# Towel [](https://travis-ci.org/knrz/towel)
Simple, unobtrusive monads that anyone can use in Elixir.
## Installation
Keep calm and add Towel to your `mix.exs` dependencies:
```elixir
def deps do
[{:towel, git: "https://github.com/knrz/towel.git", "~> 1.0.0"}]
end
```
and run `$ mix deps.get`.
You're ready to start righting more expressive code!
## Philosophy
Let's keep it Elixir. Forget Haskell's `Just` and `Nothing`; Elixir's already got its tuple-based `{:ok, value}` and `{:error, reason}` construct, so let's use that. That's why Towel's `Maybe` monad returns `{:ok, value}` and `{:error, reason}` instead of `:some` or `:none` or whatnot.
Elixir's got piping (`|>`). Piping is great, but not when you've got wrapped values. What if you've got a list in a `maybe`? Just keep your Towel handy:
```elixir
[1, 2, 3] |> bind(fn x -> x * x end)
# returns [1, 4, 9]
#
# Piping to bind(fn) passes each value of the monad
# (in this case the List monad) to fn and normalizes the result.
# Typically, you'd have to wrap your function's return value as a monad, like so:
# [1, 2, 3] |> bind(fn x -> [x * x] end)
# but Towel automatically calls the appropriate monadic return function to wrap
# the return value of your function (in this case, List.wrap).
```
You only have to explicitly return `{:error, reason}` for the Maybe monad or an empty list (`[]`) for the List monad if you want to return the monad's 'invalid' state, so to speak.
```elixir
maybe(42) |> bind(fn a -> [answer: a] end)
# returns {:ok, [answer: 42]}
#
# But if you want to error out:
maybe(39)
|> bind fn a ->
if a == 42, do: [answer: a], else: {:error, "there's no point"}
end
# returns {:error, "there's no point"}
```
## Usage
You'll have to add `use Towel` to the top of whatever module you're working in first (or just run it in `iex` to try it out):
```elixir
defmodule Hitchhiking.Galaxy do
# Don't panic
use Towel
end
```
## License
Copyright (c) 2015 Kash Nouroozi
This work is free. You can redistribute it and/or modify it under the
terms of the MIT License. See the LICENSE file for more details.
