Maybe

This library fills a bunch of important niches in Elm. A Maybe can help you with optional arguments, error handling, and records with optional fields.

Definition

data Maybe a

The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error.

The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing. A richer error monad can be built using the Either type.

Constructors

Nothing

Just a

Instances

Monad Maybe

Since: base-2.1

Defined in GHC.Base

Methods

(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

(>>) :: Maybe a -> Maybe b -> Maybe b

return :: a -> Maybe a

Functor Maybe

Since: base-2.1

Defined in GHC.Base

Methods

fmap :: (a -> b) -> Maybe a -> Maybe b

(<$) :: a -> Maybe b -> Maybe a

MonadFail Maybe

Since: base-4.9.0.0

Defined in Control.Monad.Fail

Methods

fail :: String -> Maybe a

Applicative Maybe

Since: base-2.1

Defined in GHC.Base

Methods

pure :: a -> Maybe a

(<*>) :: Maybe (a -> b) -> Maybe a -> Maybe b

liftA2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c

(*>) :: Maybe a -> Maybe b -> Maybe b

(<*) :: Maybe a -> Maybe b -> Maybe a

Foldable Maybe

Since: base-2.1

Defined in Data.Foldable

Methods

fold :: Monoid m => Maybe m -> m

foldMap :: Monoid m => (a -> m) -> Maybe a -> m

foldMap' :: Monoid m => (a -> m) -> Maybe a -> m

foldr :: (a -> b -> b) -> b -> Maybe a -> b

foldr' :: (a -> b -> b) -> b -> Maybe a -> b

foldl :: (b -> a -> b) -> b -> Maybe a -> b

foldl' :: (b -> a -> b) -> b -> Maybe a -> b

foldr1 :: (a -> a -> a) -> Maybe a -> a

foldl1 :: (a -> a -> a) -> Maybe a -> a

toList :: Maybe a -> [a]

null :: Maybe a -> Bool

length :: Maybe a -> Int

elem :: Eq a => a -> Maybe a -> Bool

maximum :: Ord a => Maybe a -> a

minimum :: Ord a => Maybe a -> a

sum :: Num a => Maybe a -> a

product :: Num a => Maybe a -> a

Traversable Maybe

Since: base-2.1

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Maybe a -> f (Maybe b)

sequenceA :: Applicative f => Maybe (f a) -> f (Maybe a)

mapM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)

sequence :: Monad m => Maybe (m a) -> m (Maybe a)

ToJSON1 Maybe

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a -> Value) -> ([a] -> Value) -> Maybe a -> Value

liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [Maybe a] -> Value

liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> Maybe a -> Encoding

liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [Maybe a] -> Encoding

FromJSON1 Maybe

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (Maybe a)

liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [Maybe a]

Alternative Maybe

Since: base-2.1

Defined in GHC.Base

Methods

empty :: Maybe a

(<|>) :: Maybe a -> Maybe a -> Maybe a

some :: Maybe a -> Maybe [a]

many :: Maybe a -> Maybe [a]

MonadPlus Maybe

Since: base-2.1

Defined in GHC.Base

Methods

mzero :: Maybe a

mplus :: Maybe a -> Maybe a -> Maybe a

Eq1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftEq :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool

Ord1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftCompare :: (a -> b -> Ordering) -> Maybe a -> Maybe b -> Ordering

Read1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Maybe a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Maybe a]

Show1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Maybe a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Maybe a] -> ShowS

MonadThrow Maybe

Defined in Control.Monad.Catch

Methods

throwM :: Exception e => e -> Maybe a

Hashable1 Maybe

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a -> Int) -> Int -> Maybe a -> Int

MonadBaseControl Maybe Maybe

Defined in Control.Monad.Trans.Control

Associated Types

type StM Maybe a

Methods

liftBaseWith :: (RunInBase Maybe Maybe -> Maybe a) -> Maybe a

restoreM :: StM Maybe a -> Maybe a

(Selector s, GToJSON' enc arity (K1 i (Maybe a) :: Type -> Type), KeyValuePair enc pairs, Monoid pairs) => RecordToPairs enc pairs arity (S1 s (K1 i (Maybe a) :: Type -> Type))

