Safe Haskell	None
Language	Haskell2010

Maybe

Contents

Definition
Common Helpers
Chaining Maybes

Description

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.

Synopsis

data Maybe a
- = Nothing
- | Just a
withDefault :: a -> Maybe a -> a
map :: (a -> b) -> Maybe a -> Maybe b
map2 :: (a -> b -> value) -> Maybe a -> Maybe b -> Maybe value
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
andThen :: (a -> Maybe b) -> Maybe a -> Maybe b

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

Instances details

FromJSON1 Maybe #

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

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

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

liftOmittedField :: Maybe a -> Maybe (Maybe a) #

ToJSON1 Maybe #

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

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

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

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

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

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

Eq1 Maybe #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

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

Ord1 Maybe #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

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

Read1 Maybe #

Since: base-4.9.0.0

Instance details

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

Instance details

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 #

NFData1 Maybe #

Since: deepseq-1.4.3.0

Instance details

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> Maybe a -> () #

MonadThrow Maybe #

Instance details

Defined in Control.Monad.Catch

Methods

throwM :: (HasCallStack, Exception e) => e -> Maybe a #

Alternative Maybe #

Picks the leftmost Just value, or, alternatively, Nothing.

Since: base-2.1

Instance details

Defined in GHC.Internal.Base

Methods

empty :: Maybe a #

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

some :: Maybe a -> Maybe [a] #

many :: Maybe a -> Maybe [a] #

Applicative Maybe #

Since: base-2.1

Instance details

Defined in GHC.Internal.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 #

Functor Maybe #

Since: base-2.1

Instance details

Defined in GHC.Internal.Base

Methods

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

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

Monad Maybe #

Since: base-2.1

Instance details

Defined in GHC.Internal.Base

Methods

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

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

return :: a -> Maybe a #

MonadPlus Maybe #

Picks the leftmost Just value, or, alternatively, Nothing.

Since: base-2.1

Instance details

Defined in GHC.Internal.Base

Methods

mzero :: Maybe a #

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

MonadFail Maybe #

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Control.Monad.Fail

Methods

fail :: String -> Maybe a #

Foldable Maybe #

Since: base-2.1

Instance details

Defined in GHC.Internal.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

Instance details

Defined in GHC.Internal.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) #

Hashable1 Maybe #

Instance details

Defined in Data.Hashable.Class

Methods

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

Generic1 Maybe #

Instance details

Defined in GHC.Internal.Generics

Associated Types

type Rep1 Maybe	Since: base-4.6.0.0
Instance details Defined in GHC.Internal.Generics type Rep1 Maybe = D1 ('MetaData "Maybe" "GHC.Internal.Maybe" "ghc-internal" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1))

Methods

from1 :: Maybe a -> Rep1 Maybe a #

to1 :: Rep1 Maybe a -> Maybe a #

MonadBaseControl Maybe Maybe #

Instance details

Defined in Control.Monad.Trans.Control

Associated Types

type StM Maybe a
Instance details Defined in Control.Monad.Trans.Control type StM Maybe a = a

Methods

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

restoreM :: StM Maybe a -> Maybe a #

ReifyFlag TyConBndrVis (Maybe BndrVis)

Instance details

Defined in GHC.Tc.Gen.Splice

Methods

reifyFlag :: TyConBndrVis -> Maybe BndrVis

OutputableP env a => OutputableP env (Maybe a) #

Instance details

Defined in GHC.Utils.Outputable

Methods

pdoc :: env -> Maybe a -> SDoc #

FromJSON a => FromJSON (Maybe a) #

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Maybe a) #

parseJSONList :: Value -> Parser [Maybe a] #

omittedField :: Maybe (Maybe a) #

ToJSON a => ToJSON (Maybe a) #

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Maybe a -> Value #

toEncoding :: Maybe a -> Encoding #

toJSONList :: [Maybe a] -> Value #

toEncodingList :: [Maybe a] -> Encoding #

omitField :: Maybe a -> Bool #

NFData a => NFData (Maybe a) #

Instance details

Defined in Control.DeepSeq

Methods

rnf :: Maybe a -> () #

ToHie a => ToHie (Maybe a)

Instance details

Defined in GHC.Iface.Ext.Ast

Methods

toHie :: Maybe a -> HieM [HieAST Type]

HasLoc a => HasLoc (Maybe a) #

Instance details

Defined in GHC.Parser.Annotation

Methods

getHasLoc :: Maybe a -> SrcSpan #

NoAnn (Maybe a) #

Instance details

Defined in GHC.Parser.Annotation

Methods

noAnn :: Maybe a #

Binary a => Binary (Maybe a) #

Instance details

Defined in GHC.Utils.Binary

Methods

put_ :: WriteBinHandle -> Maybe a -> IO () #

put :: WriteBinHandle -> Maybe a -> IO (Bin (Maybe a)) #

get :: ReadBinHandle -> IO (Maybe a) #

Outputable a => Outputable (Maybe a) #

Instance details

Defined in GHC.Utils.Outputable

Methods

ppr :: Maybe a -> SDoc #

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

Instance details

Defined in GHC.Internal.Base

Methods

mempty :: Maybe a #

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

mconcat :: [Maybe a] -> Maybe a #

Semigroup a => Semigroup (Maybe a) #

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Base

Methods

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

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

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

Generic (Maybe a) #

Instance details

Defined in GHC.Internal.Generics

Associated Types

type Rep (Maybe a)	Since: base-4.6.0.0
Instance details Defined in GHC.Internal.Generics type Rep (Maybe a) = D1 ('MetaData "Maybe" "GHC.Internal.Maybe" "ghc-internal" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))

