Freer

###A Freer Monad implemented in elixir.

So, what is a Freer Monad? Hmmmm, it's just a monad even freer than an free monad:

If you still have no idea about Free Monad Free Monad: http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.html

If you really want to know about the detail on Freer Monad Freer Monad: http://okmij.org/ftp/Haskell/extensible/more.pdf

###Basic Usage

Say you wanted to build a Error Monad to chain a series of failable functions: With Freer, well, you don't need to build any thing at all, you just write your monadic code in the charm block

require Freer
# x is some initial value, eg: a file path on the disk
# and in the charm block, you just have the 'bind' 'return' 'map' and 'apply' functions already free to use!
# eg:
f_result = Freer.charm do
  return(x) 
    |> bind(failable_fun1) 
    |> bind(failable_fun2)
    |> map(transform_fun)
end

After all your logic and operation is wrote done, you simple use the real return and bind of your Monads to interpret the final result:

#you can build or update your interpreter at the final step after all of your business logic is done.
# the first parameter of 'interpret' is the real 'return' function
# and the second parameter is the real 'bind' function
interpreter = Freer.interpret(
  &({:ok, &1}), 
  fn
    {:ok, x}, fun -> fun.(x)
    e, _fun -> e
  end
)
result = interpreter.(f_result)
# now result is {:ok, final_value} or {:error, some_reason}

This approach of interpreter pattern can be useful, take a look at the Facebook's Haxl library, and a series of talk in the community.

Rune Word

For most common use case of Monads, this library provides a runes macro, which behaviours exactly like the for or with statement in elixir.

f = Freer.charm do
  runes x <- [1,3,5],
        y <- [2,4,6],
        do: x * y
end
#[2, 4, 6, 6, 12, 18, 10, 20, 30] after interpret.

f = Freer.charm do
  runes line1 <- read_file1,
        line2 <- read_file2,
        do: line1 <> line2
end
# if both file read successfully, it returns the value, or if any fails, it returns the error reason of the failed one.