Postfix

Simple evaluator of a list of values and functions given in postfix order.

{:ok, 20} = Postfix.eval([7, 2, &-/2, 4, &*/2])

Inspired by Factor. See Postfix.Stack for examples.

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 which uses a list for the program and a stack for operands and results

It is not possible to pass a function to a higher-order function as it will be evaluated.

Given the filter example:

is_odd? = fn x -> rem(x, 2) == 0 end
Enum.filter(1..3, is_odd?)

This cannot be written as:

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:

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])

This package can be installed by adding postfix to your list of dependencies in mix.exs:

def deps do
  [
    {:postfix, "~> 1.2"}
  ]
end

Released under the MIT License