Working with Maybe - Speaker Deck

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WORKING WITH MAYBE

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There are only two certainties in life: death, and taxes.

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UNCERTAINTY IN CODE

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USER INPUT

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OTHER SYSTEMS

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INSIDE YOUR PROGRAM

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SOCIETY FOR THE PRESERVATION OF VOWEL NAMES

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> All names lowercase > Vowel names prefixed with asterisk > Display unknown donors as "Anonymous"

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JAVASCRIPT let displayName = function(user) { if (startsWithVowel(user.name)) { "*" + user.name.toLowerCase(); } else { user.name.toLowerCase(); } }

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IS THIS SAFE?

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¯\_(ϑ)_/¯

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UNCERTAINTY let user1 = null; let user2 = { name: null }; displayName(user1); displayName(user2);

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You can never trust anything

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NULL EXCEPTIONS

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TYPE SYSTEMS?

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NULL IS VIRAL

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Uncertainty begets uncertainty

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NULL IS USED TO REPRESENT MANY DIFFERENT CONCEPTS

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THE ELM APPROACH

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"Trust no one" vs "Trust everyone"

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UNCERTAINTY PROBLEMS 1. any value can be missing at any time

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UNCERTAINTY PROBLEMS 1. any value can be missing at any time

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UNCERTAINTY PROBLEMS 1. any value can be missing at any time 2. Maybe is viral

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UNCERTAINTY PROBLEMS 1. any value can be missing at any time 2. Maybe is viral 3. Programmers abuse the concept

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formatName : Maybe User -> Maybe String startsWithVowel : Maybe String -> Maybe Bool

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type alias User = { name : Maybe String } formatName : Maybe User -> Maybe String formatName maybeUser = case maybesUser of Just user -> case startsWithVowel user.name of Just True -> case user.name of Just name -> Just <| "*" ++ String.toLower name Nothing -> Nothing -- should never happen Just False -> case user.name of Just name -> Just <| String.toLower name Nothing -> Nothing -- should never happen Nothing -> Just "Anonymous" startsWithVowel : Maybe String -> Maybe Bool startsWithVowel name = -- ...

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!!!

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ELIMINATE THE MAYBE

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UNWRAPPING EARLY getNameOrDefault : Maybe User -> String getNameOrDefault optionalUser = case optionalUser of Just user -> user.name Nothing -> ""

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"DEFAULT VALUES"

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ANOTHER FORM OF OPTIONAL viewName : Maybe User -> Html a viewName optionalUser = case getNameOrDefault optionalUser of "" -> text "Anonymous" actualName -> text actualName

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EXPLICIT OPTIONAL VALUE viewName : Maybe User -> Html a viewName optionalUser = case optionalUser of Nothing -> text "Anonymous" Just user -> text user.name

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POLYMORPHISM Passing default values like "" into a chain of functions is a clever way of no-opp

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GUIDELINE 1 Separate code that checks for presence from code that calculates values.

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CONFIDENT BUSINESS FUNCTION formatName : User -> String formatName user = user.name |> String.toLower |> prefixVowelName

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HANDLE THE MAYBE Maybe.map formatName optionalUser |> Maybe.withDefault "Anonymous"

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Business functions cannot take Maybe as an argument

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2 MAYBES Uncertain user and uncertain donation amount

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BUSINESS FUNCTION WITH NO MAYBES userWithDonation : User -> Int -> String userWithDonation user donation = (formatName user) ++ " - $" ++ toString donation

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map2 Maybe.map2 userWithDonation optionalUser optionalDonation |> Maybe.withDefault "Anonymous"

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Write confident business-logic functions and only deal with Maybe at the edges of your system (with the appropriate map function)

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UNCERTAINTY FROM WITHIN

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findByName : String -> List User -> Maybe User findByName name users = users |> List.filter (\user -> user.name == name) |> List.head

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GUIDELINE 2 Business functions may return Maybe but may not accept Maybe as any of its arguments.

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QUARANTINE

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type alias Session = { searchTerm : Maybe String } session.searchTerm |> Maybe.andThen (\name -> findByName name users)

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IS Maybe BAD?

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TWO MEANINGS List.head [] -- Nothing List.head [Nothing, { name = "Arthur" }] -- Nothing

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MODEL THE DIFFERENCE type Donor = Anonymous | Named String

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GUIDELINE 0 Actively try to model your data structures to avoid Maybe

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FORCING A SINGLE SHAPE

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UNCERTAINTY FROM WITHIN type alias DonorWizard = { donor : Maybe Donor , donationAmount : Maybe Int }

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IMPOSSIBLE STATES { donor = Nothing , donationAmount = Just 100000 }

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UNION TYPES type DonorWizard = Step1 (Maybe Donor) | Step2 Donor (Maybe Int) | Complete Donor Int

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> Actively try to model your data structures to avoid Maybe > Separate code that checks for presence from code that calculates values. > Business functions may return Maybe but may not accept Maybe as any of its arguments.

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JOËL QUENNEVILLE > Github - github.com/JoelQ > Slack - @joelq > Twitter - @joelquen Come talk to me, I have postcards!