Safe Haskell	Safe-Inferred
Language	Haskell2010

Prolude.Maybe

Synopsis

data Maybe a
- = Nothing
- | Just a
maybeToList :: Maybe a -> [a]
maybe :: b -> (a -> b) -> Maybe a -> b
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
listToMaybe :: [a] -> Maybe a
isNothing :: Maybe a -> Bool
isJust :: Maybe a -> Bool
fromMaybe :: a -> Maybe a -> a
catMaybes :: [Maybe a] -> [a]
note :: a -> Maybe b -> Either a b
hush :: Either a b -> Maybe b

Documentation

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

Arbitrary1 Maybe
Instance details Defined in Test.QuickCheck.Arbitrary Methods liftArbitrary :: Gen a -> Gen (Maybe a) # liftShrink :: (a -> [a]) -> Maybe a -> [Maybe a] #
FromJSON1 Maybe
Instance details 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] #
ToJSON1 Maybe
Instance details 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 #
MonadFail Maybe	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> Maybe a #
Foldable Maybe	Since: base-2.1
Instance details 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
Instance details 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) #
Alternative Maybe	Since: base-2.1
Instance details Defined in GHC.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.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.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.Base Methods (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b # (>>) :: Maybe a -> Maybe b -> Maybe b # return :: a -> Maybe a #
MonadPlus Maybe	Since: base-2.1
Instance details Defined in GHC.Base Methods mzero :: Maybe a # mplus :: Maybe a -> Maybe a -> Maybe a #
MonadFailure Maybe
Instance details Defined in Basement.Monad Associated Types type Failure Maybe # Methods mFail :: Failure Maybe -> Maybe () #
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 :: Exception e => e -> 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.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 #
MonadBaseControl Maybe Maybe
Instance details 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 LiftedRep -> Type), KeyValuePair enc pairs, Monoid pairs) => RecordToPairs enc pairs arity (S1 s (K1 i (Maybe a) :: TYPE LiftedRep -> Type))
Instance details Defined in Data.Aeson.Types.ToJSON Methods recordToPairs :: Options -> ToArgs enc arity a0 -> S1 s (K1 i (Maybe a)) a0 -> pairs
(PersistEntity rec, PersistField typ, SymbolToField sym rec typ) => HasField (sym :: Symbol) (SqlExpr (Maybe (Entity rec))) (SqlExpr (Value (Maybe typ)))	This instance allows you to use `record.field` notation with GC 9.2's `OverloadedRecordDot` extension. Example: -- persistent model: Person name Text BlogPost title Text authorId PersonId -- query: `select` $ do (p :& bp) <- `from` $ `table` `Person leftJoin table`BlogPost `on` do \(p :& bp) -> just p.id ==. bp.authorId pure (p.name, bp.title) The following forms are all equivalent: blogPost :: SqlExpr (Maybe (Entity BlogPost)) blogPost ?. BlogPostTitle blogPost ?. #title blogPost.title Since: esqueleto-3.5.4.0
Instance details Defined in Database.Esqueleto.Internal.Internal Methods getField :: SqlExpr (Maybe (Entity rec)) -> SqlExpr (Value (Maybe typ)) #
(Selector s, FromHttpApiData c) => GFromForm (t :: k) (M1 S s (K1 i (Maybe c) :: Type -> Type))
Instance details Defined in Web.Internal.FormUrlEncoded Methods gFromForm :: Proxy t -> FormOptions -> Form -> Either Text (M1 S s (K1 i (Maybe c)) x) #
(Selector s, ToHttpApiData c) => GToForm (t :: k) (M1 S s (K1 i (Maybe c) :: Type -> Type))
Instance details Defined in Web.Internal.FormUrlEncoded Methods gToForm :: Proxy t -> FormOptions -> M1 S s (K1 i (Maybe c)) x -> Form #
HasAdditionalProperties Schema (Maybe AdditionalProperties)
Instance details Defined in Data.Swagger.Lens Methods additionalProperties :: Lens' Schema (Maybe AdditionalProperties) #
HasAllOf Schema (Maybe [Referenced Schema])
Instance details Defined in Data.Swagger.Lens Methods allOf :: Lens' Schema (Maybe [Referenced Schema]) #
HasAllowEmptyValue ParamOtherSchema (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods allowEmptyValue :: Lens' ParamOtherSchema (Maybe Bool) #
