MonadEx v1.0.2 → Overview → Monad → Reader

Monad.Reader

The reader monad is great for situations when you need to share some environment or state between several operations.

Reader monads are great for dependency injection. The environment is specified separately from the operations.

Example

iex> import Curry
iex> use Monad.Operators
iex> env = %{dev: %{url: "http://www.example.com/dev"}, prod: %{url: "https://www.example.com/prod"}}
iex> r = reader(& env[&1])
...> ~>> (& return(&1[:url]))
...> ~>> (& return(&1 <> "/index.html"))
iex> fun = runReader r
iex> fun.(:dev)
"http://www.example.com/dev/index.html"
iex> fun.(:prod)
"https://www.example.com/prod/index.html"

iex> reader = curry(& &1 + &2) <|> reader(& &1 * 2) <~> reader(& &1 * &1)
iex> fun = runReader reader
iex> fun.(2)
8
iex> fun.(-12)
120
Source

Summary↑

bind(reader, fun)

Callback implementation of Monad.Behaviour.bind/2
reader(fun)

Wraps fun in a reader monad
return(value)

Callback implementation of Monad.Behaviour.return/1
runReader(reader)

Unwraps the function from the reader

Types ↑

t

Functions

bind(reader, fun)
(function) # ↑

Specs:

  • bind(t, (term -> t)) :: t

Callback implementation of Monad.Behaviour.bind/2.

iex> r = reader &(&1 * 2)
iex> bind r, &(&1 + 3)
iex> f = runReader r
iex> f.(10)
20
Source
reader(fun)
(function) # ↑

Specs:

  • reader((term -> term)) :: t

Wraps fun in a reader monad.

iex> r = reader &(&1)
iex> r.fun.(4)
4
Source
return(value)
(function) # ↑

Specs:

  • return(term) :: t

Callback implementation of Monad.Behaviour.return/1.

Converts value into a function that always returns value. Then wraps that function into a reader monad.

iex> r = return 42
iex> r.fun.(:unused)
42
Source
runReader(reader)
(function) # ↑

Specs:

  • runReader(t) :: (term -> term)

Unwraps the function from the reader.

iex> r = reader &(&1 * 2) iex> f = runReader r iex> f.(3) 6

Source