Methods

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

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

SingKind a => SingKind (Maybe a)

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Associated Types

type DemoteRep (Maybe a)
Instance details Defined in GHC.Internal.Generics type DemoteRep (Maybe a) = Maybe (DemoteRep a)

Methods

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

Read a => Read (Maybe a) #

Since: base-2.1

Instance details

Defined in GHC.Internal.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

Instance details

Defined in GHC.Internal.Show

Methods

showsPrec :: Int -> Maybe a -> ShowS #

show :: Maybe a -> String #

showList :: [Maybe a] -> ShowS #

Eq (DeBruijn a) => Eq (DeBruijn (Maybe a)) #

Instance details

Defined in GHC.Core.Map.Type

Methods

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

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

Eq a => Eq (Maybe a) #

Since: base-2.1

Instance details

Defined in GHC.Internal.Maybe

Methods

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

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

Ord a => Ord (Maybe a) #

Since: base-2.1

Instance details

Defined in GHC.Internal.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 #

Hashable a => Hashable (Maybe a) #

Instance details

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Maybe a -> Int #

hash :: Maybe a -> Int #

At (Maybe a) #

Instance details

Defined in Control.Lens.At

Methods

at :: Index (Maybe a) -> Lens' (Maybe a) (Maybe (IxValue (Maybe a))) #

Ixed (Maybe a) #

Instance details

Defined in Control.Lens.At

Methods

ix :: Index (Maybe a) -> Traversal' (Maybe a) (IxValue (Maybe a)) #

AsEmpty (Maybe a) #

Instance details

Defined in Control.Lens.Empty

Methods

_Empty :: Prism' (Maybe a) () #

HasTypeVars t => HasTypeVars (Maybe t) #

Instance details

Defined in Language.Haskell.TH.Lens

Methods

typeVarsEx :: Set Name -> Traversal' (Maybe t) Name #

MonoFoldable (Maybe a) #

Instance details

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 #

MonoFunctor (Maybe a) #

Instance details

Defined in Data.MonoTraversable

Methods

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

MonoPointed (Maybe a) #

Instance details

Defined in Data.MonoTraversable

Methods

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

MonoTraversable (Maybe a) #

Instance details

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) #

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.

Instance details

Defined in Data.Validity

Methods

validate :: Maybe a -> Validation #

Pretty a => Pretty (Maybe a) #

Instance details

Defined in Text.PrettyPrint.Annotated.WL

Methods

pretty :: Maybe a -> Doc b #

prettyList :: [Maybe a] -> Doc b #

HasType ty orig g => HasTypeProd ty ('Nothing :: Maybe a) (orig :: Symbol) (f :: Type -> Type) (g :: Type -> Type)

Instance details

Defined in GHC.Types.Error.Codes

Methods

getTypeProd :: (f :*: g) a0 -> ty

HasType ty orig f => HasTypeProd ty ('Just l :: Maybe a) (orig :: Symbol) (f :: Type -> Type) (g :: Type -> Type)

Instance details

Defined in GHC.Types.Error.Codes

Methods

getTypeProd :: (f :*: g) a0 -> ty

SingI ('Nothing :: Maybe a)

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

sing :: Sing ('Nothing :: Maybe a)

Each (Maybe a) (Maybe b) a b #

each :: Traversal (Maybe a) (Maybe b) a b

Instance details

Defined in Control.Lens.Each

Methods

each :: Traversal (Maybe a) (Maybe b) a b #

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

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

sing :: Sing ('Just a2)

type Rep1 Maybe #

Since: base-4.6.0.0

Instance details

Defined in GHC.Internal.Generics

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

type StM Maybe a #

Instance details

Defined in Control.Monad.Trans.Control

type StM Maybe a = a

type Anno (Maybe Role) #

Instance details

Defined in GHC.Hs.Decls

type Anno (Maybe Role) = EpAnnCO

type DemoteRep (Maybe a) #

Instance details

Defined in GHC.Internal.Generics

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

type Rep (Maybe a) #

Since: base-4.6.0.0

Instance details

Defined in GHC.Internal.Generics

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

data Sing (b :: Maybe a) #

Instance details

Defined in GHC.Internal.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 Index (Maybe a) #

Instance details

Defined in Control.Lens.At

type Index (Maybe a) = ()

type IxValue (Maybe a) #

Instance details

Defined in Control.Lens.At

type IxValue (Maybe a) = a

type Element (Maybe a) #

Instance details

Defined in Data.MonoTraversable

type Element (Maybe a) = a

Common Helpers

withDefault :: a -> Maybe a -> a Source #

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 Source #

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 Source #

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 Source #

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

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

Chaining Maybes

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

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.