HasAttribute Xml (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods attribute :: Lens' Xml (Maybe Bool) #
HasBasePath Swagger (Maybe FilePath)
Instance details Defined in Data.Swagger.Lens Methods basePath :: Lens' Swagger (Maybe FilePath) #
HasConsumes Operation (Maybe MimeList)
Instance details Defined in Data.Swagger.Lens Methods consumes :: Lens' Operation (Maybe MimeList) #
HasContact Info (Maybe Contact)
Instance details Defined in Data.Swagger.Lens Methods contact :: Lens' Info (Maybe Contact) #
HasDefault Header (Maybe Value)
Instance details Defined in Data.Swagger.Lens Methods default_ :: Lens' Header (Maybe Value) #
HasDefault ParamOtherSchema (Maybe Value)
Instance details Defined in Data.Swagger.Lens Methods default_ :: Lens' ParamOtherSchema (Maybe Value) #
HasDefault Responses (Maybe (Referenced Response))
Instance details Defined in Data.Swagger.Lens Methods default_ :: Lens' Responses (Maybe (Referenced Response)) #
HasDefault Schema (Maybe Value)
Instance details Defined in Data.Swagger.Lens Methods default_ :: Lens' Schema (Maybe Value) #
HasDelete PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods delete :: Lens' PathItem (Maybe Operation) #
HasDeprecated Operation (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods deprecated :: Lens' Operation (Maybe Bool) #
HasDescription ExternalDocs (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' ExternalDocs (Maybe Text) #
HasDescription Header (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' Header (Maybe Text) #
HasDescription Info (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' Info (Maybe Text) #
HasDescription Operation (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' Operation (Maybe Text) #
HasDescription Param (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' Param (Maybe Text) #
HasDescription Schema (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' Schema (Maybe Text) #
HasDescription SecurityScheme (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' SecurityScheme (Maybe Text) #
HasDescription Tag (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods description :: Lens' Tag (Maybe Text) #
HasDiscriminator Schema (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods discriminator :: Lens' Schema (Maybe Text) #
HasEmail Contact (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods email :: Lens' Contact (Maybe Text) #
HasParamSchema s (ParamSchema t) => HasEnum s (Maybe [Value])
Instance details Defined in Data.Swagger.Lens Methods enum_ :: Lens' s (Maybe [Value]) #
HasExample Schema (Maybe Value)
Instance details Defined in Data.Swagger.Lens Methods example :: Lens' Schema (Maybe Value) #
HasExamples Response (Maybe Example)
Instance details Defined in Data.Swagger.Lens Methods examples :: Lens' Response (Maybe Example) #
HasParamSchema s (ParamSchema t) => HasExclusiveMaximum s (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods exclusiveMaximum :: Lens' s (Maybe Bool) #
HasParamSchema s (ParamSchema t) => HasExclusiveMinimum s (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods exclusiveMinimum :: Lens' s (Maybe Bool) #
HasExternalDocs Operation (Maybe ExternalDocs)
Instance details Defined in Data.Swagger.Lens Methods externalDocs :: Lens' Operation (Maybe ExternalDocs) #
HasExternalDocs Schema (Maybe ExternalDocs)
Instance details Defined in Data.Swagger.Lens Methods externalDocs :: Lens' Schema (Maybe ExternalDocs) #
HasExternalDocs Swagger (Maybe ExternalDocs)
Instance details Defined in Data.Swagger.Lens Methods externalDocs :: Lens' Swagger (Maybe ExternalDocs) #
HasExternalDocs Tag (Maybe ExternalDocs)
Instance details Defined in Data.Swagger.Lens Methods externalDocs :: Lens' Tag (Maybe ExternalDocs) #
HasParamSchema s (ParamSchema t) => HasFormat s (Maybe Format)
Instance details Defined in Data.Swagger.Lens Methods format :: Lens' s (Maybe Format) #
HasGet PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods get :: Lens' PathItem (Maybe Operation) #
