Reather
is a shortcut of Reader
+ Either
monads pattern.
It makes you define and unwrap the Reather
easiliy by using macros.
The original idea is from jechol/reather, and this is a lite version without using Witchcraft.
def deps do
[
{:reather_lite, "~> 0.3.0"}
]
end
def
macro marked @reather true
defines a function returns Reather
.
defmodule Target do
use Reather
@reather true
def foo(a, b) do
a + b
end
end
iex> Target.foo(1, 1)
%Reather{...}
Since the Reather
is lazily evaluated, it does nothing until call Reather.run/2
.
iex> Target.foo(1, 1) |> Reather.run()
{:ok, 2}
The result of Reather
is always {:ok, value}
or {:error, error}
.
In a reather function, the ok
tuple will be automatically unwrapped by a <-
operator.
defmodule Target do
use Reather
@reather true
def foo() do
a <- {:ok, 1} # a = 1
{b, c} <- {:ok, 2, 3} # b = 2, c = 3
d = nil
^d <- :ok
a + b + c
end
end
iex> Target.foo() |> Reather.run()
{:ok, 6}
Also, a Reather
unwrap into a value with a <-
operator.
defmodule Target do
use Reather
@reather true
def foo(a, b) do
x <- bar(a) # The result of bar(a) is {:ok, a + 1} and x will be bound to a + 1.
x + b
end
@reather true
def bar(a), do: a + 1
end
iex> Target.foo(1, 1) |> Reather.run()
{:ok, 3}
Because of the either monad, when the <-
operator meets an error tuple,
the reather will return it immediately.
defmodule Target do
use Reather
@reather true
def foo() do
x <- {:ok, 1}
y <- {:error, "asdf", 1} # foo will return {:error, {"asdf", 1}}
x + y
end
end
iex> Target.foo() |> Reather.run()
{:error, {"asdf", 1}}
reather
also can be inlined.
iex> r =
...> reather do
...> x <- {:ok, 1}
...> y <- {:ok, 2}
...>
...> x + y
...> end
%Reather{...}
iex> r |> Reather.run()
{:ok, 3}
The macros also accepts above clauses.
defmodule Target do
use Reather
@reather true
def foo(a, b) do
x <- bar(a)
y <- baz(b)
x + y
else
{:error, _} -> {:ok, a + b}
ok -> ok
rescue
ArithmeticError -> {:error, :div_by_zero}
after
IO.puts("Target.foo/2")
end
end
Because of the Reather
is a combination of reader and either monads,
it also provides an environment.
The providen environment can be accessed with Reather.ask/0
.
defmodule Target do
use Reather
@reather true
def foo() do
%{a: a} <- Reather.ask()
%{b: b} <- Reather.ask()
1 + a + b
end
@reather true
def bar() do
x <- foo()
x + 1
end
end
iex> Target.foo() |> Reather.run(%{a: 10, b: 100})
{:ok, 111}
# The environment can be accessed in nested reathers.
iex> Target.bar() |> Reather.run(%{a: 10, b: 100})
{:ok, 112}
If you want to define a private reather, use defp
macro instead.
defmodule Target do
use Reather
@reather true
defp foo() do
1
end
end
You can map
a function to a Reather
.
The given function will be applied lazily when the result of
the reather is an ok
tuple.
defmodule Target do
use Reather
@reather true
def foo() do
x <- {:ok, 1}
x
end
@reather true
def bar() do
x <- {:error, 1}
x
end
end
iex> Target.foo()
...> |> Reather.map(fn x -> x + 1 end)
...> |> Reather.run()
{:ok, 2}
iex> Target.bar()
...> |> Reather.map(fn x -> x + 1 end)
...> |> Reather.run()
{:error, 1}
Transform a list of reathers to an reather of a list.
This operation is lazy, so it's never computed until
explicitly call Reather.run/2
.
iex> r = [{:ok, 1}, {:ok, 2}, {:ok, 3}]
...> |> Enum.map(&Reather.of/1) # Make reathers return each elements.
...> |> Reather.traverse()
iex> Reather.run(r)
{:ok, [1, 2, 3]}
iex> r = [{:ok, 1}, {:error, "error"}, {:ok, 3}]
...> |> Enum.map(&Reather.of/1) # Make reathers return each elements.
...> |> Reather.traverse()
iex> Reather.run(r)
{:error, "error"}
Convert a value into ok
or error
tuple. The result is a tuple having
an :ok
or :error
atom for the first element, and a value for the second
element.
Make an error tuple from a value.
map
a function to an either tuple.
The given function will be applied lazily
when the either is an ok
tuple.
Transform a list of eithers to an either of a list.
If any of the eithers is error
, the result is error
.
iex> [{:ok, 1}, {:ok, 2}] |> Either.traverse()
{:ok, [1, 2]}
iex> [{:ok, 1}, {:error, "error!"}, {:ok, 2}]
...> |> Either.traverse()
{:error, "error!"}