Maybe and null safety

Why a missing value is a type

There is no null in Haskell. A value that may be absent is written with a type that says so, and the compiler refuses to use it without handling the empty case:

data Maybe a = Nothing | Just a findUser :: Int -> Maybe String findUser 1 = Just "Ada" findUser _ = Nothing

Maybe String means a string or nothing. Compare that with a signature of String, where the absence has to be smuggled in as an empty string or a magic value.

Building and unwrapping

Wrap with Just, and unwrap with pattern matching or case. Both branches are mandatory, which is the whole point:

greet :: Maybe String -> String greet (Just name) = "Hello, " ++ name greet Nothing = "Hello, stranger" -- greet (Just "Ada") == "Hello, Ada"

A pattern match that forgets Nothing produces a non-exhaustive warning under -Wall and an exception at runtime, so the compiler pushes you towards handling it.

fromMaybe and friends

Three standard functions cover the common ways to leave the wrapper behind. All of them are total:

fromMaybe 0 (Just 5) -- 5 fromMaybe 0 Nothing -- 0 maybe "none" show (Just 5) -- "5" isJust (Just 5) -- True

fromMaybe needs the import Data.Maybe. Reach for it when a sensible default exists, and keep the wrapper when the caller must decide.

Chaining lookups

Two lookups in a row would nest two cases. The >>= operator flattens the chain and short-circuits on the first Nothing:

cityOf :: Int -> Maybe String cityOf uid = findUser uid >>= \name -> lookup name phoneBook

If findUser returns Nothing, the lambda never runs and the result is Nothing. Lesson 31 explains why this works for every monad, not only for Maybe.

Next: Either and recoverable errors

Article author: Arthur Isaev