HasHead PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods head_ :: Lens' PathItem (Maybe Operation) #
HasHost Swagger (Maybe Host)
Instance details Defined in Data.Swagger.Lens Methods host :: Lens' Swagger (Maybe Host) #
HasParamSchema s (ParamSchema t) => HasItems s (Maybe (SwaggerItems t))
Instance details Defined in Data.Swagger.Lens Methods items :: Lens' s (Maybe (SwaggerItems t)) #
HasLicense Info (Maybe License)
Instance details Defined in Data.Swagger.Lens Methods license :: Lens' Info (Maybe License) #
HasParamSchema s (ParamSchema t) => HasMaxItems s (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods maxItems :: Lens' s (Maybe Integer) #
HasParamSchema s (ParamSchema t) => HasMaxLength s (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods maxLength :: Lens' s (Maybe Integer) #
HasMaxProperties Schema (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods maxProperties :: Lens' Schema (Maybe Integer) #
HasParamSchema s (ParamSchema t) => HasMaximum s (Maybe Scientific)
Instance details Defined in Data.Swagger.Lens Methods maximum_ :: Lens' s (Maybe Scientific) #
HasParamSchema s (ParamSchema t) => HasMinItems s (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods minItems :: Lens' s (Maybe Integer) #
HasParamSchema s (ParamSchema t) => HasMinLength s (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods minLength :: Lens' s (Maybe Integer) #
HasMinProperties Schema (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods minProperties :: Lens' Schema (Maybe Integer) #
HasParamSchema s (ParamSchema t) => HasMinimum s (Maybe Scientific)
Instance details Defined in Data.Swagger.Lens Methods minimum_ :: Lens' s (Maybe Scientific) #
HasParamSchema s (ParamSchema t) => HasMultipleOf s (Maybe Scientific)
Instance details Defined in Data.Swagger.Lens Methods multipleOf :: Lens' s (Maybe Scientific) #
HasName Contact (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods name :: Lens' Contact (Maybe Text) #
HasName NamedSchema (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods name :: Lens' NamedSchema (Maybe Text) #
HasName Xml (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods name :: Lens' Xml (Maybe Text) #
HasNamespace Xml (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods namespace :: Lens' Xml (Maybe Text) #
HasOperationId Operation (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods operationId :: Lens' Operation (Maybe Text) #
HasOptions PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods options :: Lens' PathItem (Maybe Operation) #
HasPatch PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods patch :: Lens' PathItem (Maybe Operation) #
HasParamSchema s (ParamSchema t) => HasPattern s (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods pattern :: Lens' s (Maybe Text) #
HasPort Host (Maybe PortNumber)
Instance details Defined in Data.Swagger.Lens Methods port :: Lens' Host (Maybe PortNumber) #
HasPost PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods post :: Lens' PathItem (Maybe Operation) #
HasPrefix Xml (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods prefix :: Lens' Xml (Maybe Text) #
HasProduces Operation (Maybe MimeList)
Instance details Defined in Data.Swagger.Lens Methods produces :: Lens' Operation (Maybe MimeList) #
HasPut PathItem (Maybe Operation)
Instance details Defined in Data.Swagger.Lens Methods put :: Lens' PathItem (Maybe Operation) #
HasReadOnly Schema (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods readOnly :: Lens' Schema (Maybe Bool) #
HasRequired Param (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods required :: Lens' Param (Maybe Bool) #
HasSchema Response (Maybe (Referenced Schema))
Instance details Defined in Data.Swagger.Lens Methods schema :: Lens' Response (Maybe (Referenced Schema)) #
HasSchemes Operation (Maybe [Scheme])
Instance details Defined in Data.Swagger.Lens Methods schemes :: Lens' Operation (Maybe [Scheme]) #
HasSchemes Swagger (Maybe [Scheme])
Instance details Defined in Data.Swagger.Lens Methods schemes :: Lens' Swagger (Maybe [Scheme]) #
