ErrorMonad.exs

defmodule ErrorMonad do
    def return(x) do
        {:ok, x}
    end
        
    def and_then(m, f) do
        case m do
            {:ok, v} -> f.(v)
            err -> err
        end
    end
end

defmodule Main do
    def addFive(x) do
        {:ok, x + 5}
    end
    
    def parseInt(x) do
        case Integer.parse(x) do
            {num, _} -> {:ok, num}
            :error -> {:err, "Could not parse: #{x}"}
            _ -> {:err, "Unkown error in parseInt/1"}
        end
    rescue
        e -> {:err, e}
    end
end

5
    |> ErrorMonad.return
    |> ErrorMonad.and_then(&Main.parseInt/1)
    |> ErrorMonad.and_then(&Main.addFive/1)
    |> IO.inspect
    
"abc"
    |> ErrorMonad.return
    |> ErrorMonad.and_then(&Main.parseInt/1)
    |> ErrorMonad.and_then(&Main.addFive/1)
    |> IO.inspect
    
"23"
    |> ErrorMonad.return
    |> ErrorMonad.and_then(&Main.parseInt/1)
    |> ErrorMonad.and_then(&Main.addFive/1)
    |> IO.inspect
   
# In Haskell you combine the `|>` and the `ErrorMonad.and_then` to form
# the `>>=` operator, so this example might look like the following pseudo
# code:
#   "23"
#       & ErrorMonad.return
#       >>= Main.parseInt
#       >>= Main.addFive
#       >>= IO.inspect
#
# Note: In Haskell `&` is the same as `|>`