitFun.with_units(80, Miles)```

## Example - Composite unit conversions

If there's a single unit way of representing some composite units this conversion can also be defined (N.B. there's currently no way of defining a conversion from a simple unit to a composite one):

```defimpl UnitFun.Convertor, for: UnitFun.ConvertorComplexTest.Pascals do
alias UnitFun.ConvertorComplexTest.Meters
alias UnitFun.ConvertorComplexTest.Newtons
alias UnitFun.Units.CompositeUnit

def convert(_, %CompositeUnit{numerators: [%Newtons{}], denominators: [%Meters{}]}, value) do
value
end
end```

## Example - Custom mathematic functions.

All the maths is controlled by protocols. So for example if you decided pence should only be handled as integers (so rounding isn't an issue) the following protocol could be defined:

```defimpl UnitFun.Maths.AddSubtractMaths, for: UnitFun.ExampleTest.Pence do
def add(_, left, right) when is_integer(left) and is_integer(right) do
left + right
end
def subtract(_, left, right) when is_integer(left) and is_integer(right) do
left - right
end
end```

Now any addition using non integer quantities will raise a FunctionClauseError. For convinience if nothing is defined then the kernel +-/* are used.

## Example - Facts about units

It's possible to define units with facts that must always hold true. This is handled by defining a list of functions that return true or false.

```defmodule UnitFun.Examples.PositiveUnit do
@moduledoc false
use UnitFun.Unit

defp greater_than_zero(x), do: x >= 0

facts [
&greater_than_zero/1
]

end```

Now whenever a `PositiveUnit` value is constructed the greater_than_zero callback will be executed. If this returns False then an InvalidValueError will be raised.

π Dimension based safety in elixir

V0.6.1 Latest
Dec 16, 2017

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