HasSummary Operation (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods summary :: Lens' Operation (Maybe Text) #
HasTermsOfService Info (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods termsOfService :: Lens' Info (Maybe Text) #
HasTitle Schema (Maybe Text)
Instance details Defined in Data.Swagger.Lens Methods title :: Lens' Schema (Maybe Text) #
HasType Header (Maybe (SwaggerType ('SwaggerKindNormal Header)))
Instance details Defined in Data.Swagger.Lens Methods type_ :: Lens' Header (Maybe (SwaggerType ('SwaggerKindNormal Header))) #
HasType NamedSchema (Maybe (SwaggerType ('SwaggerKindSchema :: SwaggerKind Type)))
Instance details Defined in Data.Swagger.Lens Methods type_ :: Lens' NamedSchema (Maybe (SwaggerType 'SwaggerKindSchema)) #
HasType ParamOtherSchema (Maybe (SwaggerType ('SwaggerKindParamOtherSchema :: SwaggerKind Type)))
Instance details Defined in Data.Swagger.Lens Methods type_ :: Lens' ParamOtherSchema (Maybe (SwaggerType 'SwaggerKindParamOtherSchema)) #
HasType Schema (Maybe (SwaggerType ('SwaggerKindSchema :: SwaggerKind Type)))
Instance details Defined in Data.Swagger.Lens Methods type_ :: Lens' Schema (Maybe (SwaggerType 'SwaggerKindSchema)) #
HasParamSchema s (ParamSchema t) => HasUniqueItems s (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods uniqueItems :: Lens' s (Maybe Bool) #
HasUrl Contact (Maybe URL)
Instance details Defined in Data.Swagger.Lens Methods url :: Lens' Contact (Maybe URL) #
HasUrl License (Maybe URL)
Instance details Defined in Data.Swagger.Lens Methods url :: Lens' License (Maybe URL) #
HasWrapped Xml (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods wrapped :: Lens' Xml (Maybe Bool) #
HasXml Schema (Maybe Xml)
Instance details Defined in Data.Swagger.Lens Methods xml :: Lens' Schema (Maybe Xml) #
Lift a => Lift (Maybe a :: Type)
Instance details Defined in Language.Haskell.TH.Syntax Methods lift :: Quote m => Maybe a -> m Exp # liftTyped :: forall (m :: Type -> Type). Quote m => Maybe a -> Code m (Maybe a) #
(Selector s, FromJSON a) => RecordFromJSON' arity (S1 s (K1 i (Maybe a) :: Type -> Type))
Instance details Defined in Data.Aeson.Types.FromJSON Methods recordParseJSON' :: (ConName :* (TypeName :* (Options :* FromArgs arity a0))) -> Object -> Parser (S1 s (K1 i (Maybe a)) a0)
Arbitrary a => Arbitrary (Maybe a)
Instance details Defined in Test.QuickCheck.Arbitrary Methods arbitrary :: Gen (Maybe a) # shrink :: Maybe a -> [Maybe a] #
CoArbitrary a => CoArbitrary (Maybe a)
Instance details Defined in Test.QuickCheck.Arbitrary Methods coarbitrary :: Maybe a -> Gen b -> Gen b #
FromJSON a => FromJSON (Maybe a)
Instance details Defined in Data.Aeson.Types.FromJSON Methods parseJSON :: Value -> Parser (Maybe a) # parseJSONList :: Value -> Parser [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 #
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 `ee = e` and `es = s = se` 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.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.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.Generics Associated Types type Rep (Maybe a) :: Type -> Type # 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.Generics Associated Types type DemoteRep (Maybe 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.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.Show Methods showsPrec :: Int -> Maybe a -> ShowS # show :: Maybe a -> String # showList :: [Maybe a] -> ShowS #
Val a => Val (Maybe a)
Instance details Defined in Data.Bson Methods val :: Maybe a -> Value # valList :: [Maybe a] -> Value # valMaybe :: Maybe (Maybe a) -> Value # cast' :: Value -> Maybe (Maybe a) # cast'List :: Value -> Maybe [Maybe a] # cast'Maybe :: Value -> Maybe (Maybe (Maybe a)) #
FromField a => FromField (Maybe a)	`Nothing` if the `Field` is `empty`, `Just` otherwise.
Instance details Defined in Data.Csv.Conversion Methods parseField :: Field -> Parser (Maybe a) #
ToField a => ToField (Maybe a)	`Nothing` is encoded as an `empty` field.
