Example for https://twitter.com/sylvanica/status/975038763180027904

WitchcraftMonadExample.ex

Interactive Elixir (1.6.4) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> use Witchcraft
iex(2)> alias Algae.Maybe

# With a Just
iex(5)> monad %Maybe.Just{} do  
...(5)>   a <- Maybe.Just.new(1)
...(5)>   b <- Maybe.Just.new(2)
...(5)>   return(a + b)         
...(5)> end
%Algae.Maybe.Just{just: 3}

# Same with Nothing
iex(6)> monad %Maybe.Nothing{} do
...(6)>   a <- Maybe.Just.new(1) 
...(6)>   b <- Maybe.Just.new(2) 
...(6)>   return(a + b)          
...(6)> end
%Algae.Maybe.Just{just: 3}

# Makes no difference if there's something in the Just
iex(5)> monad %Maybe.Just{just: 42} do  
...(5)>   a <- Maybe.Just.new(1)
...(5)>   b <- Maybe.Just.new(2)
...(5)>   return(a + b)         
...(5)> end
%Algae.Maybe.Just{just: 3}

# Because of how Nothing short circuits, we skip the return function
iex(7)> monad 12.3456 do # floats are not monads
...(7)>   a <- Maybe.Just.new(1)
...(7)>   b <- Maybe.Nothing.new() 
...(7)>   return(a + b)
...(7)> end
%Algae.Maybe.Nothing{}

# `Monad.return/1` is just `Applicative.of/1`, ie: partially applied `of/2` with whatever we hand to the `monad` macro.
# There is no type environment carried around in Elixir, so `Monad %Sample{} do` is a way of emulating type metadata for the `return`
#
# Strictly speaking, all this gets you the ability to:
# 1. Write pure-yet-imperative-feeling code, because `return` is familiar
# 2. Use `return` in several places without needing to always specify the context that you're in

# The example above but using more `return`:
iex(8)> monad %Maybe.Nothing{} do
...(8)>   a <- return(1)
...(8)>   b <- return(2)
...(8)>   return(a + b) 
...(8)> end
%Algae.Maybe.Just{just: 3}

# This code is far more general (and thus portable) than the one with explicit values on each line.

#For example:
iex(10)> context_add = 
...(10)>   fn(sample) ->
...(10)>     monad sample do
...(10)>       a <- return(1)
...(10)>       b <- return(2)
...(10)>       return(a + b)
...(10)>      end
...(10)>    end
#Function<6.99386804/1 in :erl_eval.expr/5>

iex(11)> context_add.([]) 
[3]

iex(12)> context_add.(["these", "values", "are", "thrown out"]) 
[3]

iex(13)> context_add.(%Algae.Either.Right{})
%Algae.Either.Right{right: 3} 

iex(14)> context_add.(%Algae.Maybe.Nothing{})
%Algae.Maybe.Just{just: 3}

iex(15)> context_add.(%Algae.Tree.Rose{})    
%Algae.Tree.Rose{forest: [], rose: 3}

# Obviously that's a somewhat silly example, but it should help illustrate the behaviour :)