# ReatherLite
[](https://github.com/SeokminHong/reather_lite/actions/workflows/test.yml)
`Reather` is a shortcut of `Reader` + `Either` monads pattern.
It makes you define and unwrap the `Reather` easiliy by using `reather` macro.
## Installation
```elixir
def deps do
[
{:reather_lite, "~> 0.1.0"}
]
end
```
## Usage
### Basic usage
`reather` macro defines a function returns `Reather`.
```elixir
defmodule Target do
use Reather
reather foo(a, b) do
a + b
end
end
iex> Target.foo(1, 1)
%Reather{...}
```
The `Reather` is lazily evaluated, so it do nothing until call `Reather.run/1`.
```elixir
iex> Target.foo(1, 1) |> Reather.run()
{:ok, 2}
```
The result of `Reather` is always `{:ok, value}` or `{:error, error}`.
In `reather` block, the `ok` tuple will be automatically unwrapped with `<-` operator.
```elixir
defmodule Target do
use Reather
reather 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 `<-` operator.
```elixir
defmodule Target do
use Reather
reather 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 bar(a), do: a + 1
end
iex> Target.foo(1, 1) |> Reather.run()
{:ok, 3}
```
Because of the either monad, when the `<-` operator meets the error tuple,
the reather will return immediately.
```elixir
defmodule Target do
use Reather
reather 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}}
```
### Inline `reather`
`reather` also can be inlined.
```elixir
iex> r =
...> reather do
...> x <- {:ok, 1}
...> y <- {:ok, 2}
...>
...> x + y
...> end
%Reather{...}
iex> r |> Reather.run()
{:ok, 3}
```
### `Reather.ask`
Because of the `Reather` is a combination of reader and either monads,
it also provide an environment.
The providen environment can be accessed with `Reather.ask/0`.
```elixir
defmodule Target do
use Reather
reather foo() do
%{a: a} <- Reather.ask()
%{b: b} <- Reather.ask()
1 + a + b
end
reather 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}
```
### `reatherp`
If you want to define a private reather, use `reatherp` macro instead.
```elixir
defmodule Target do
use Reather
reatherp foo() do
1
end
end
```