Instance details Defined in Data.Csv.Conversion Methods toField :: Maybe a -> Field #
NFData a => NFData (Maybe a)
Instance details Defined in Control.DeepSeq Methods rnf :: Maybe a -> () #
ToMaybe (SqlExpr (Maybe a))
Instance details Defined in Database.Esqueleto.Experimental.ToMaybe Associated Types type ToMaybeT (SqlExpr (Maybe a)) # Methods toMaybe :: SqlExpr (Maybe a) -> ToMaybeT (SqlExpr (Maybe a)) #
FromPreprocess (SqlExpr (Maybe (Entity val))) => From (SqlExpr (Maybe (Entity val)))
Instance details Defined in Database.Esqueleto.Internal.Internal Methods from_ :: SqlQuery (SqlExpr (Maybe (Entity val))) #
(PersistEntity val, BackendCompatible SqlBackend (PersistEntityBackend val)) => FromPreprocess (SqlExpr (Maybe (Entity val)))
Instance details Defined in Database.Esqueleto.Internal.Internal Methods fromPreprocess :: SqlQuery (PreprocessedFrom (SqlExpr (Maybe (Entity val)))) #
SqlString a => SqlString (Maybe a)	Since: esqueleto-2.4.0
Instance details Defined in Database.Esqueleto.Internal.Internal
Eq a => Eq (Maybe a)	Since: base-2.1
Instance details Defined in GHC.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.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 #
FromHttpApiData a => FromHttpApiData (Maybe a)	`>>>` `parseUrlPiece "Just 123" :: Either Text (Maybe Int)`Right (Just 123)
Instance details Defined in Web.Internal.HttpApiData Methods parseUrlPiece :: Text -> Either Text (Maybe a) # parseHeader :: ByteString -> Either Text (Maybe a) # parseQueryParam :: Text -> Either Text (Maybe a) #
ToHttpApiData a => ToHttpApiData (Maybe a)	`>>>` `toUrlPiece (Just "Hello")`"just Hello"
Instance details Defined in Web.Internal.HttpApiData Methods toUrlPiece :: Maybe a -> Text # toEncodedUrlPiece :: Maybe a -> Builder # toHeader :: Maybe a -> ByteString # toQueryParam :: Maybe a -> Text #
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)) #
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) #
At (Maybe a)
Instance details Defined in Optics.At.Core Methods at :: Index (Maybe a) -> Lens' (Maybe a) (Maybe (IxValue (Maybe a))) #
Ixed (Maybe a)
Instance details Defined in Optics.At.Core Associated Types type IxKind (Maybe a) # Methods ix :: Index (Maybe a) -> Optic' (IxKind (Maybe a)) NoIx (Maybe a) (IxValue (Maybe a)) #
PersistField a => PersistField (Maybe a)
Instance details Defined in Database.Persist.Class.PersistField Methods toPersistValue :: Maybe a -> PersistValue # fromPersistValue :: PersistValue -> Either Text (Maybe a) #
PersistFieldSql a => PersistFieldSql (Maybe a)
Instance details Defined in Database.Persist.Sql.Class Methods sqlType :: Proxy (Maybe a) -> SqlType #
RawSql a => RawSql (Maybe a)	Since: persistent-1.0.1
Instance details Defined in Database.Persist.Sql.Class Methods rawSqlCols :: (Text -> Text) -> Maybe a -> (Int, [Text]) # rawSqlColCountReason :: Maybe a -> String # rawSqlProcessRow :: [PersistValue] -> Either Text (Maybe a) #
MongoRegexSearchable rs => MongoRegexSearchable (Maybe rs)
Instance details Defined in Database.Persist.MongoDB
ToSchema a => ToSchema (Maybe a)
Instance details Defined in Data.Swagger.Internal.Schema Methods declareNamedSchema :: Proxy (Maybe a) -> Declare (Definitions Schema) NamedSchema #
SingI ('Nothing :: Maybe a)	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing 'Nothing
PersistEntity a => SqlSelect (SqlExpr (Maybe (Entity a))) (Maybe (Entity a))	You may return a possibly-`NULL` `Entity` from a `select` query.