Defined in Data.Aeson.Types.ToJSON

Methods

recordToPairs :: Options -> ToArgs enc arity a0 -> S1 s (K1 i (Maybe a)) a0 -> pairs

Lift a => Lift (Maybe a :: Type)

Defined in Language.Haskell.TH.Syntax

Methods

lift :: Maybe a -> Q Exp

liftTyped :: Maybe a -> Q (TExp (Maybe a))

(Selector s, FromJSON a) => RecordFromJSON' arity (S1 s (K1 i (Maybe a) :: Type -> Type))

Defined in Data.Aeson.Types.FromJSON

Methods

recordParseJSON' :: (ConName :* (TypeName :* (Options :* FromArgs arity a0))) -> Object -> Parser (S1 s (K1 i (Maybe a)) a0)

Eq a => Eq (Maybe a)

Since: base-2.1

Defined in GHC.Maybe

Methods

(==) :: Maybe a -> Maybe a -> Bool

(/=) :: Maybe a -> Maybe a -> Bool

Ord a => Ord (Maybe a)

Since: base-2.1

Defined in GHC.Maybe

Methods

compare :: Maybe a -> Maybe a -> Ordering

(<) :: Maybe a -> Maybe a -> Bool

(<=) :: Maybe a -> Maybe a -> Bool

(>) :: Maybe a -> Maybe a -> Bool

(>=) :: Maybe a -> Maybe a -> Bool

max :: Maybe a -> Maybe a -> Maybe a

min :: Maybe a -> Maybe a -> Maybe a

Read a => Read (Maybe a)

Since: base-2.1

Defined in GHC.Read

Methods

readsPrec :: Int -> ReadS (Maybe a)

readList :: ReadS [Maybe a]

readPrec :: ReadPrec (Maybe a)

readListPrec :: ReadPrec [Maybe a]

Show a => Show (Maybe a)

Since: base-2.1

Defined in GHC.Show

Methods

showsPrec :: Int -> Maybe a -> ShowS

show :: Maybe a -> String

showList :: [Maybe a] -> ShowS

Generic (Maybe a)

Since: base-4.6.0.0

Defined in GHC.Generics

Associated Types

type Rep (Maybe a) :: Type -> Type

Methods

from :: Maybe a -> Rep (Maybe a) x

to :: Rep (Maybe a) x -> Maybe a

Semigroup a => Semigroup (Maybe a)

Since: base-4.9.0.0

Defined in GHC.Base

Methods

(<>) :: Maybe a -> Maybe a -> Maybe a

sconcat :: NonEmpty (Maybe a) -> Maybe a

stimes :: Integral b => b -> Maybe a -> Maybe a

Semigroup a => Monoid (Maybe a)

Lift a semigroup into Maybe forming a Monoid according to http://en.wikipedia.org/wiki/Monoid: "Any semigroup S may be turned into a monoid simply by adjoining an element e not in S and defining e*e = e and e*s = s = s*e for all s ∈ S."

Since 4.11.0: constraint on inner a value generalised from Monoid to Semigroup.

Since: base-2.1

Defined in GHC.Base

Methods

mempty :: Maybe a

mappend :: Maybe a -> Maybe a -> Maybe a

mconcat :: [Maybe a] -> Maybe a

Hashable a => Hashable (Maybe a)

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Maybe a -> Int

hash :: Maybe a -> Int

ToJSON a => ToJSON (Maybe a)

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Maybe a -> Value

toEncoding :: Maybe a -> Encoding

toJSONList :: [Maybe a] -> Value

toEncodingList :: [Maybe a] -> Encoding

FromJSON a => FromJSON (Maybe a)

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Maybe a)

parseJSONList :: Value -> Parser [Maybe a]

Outputable a => Outputable (Maybe a)

Defined in Outputable

Methods

ppr :: Maybe a -> SDoc

pprPrec :: Rational -> Maybe a -> SDoc

MonoFunctor (Maybe a)

