expede

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)         

expede

# ========
# NOTATION
# ========

# Control.Monad (Haskell) Bind
>>=

# Witchcraft.Chain.bind (Elixir)
>>>

expede

{-
  Vectors are fixed-length data structures
  They're written in this format: `Vect length type`
  
  ex. `Vect 3 Int` could represent [1,2,3], but not [1,2] or ['a', 'b', 'c']
-} 

concat : Vect lengthX a -> Vect lengthY a -> Vect (lengthX + lengthY) a
concat Nil       ys = ys
concat (x :: xs) ys = x :: app xs ys

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x == x -- Reflixivity 

x * 0     == 0 -- Origin
x * 0 * 0 == 0 -- Idempotence

x + 0 == x -- Additive identity
x / 1 == x -- Divisive identity
x * 1 == x -- Multiplicative identity

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return a >>= f  == f a -- Left Identity
m >>= return    == m   -- Right Identity
(m >>= f) >>= g == m >>= (\x -> f x >>= g) -- Associativity
-- ...plus all applicative functor laws

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# =====================
# Explicit branch names
# =====================

branch OtherFile.read("./existing_file.txt"),
  value_do: String.length,
  exception_do: fn %{message: msg} -> msg end.()
# => 1000

branch OtherFile.read("./missing.file"),

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OtherFile.read("./existing_file.txt")
~> fn text -> 
  text
  |> String.length
  |> fn x -> x / 2 end.()
end.()
|> to_tagged_status
#=> {:ok, 500}

# `ok` alias for tagged_status

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search_result = find_book(title: title)
case search_result do
  page = %Page{} -> render(page)
  %Ecto.MultipleResultsError{} when is_series(title) ->
    %Book.SeriesAreNotBooksError{
      message: "#{title} is a series, not a book.",
      title: title,
      series: search_result,
      plug_status: 422
    }

expede

use Exceptional

OtherFile.read("./existing_file.txt") ~> String.length
# 19

OtherFile.read("./missing.file") ~> String.length
# %OtherFile.NotFoundError{
#   message: "File not found at ./missing.file",
#   path: "./missing.file"
# }

expede

# Hypothetical rewrite of `File.read`
# Not the lack of a `OtherFile.read!` (with a bang),
# but we get the same effect with the caller via `>>>`

# Success
iex> OtherFile.read("./existing_file.txt")
"I'm a little teapot short and stout ..."

# Error
# Note that this returns an Exception struct as a value