[](https://github.com/moritzploss/do/actions/workflows/tests.yml)
## do
This package brings monads, applicatives, functors, and `do`-notation to
Erlang. It provides behaviours for related type classes, as well as
implementations for commonly used class instances.
### Installation
To install the latest version of `do` from [`hex`](https://hex.pm/packages/do),
add `do` to the `deps` in your rebar config file:
{do, "1.5.0"}
### What's in the box
The `do` package provides implementations for `either`, `list` and `maybe`
monads, as well as a range of useful functors. See
[do_types.hrl](./include/do_types.hrl) for type definitions.
### The fmap macro
The `?fmap` macro can be used to map functions over functors:
```erlang
-include_lib("do/include/do.hrl").
add1(N) -> N + 1.
fmap_example() ->
{ok, 2} = ?fmap(fun add1/1, {ok, 1}),
{error, rsn} = ?fmap(fun add1/1, {error, rsn}),
[2, 3, 4] = ?fmap(fun add1/1, [1, 2, 3]),
#{a => 2} = ?fmap(fun add1/1, #{a => 1}).
```
### The do macro
The `?do` macro consecutively executes functions inside monads. The macro
takes a start value (a monad), and a list of functions. The functions must
each take either 0 or 1 argument(s) and must return a monad. On execution,
the start value is passed to the first function in the provided list, and is
then piped through consecutive functions using `bind`.
```erlang
-include_lib("do/include/do.hrl").
maybe_add1(N) when N > 2 -> {error, N};
maybe_add1(N) -> {ok, N + 1}.
do_either() ->
{error, 3} = ?do({ok, 1}, [ fun maybe_add1/1,
fun maybe_add1/1,
fun maybe_add1/1,
fun maybe_add1/1]).
```