Defined in Data.MonoTraversable

Methods

omap :: (Element (Maybe a) -> Element (Maybe a)) -> Maybe a -> Maybe a

MonoFoldable (Maybe a)

Defined in Data.MonoTraversable

Methods

ofoldMap :: Monoid m => (Element (Maybe a) -> m) -> Maybe a -> m

ofoldr :: (Element (Maybe a) -> b -> b) -> b -> Maybe a -> b

ofoldl' :: (a0 -> Element (Maybe a) -> a0) -> a0 -> Maybe a -> a0

otoList :: Maybe a -> [Element (Maybe a)]

oall :: (Element (Maybe a) -> Bool) -> Maybe a -> Bool

oany :: (Element (Maybe a) -> Bool) -> Maybe a -> Bool

onull :: Maybe a -> Bool

olength :: Maybe a -> Int

olength64 :: Maybe a -> Int64

ocompareLength :: Integral i => Maybe a -> i -> Ordering

otraverse_ :: Applicative f => (Element (Maybe a) -> f b) -> Maybe a -> f ()

ofor_ :: Applicative f => Maybe a -> (Element (Maybe a) -> f b) -> f ()

omapM_ :: Applicative m => (Element (Maybe a) -> m ()) -> Maybe a -> m ()

oforM_ :: Applicative m => Maybe a -> (Element (Maybe a) -> m ()) -> m ()

ofoldlM :: Monad m => (a0 -> Element (Maybe a) -> m a0) -> a0 -> Maybe a -> m a0

ofoldMap1Ex :: Semigroup m => (Element (Maybe a) -> m) -> Maybe a -> m

ofoldr1Ex :: (Element (Maybe a) -> Element (Maybe a) -> Element (Maybe a)) -> Maybe a -> Element (Maybe a)

ofoldl1Ex' :: (Element (Maybe a) -> Element (Maybe a) -> Element (Maybe a)) -> Maybe a -> Element (Maybe a)

headEx :: Maybe a -> Element (Maybe a)

lastEx :: Maybe a -> Element (Maybe a)

unsafeHead :: Maybe a -> Element (Maybe a)

unsafeLast :: Maybe a -> Element (Maybe a)

maximumByEx :: (Element (Maybe a) -> Element (Maybe a) -> Ordering) -> Maybe a -> Element (Maybe a)

minimumByEx :: (Element (Maybe a) -> Element (Maybe a) -> Ordering) -> Maybe a -> Element (Maybe a)

oelem :: Element (Maybe a) -> Maybe a -> Bool

onotElem :: Element (Maybe a) -> Maybe a -> Bool

MonoTraversable (Maybe a)

Defined in Data.MonoTraversable

Methods

otraverse :: Applicative f => (Element (Maybe a) -> f (Element (Maybe a))) -> Maybe a -> f (Maybe a)

omapM :: Applicative m => (Element (Maybe a) -> m (Element (Maybe a))) -> Maybe a -> m (Maybe a)

MonoPointed (Maybe a)

Defined in Data.MonoTraversable

Methods

opoint :: Element (Maybe a) -> Maybe a

Validity a => Validity (Maybe a)

A Maybe thing is valid if the thing inside is valid or it's nothing It makes sense to assume that Nothing is valid. If Nothing wasn't valid, you wouldn't have used a Maybe in the datastructure.

Defined in Data.Validity

Methods

validate :: Maybe a -> Validation

Pretty a => Pretty (Maybe a)

Defined in Text.PrettyPrint.Annotated.WL

Methods

pretty :: Maybe a -> Doc b

prettyList :: [Maybe a] -> Doc b

SingKind a => SingKind (Maybe a)

Since: base-4.9.0.0

Defined in GHC.Generics

Associated Types

type DemoteRep (Maybe a)

Methods

fromSing :: forall (a0 :: Maybe a). Sing a0 -> DemoteRep (Maybe a)

