f_context

Simple functional programming library for CoffeeScript.

1. Overview
2. Installation
3. Modules
4. Pattern matching
5. Destructuring
6. Guards
7. Real life examples
8. How it works
9. Testing
10. Benchmark

Overview

Those who had some experience with functional programming languages such as Erlang did notice its excessive pattern matching abilities:

fact(0) -> 1
fact(N) -> N * fact(N - 1)

This is a classical implementation of factorial compilation using patter matching. Now you can use code it exactly the same style, but in CoffeeScript using f_context wrapper function. For example:

f_context ->
    fact(0) -> 1
    fact(N) -> N * fact(N - 1)

See more examples.

Try it yourself:

git clone git@github.com:nogizhopaboroda/f_context.git
cd f_context/example
open example.html

If you still think there's incompliance or don't understand why it should work, you can read how it works before.

Disclaimer

This doesn't contain any hacks and evals whatsoever.

Functions in examples are prefixed with f_ so it is easy to distinguish between functional and imperative counterparts, i.e. f_ prefix is not enforced.

Installation

You can link directly to this repo:

<script src="https://raw.githubusercontent.com/nogizhopaboroda/f_context/master/dist/f_context.js"></script>

Or you can clone the repo instead: the release resides in dist/f_context.js file.

git clone git@github.com:nogizhopaboroda/f_context.git

Using npm:

npm install f_context

Then

f_context = require('f_context')

Modules

f_context returns object with generated functions by default, so you can use it like this:

examples = f_context ->

  f_range(I) ->
    f_range(I, 0, [])

  f_range(I, I, Accum) -> Accum
  f_range(I, Iterator, Accum) ->
    f_range(I, Iterator + 1, [Accum..., Iterator])

examples.f_range(10) #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Using module directive you can specify the name of the object module that will contain your functions. As a result module will be available in global scope (window or global):

f_context ->
  module("examples")

  f_range(I) ->
    f_range(I, 0, [])

  f_range(I, I, Accum) -> Accum
  f_range(I, Iterator, Accum) ->
    f_range(I, Iterator + 1, [Accum..., Iterator])

examples.f_range(10) #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Features

Pattern matching

Wikipedia

Example with one argument being pattern matched:

f_context ->
    matching_example_1("foo") -> "foo matches"
    matching_example_1("bar") -> "bar matches"
    matching_example_1(Str)   -> "nothing matches, argument: #{Str}"

The same, but with two arguments:

f_context ->
    matching_example_2("foo", "bar") -> "foo and bar matches"
    matching_example_2("bar", "bla") -> "bar and bla matches"
    matching_example_2("bar", "bar") -> "bar and bar matches"
    matching_example_2(Str, Str2)    -> "no matching pairs, arguments: #{Str}, #{Str2}"

Destructuring

Wikipedia

f_context ->
    test_destruct_1([Head, Tail...]) -> {Head, Tail}
    test_destruct_1_1([Head, Head1, Tail...]) -> {Head, Head1, Tail}

f_context ->
    test_destruct_2([Head..., Last]) -> {Head, Last}
    test_destruct_2_1([Head..., Last, Last1]) -> {Head, Last, Last1}

f_context ->
    test_destruct_3([Head, Middle..., Last]) -> {Head, Middle, Last}
    test_destruct_3_1([Head, Head2, Middle..., Last, Last2]) -> {Head, Head2, Middle, Last, Last2}

Guards

Wikipedia

Use where(%condition%) syntax to define a guard. For example: here is a function that produces fibonacci sequence, implemented without guards:

f_context ->
  fibonacci_range(Count) ->
    fibonacci_range(Count, 0, [])

  fibonacci_range(Count, Count, Accum) -> Accum

  fibonacci_range(Count, 0, Accum) ->
    fibonacci_range(Count, 1, [Accum..., 0])

  fibonacci_range(Count, 1, Accum) ->
    fibonacci_range(Count, 2, [Accum..., 1])

  fibonacci_range(Count, Iterator, [AccumHead..., A, B]) ->
    fibonacci_range(Count, Iterator + 1, [AccumHead..., A, B, A + B])

Using guards you can make it considerably shorter:

f_context ->
    fibonacci_range(Count) ->
      fibonacci_range(Count, 0, [])

    fibonacci_range(Count, Count, Accum) -> Accum

    fibonacci_range(Count, Iterator, Accum) where(Iterator is 0 or Iterator is 1) ->
      fibonacci_range(Count, Iterator + 1, [Accum..., Iterator])

    fibonacci_range(Count, Iterator, [AccumHead..., A, B]) ->
      fibonacci_range(Count, Iterator + 1, [AccumHead..., A, B, A + B])

Real life examples

Here are reduce and qsort functions implemented using f_context:

f_context ->
  f_reduce(List, F) ->
    f_reduce(List, F, 0)

  f_reduce([], _, Memo) -> Memo

  f_reduce([X, List...], F, Memo) ->
    f_reduce(List, F, F(X, Memo))

f_context ->
  f_qsort([]) -> []
  f_qsort([Pivot, Rest...]) ->
    [f_qsort((X for X in Rest when X < Pivot))..., Pivot, f_qsort((Y for Y in Rest when Y >= Pivot))...]

How it works

fact(0) -> 1
fact(N) -> N * fact(N - 1)

If you compile this from CoffeeScript to JS, you'll get this:

fact(0)(function() {
    return 1;
});

fact(N)(function() {
    return N * fact(N - 1);
});

As you can see this is absolutely valid, and that means you can evaluate it. Though now it probably will throw an error.

But if you wrap it in a function, like that:

function_wrapper ->
    fact(0) -> 1
    fact(N) -> N * fact(N - 1)

You'll get:

function_wrapper(function() {
    fact(0)(function() {
      return 1;
    });
    fact(N)(function() {
      return N * fact(N - 1);
    });
});

Now fact evaluates inside function_wrapper that can analyse the argument function, then modify and extend it before execution. Like this:

var fact_stub = function(){ return function(){}; },
    N_stub;
(function(fact, N) {
    fact(0)(function() {
      return 1;
    });
    fact(N)(function() {
      return N * fact(N - 1);
    });
})(fact_stub, N_stub);

Ok, now argument function executes without an errors. What's next?

We can mark N_stub as a "variable" and design fact_stub so it looks at itself arguments and generate a part of future wanted fact.

var N_stub = function(){};
N_stub.type = "variable";
N_stub.name = "N";

var fact_stub = function(argument){
    return function(fact_computing_function){
        if(typeof argument === "function" && argument.type === "variable"){
            var generated_part = "var " + argument.name + " = arguments[0];" +
                + "return (" + fact_computing_function + ")()";
            return generated_part;
        } else {
            var generated_part = "if(arguments[0] === " + argument + "){" +
                + "return (" + fact_computing_function + ")()" +
                + "}";
            return generated_part;
        }
    };
}

So, generated fact will look like this:

if(arguments[0] === 0){
  return (function () {
    return 1;
  })();

var N = arguments[0];
return (function () {
    return N * f_fact(N - 1);
})();

Ok, now function_wrapper has all data to build up new fact function with required checks and assign it to a key of an object (window for example). That's how it works it a few words.

And if you still don't understand how it works, you can read the sources

Testing

gulp test

Benchmarking

gulp bench

Other

Thanks a lot to yegortimoschenko for translation and correction of this document.