do

This package brings monads, applicatives, functors, and do-notation to Erlang. It provides behaviours for relevant type classes, as well as implementations for commonly used class instances.

Quick Start

The either type is defined as a tuple of either {ok, Value} or {error, Reason}, and is a member of the functor, applicative and monad type class. The ?fmap macro can be used to map functions over functors:

-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 consecutively executes functions inside the either monad. The macro takes a start value, and a list of functions. The functions must each take either 0 or 1 argument(s) and must return an either monad. On execution, the start value is passed to the first function in the provided list. If the function returns {ok, Val}, the second function is called with Val, and so on:

-include_lib("do/include/do.hrl").

maybe_add1(N) when N > 5 -> {error, greater_five};
maybe_add1(N)            -> {ok, N + 1}.

do_example1() ->
  {ok, 3} = ?do({ok, 0}, [ fun maybe_add1/1,
                           fun maybe_add1/1,
                           fun maybe_add1/1]).

If a function returns an error tuple {error, Reason}, the error is returned immediately.

do_example2() ->
  {error, greater_five} = ?do({ok, 0}, [ fun maybe_add1/1,
                                         fun maybe_add1/1,
                                         fun maybe_add1/1,
                                         fun maybe_add1/1,
                                         fun maybe_add1/1,
                                         fun maybe_add1/1]).