Rex - Concatenative elixir macro language.

Rex is a concatenative language built with Elixir macros.

Being powered by Elixir macros means that Rex has no parser of its own as it just uses valid Elixir syntax and thus can be embedded on any elixir program.

Installation

Available in Hex, the package can be installed as:

1. Add rex to your list of dependencies in mix.exs:

```elixir
def deps do
  [{:rex, "~> 0.1.0"}]
end
```

Stack based

Rex is stack based, that is all Rex functions take a list and return another list.

The topmost element in a stack is the one at index zero, for example, for [1, 2, 3] the top of the stack is 1. Most Rex functions will operate on top-most elements as they need and then push the result back to the top.

Inside the rex macro, the ~> operator indicates the order in which to push values or operators on to the top of the stack.

For example, rex(1 ~> 2 ~> add) would result on the following [add, 2, 1] stack that when executed will yield [3].

So the ~> allows post-fix notation (operands first, operator last) syntax. This is the preferred way as most concatenative languages are postfix, but Rex also has a <~ operator which just pushes values in the reverse order: rex(add <~ 2 <~ 1) will result on [add, 2, 1].

The following example uses functions from Rex.Core for stack manipulation and Rex.Math that defines aliases to standard elixir math operators.

iex> [4, 5] |> rex(3 ~> 2 ~> 1 ~> add ~> mul ~> swap)
[4, 9, 5] 

More examples available as tests

Words

In concatenative languages, functions are refered to as words.

Inside an Elixir module, once you have include Rex'd you can call drex to define a new word that acts either as a stack shuffler or as an operator on the stack.

To define a stack shuffling word, the syntax is:

    # (example from `Rex.Stack.swap/1`)
    drex swap(a, b)     (b, a)

To define a stack operator you use the ~> or <~ syntax:

    # pushes 1 then 2 then performs adition
    drex three        1 ~> 2 ~> Kernel.+/2

    # pushes 2 then performs multiplication
    # expecting a first value already on stack (ie. partial function)
    drex double       Kernel.*/2 <~ 2

As operators are the most frequent types of words you will be creating, the following concatenative syntax is supported:

    # This will multiply the second element on the stack
    # and then print the final stack state to stdout.
    drex double_second  swap double swap show

However, if you want to also push an integer or any other Elixir literal, trying something like 3 double wont work because its not valid Elixir syntax. But you can use the do notation for drex:

      drex sixsix do
        3
        double dup Kernel.*/2
      end

is exactly the same as:

      drex sixsix  3 ~> double ~> dup ~> Kernel.*/2

The do form is peferred for large words. Most likely you'll just want to keep them short as concatenative programs are very composable.

Pointless programming

With Rex you can write point-free functions.

drex sumr     List.foldr/3 <~ (&Kernel.+/2) <~ 0

assert [6] == sumr([1, 2, 3])