Generic1 Maybe

Since: base-4.6.0.0

Defined in GHC.Generics

Associated Types

type Rep1 Maybe :: k -> Type

Methods

from1 :: forall (a :: k). Maybe a -> Rep1 Maybe a

to1 :: forall (a :: k). Rep1 Maybe a -> Maybe a

SingI ('Nothing :: Maybe a)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

sing :: Sing 'Nothing

SingI a2 => SingI ('Just a2 :: Maybe a1)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

sing :: Sing ('Just a2)

type StM Maybe a

Defined in Control.Monad.Trans.Control

type StM Maybe a = a

type Rep (Maybe a)

Defined in GHC.Generics

type Rep (Maybe a) = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))

type Element (Maybe a)

Defined in Data.MonoTraversable

type Element (Maybe a) = a

type DemoteRep (Maybe a)

Defined in GHC.Generics

type DemoteRep (Maybe a) = Maybe (DemoteRep a)

data Sing (b :: Maybe a)

Defined in GHC.Generics

data Sing (b :: Maybe a) where

• SNothing :: forall a. Sing ('Nothing :: Maybe a)
• SJust :: forall a (a1 :: a). Sing a1 -> Sing ('Just a1)

type Rep1 Maybe

Defined in GHC.Generics

type Rep1 Maybe = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1))

Common Helpers

withDefault :: a -> Maybe a -> a

Provide a default value, turning an optional value into a normal value. This comes in handy when paired with functions like get which gives back a Maybe.

>>> withDefault 100 (Just 42)
42
>>> withDefault 100 Nothing
100
>>> withDefault "unknown" (Dict.get "Tom" Dict.empty)
"unknown"

Note: This can be overused! Many cases are better handled by a case expression. And if you end up using withDefault a lot, it can be a good sign that a custom type will clean your code up quite a bit!

map :: (a -> b) -> Maybe a -> Maybe b

Transform a Maybe value with a given function:

>>> map sqrt (Just 9)
Just 3
>>> map sqrt Nothing
Nothing
>>> map sqrt (String.toFloat "9")
Just 3
>>> map sqrt (String.toFloat "x")
Nothing

map2 :: (a -> b -> value) -> Maybe a -> Maybe b -> Maybe value

Apply a function if all the arguments are Just a value.

>>> map2 (+) (Just 3) (Just 4)
Just 7
>>> map2 (+) (Just 3) Nothing
Nothing
>>> map2 (+) Nothing (Just 4)
Nothing
>>> map2 (+) (String.toInt "1") (String.toInt "123")
Just 124
>>> map2 (+) (String.toInt "x") (String.toInt "123")
Nothing
>>> map2 (+) (String.toInt "1") (String.toInt "1.3")
Nothing

map3 :: (a -> b -> c -> value) -> Maybe a -> Maybe b -> Maybe c -> Maybe value

map4 :: (a -> b -> c -> d -> value) -> Maybe a -> Maybe b -> Maybe c -> Maybe d -> Maybe value

map5 :: (a -> b -> c -> d -> e -> value) -> Maybe a -> Maybe b -> Maybe c -> Maybe d -> Maybe e -> Maybe value

Chaining Maybes

andThen :: (a -> Maybe b) -> Maybe a -> Maybe b

Chain together many computations that may fail. It is helpful to see an equivalent definition:

andThen :: (a -> Maybe b) -> Maybe a -> Maybe b
andThen callback maybe =
  case maybe of
    Just value ->
      callback value

    Nothing ->
      Nothing

This means we only continue with the callback if things are going well. For example, say you need to parse some user input as a month:

parseMonth :: String -> Maybe Int
parseMonth userInput =
  String.toInt userInput
    |> andThen toValidMonth

toValidMonth :: Int -> Maybe Int
toValidMonth month =
  if 1 <= month && month <= 12 then
    Just month

  else
    Nothing

In the parseMonth' function, if 'String.toInt produces Nothing (because the userInput was not an integer) this entire chain of operations will short-circuit and result in Nothing. If toValidMonth results in Nothing, again the chain of computations will result in Nothing.