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How to Find the Bar

How to Find the Bar

The Maybe Monad in Elm

Lightning talk for SocratesBe
http://verraes.net

330627d5f564b710721236077903ed60?s=128

Mathias Verraes

August 17, 2016
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  1. how to find the bar @mathiasverraes

  2. disclaimer i don't actually know elm

  3. joining some lookup tables Integer to Character 1 => 'a'

    2 => 'b' 3 => 'c' Character to String 'a' => "foo" 'b' => "bar" String to Msg "foo" => "No bar here" "bar" => "You found the bar"
  4. import Dict exposing (get, fromList) findC n = get n

    (fromList [ ( 1, 'a' ), ( 2, 'b' ), (3, 'c') ]) findS c = get c (fromList [ ( 'a', "foo" ), ( 'b', "bar" ) ]) findM s = get s (fromList [ ( "foo", "No bar here" ) , ( "bar", "You found the bar" ) ] )
  5. regular programming find n = findM (findS (findC n)) mediocre

    functional style find n = findC n |> findS |> findM composing computations like a boss find = findC >> findS >> findM
  6. ==================================== ERRORS ==================================== -- TYPE MISMATCH --------------------------------------------- repl-temp-000.elm The right

    argument of (|>) is causing a type mismatch. 5| findC 2 |> findS ^^^^^ (|>) is expecting the right argument to be a: Maybe Char -> a But the right argument is: Char -> Maybe String Hint: I always figure out the type of the left argument first and if it is acceptable on its own, I assume it is "correct" in subsequent checks. So the problem may actually be in how the left and right arguments interact.
  7. Y YOU NO COMPOSE?! > findC 1 Just 'a' :

    Maybe.Maybe Char > findC 5 Nothing : Maybe.Maybe Char lookups have 0 or 1 results! findC : Int -> Maybe Char findS : Char -> Maybe String findM : String -> Maybe Msg
  8. find : Int -> Maybe Msg find n = case

    findC n of Nothing -> Nothing Just c -> case findS c of Nothing -> Nothing Just s -> findM s
  9. andThen : Maybe a -> (a -> Maybe b) ->

    Maybe b andThen x f = case x of Nothing -> Nothing Just a -> f a find n = findC n `andThen` findS `andThen` findM > find 1 Nothing : Maybe.Maybe Msg > find 2 Just "You found the bar" : Maybe.Maybe Msg
  10. elm: generalised andThen for all computations that can return nothing

    or fail haskell: generalised for even more contexts (>>=) :: m a -> (a -> m b) -> m b aka "bind"
  11. you understand monads now (>>=) :: m a -> (a

    -> m b) -> m b m can be many many things
  12. None
  13. thanks ❤ @mathiasverraes verraes.net