# Postfix
Simple evaluator of a list of values and functions given in [postfix][postfix] order.
```elixir
{:ok, 20} = Postfix.eval([7, 2, &-/2, 4, &*/2])
```
Inspired by [Factor][factor]. See [Postfix.Stack][stack] for examples.
[](https://hex.pm/packages/postfix)
[](https://github.com/devstopfix/postfix-elixir/actions/workflows/ci.yml)
## Usage
Functions are evaluated with all the operands to their left, and the result
replaces the function it's operands in the list and evaluation continues.
If any function returns an `{:ok, value}` tuple then the value is unwrapped
and used as the result.
Any function that fails with an `{:error, _}` tuple short-circuits and
becomes the result of the evaluation.
There are two implementations:
* `Postfix.Simple` for simple pipelines using a list
* [Postfix.Stack][stack] which uses a list for the program and a stack for operands and results
### Limitations
It is not possible to pass a function to a higher-order function as it will be evaluated.
Given the filter example:
```elixir
is_odd? = fn x -> rem(x, 2) == 0 end
Enum.filter(1..3, is_odd?)
```
This *cannot* be written as:
```elixir
is_odd? = fn x -> rem(x, 2) == 0 end
{:ok, [2]} = Postfix.eval([1..3, is_odd?, &Enum.filter/2])
```
However it could be written as:
```elixir
is_odd? = fn x -> rem(x, 2) == 0 end
filter_odd = fn xs -> Enum.filter(xs, is_odd?) end
{:ok, [2]} = Postfix.eval([1..3, filter_odd])
```
## Installation
This package can be installed
by adding `postfix` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:postfix, "~> 1.2"}
]
end
```
## License
Copyright 2022 J Every
Released under the [MIT License][mit]
[factor]: https://factorcode.org/
[mit]: https://opensource.org/licenses/MIT
[postfix]: https://en.wikipedia.org/wiki/Reverse_Polish_notation
[stack]: https://hexdocs.pm/postfix/Postfix.Stack.html