Instance details Defined in Database.Esqueleto.Internal.Internal Methods sqlSelectCols :: IdentInfo -> SqlExpr (Maybe (Entity a)) -> (Builder, [PersistValue]) # sqlSelectColCount :: Proxy (SqlExpr (Maybe (Entity a))) -> Int # sqlSelectProcessRow :: [PersistValue] -> Either Text (Maybe (Entity a)) # sqlInsertInto :: IdentInfo -> SqlExpr (Maybe (Entity a)) -> (Builder, [PersistValue]) #
HasDefault (ParamSchema t) (Maybe Value)
Instance details Defined in Data.Swagger.Lens Methods default_ :: Lens' (ParamSchema t) (Maybe Value) #
HasEnum (ParamSchema t) (Maybe [Value])
Instance details Defined in Data.Swagger.Lens Methods enum_ :: Lens' (ParamSchema t) (Maybe [Value]) #
HasExclusiveMaximum (ParamSchema t) (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods exclusiveMaximum :: Lens' (ParamSchema t) (Maybe Bool) #
HasExclusiveMinimum (ParamSchema t) (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods exclusiveMinimum :: Lens' (ParamSchema t) (Maybe Bool) #
HasFormat (ParamSchema t) (Maybe Format)
Instance details Defined in Data.Swagger.Lens Methods format :: Lens' (ParamSchema t) (Maybe Format) #
HasItems (ParamSchema t) (Maybe (SwaggerItems t))
Instance details Defined in Data.Swagger.Lens Methods items :: Lens' (ParamSchema t) (Maybe (SwaggerItems t)) #
HasMaxItems (ParamSchema t) (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods maxItems :: Lens' (ParamSchema t) (Maybe Integer) #
HasMaxLength (ParamSchema t) (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods maxLength :: Lens' (ParamSchema t) (Maybe Integer) #
HasMaximum (ParamSchema t) (Maybe Scientific)
Instance details Defined in Data.Swagger.Lens Methods maximum_ :: Lens' (ParamSchema t) (Maybe Scientific) #
HasMinItems (ParamSchema t) (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods minItems :: Lens' (ParamSchema t) (Maybe Integer) #
HasMinLength (ParamSchema t) (Maybe Integer)
Instance details Defined in Data.Swagger.Lens Methods minLength :: Lens' (ParamSchema t) (Maybe Integer) #
HasMinimum (ParamSchema t) (Maybe Scientific)
Instance details Defined in Data.Swagger.Lens Methods minimum_ :: Lens' (ParamSchema t) (Maybe Scientific) #
HasMultipleOf (ParamSchema t) (Maybe Scientific)
Instance details Defined in Data.Swagger.Lens Methods multipleOf :: Lens' (ParamSchema t) (Maybe Scientific) #
HasPattern (ParamSchema t) (Maybe Pattern)
Instance details Defined in Data.Swagger.Lens Methods pattern :: Lens' (ParamSchema t) (Maybe Pattern) #
HasType (ParamSchema t) (Maybe (SwaggerType t))
Instance details Defined in Data.Swagger.Lens Methods type_ :: Lens' (ParamSchema t) (Maybe (SwaggerType t)) #
HasUniqueItems (ParamSchema t) (Maybe Bool)
Instance details Defined in Data.Swagger.Lens Methods uniqueItems :: Lens' (ParamSchema t) (Maybe Bool) #
SingI a2 => SingI ('Just a2 :: Maybe a1)	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing ('Just a2)
a ~ a' => FindGen ('Match 'INCOHERENT) ('S _fg _coh '(con, i, 'Just s)) (FieldGen s a) gs a'	Matching custom generator for field `s`.
Instance details Defined in Generic.Random.Internal.Generic Methods findGen :: (Proxy ('Match 'INCOHERENT), Proxy ('S _fg _coh '(con, i, 'Just s)), FullGenListOf ('S _fg _coh '(con, i, 'Just s))) -> FieldGen s a -> gs -> Gen a' #
a ~ a' => FindGen ('Match 'INCOHERENT) ('S _fg _coh '('Just c, i, s)) (ConstrGen c i a) gs a'	Matching custom generator for `i`-th field of constructor `c`.
Instance details Defined in Generic.Random.Internal.Generic Methods findGen :: (Proxy ('Match 'INCOHERENT), Proxy ('S _fg _coh '('Just c, i, s)), FullGenListOf ('S _fg _coh '('Just c, i, s))) -> ConstrGen c i a -> gs -> Gen a' #
ToSchema c => GToSchema (K1 i (Maybe c) :: Type -> Type)
Instance details Defined in Data.Swagger.Internal.Schema Methods gdeclareNamedSchema :: SchemaOptions -> Proxy (K1 i (Maybe c)) -> Schema -> Declare (Definitions Schema) NamedSchema #
(Selector s, ToSchema c) => GToSchema (S1 s (K1 i (Maybe c) :: Type -> Type))	Optional record fields.
Instance details Defined in Data.Swagger.Internal.Schema Methods gdeclareNamedSchema :: SchemaOptions -> Proxy (S1 s (K1 i (Maybe c))) -> Schema -> Declare (Definitions Schema) NamedSchema #
type Failure Maybe
Instance details Defined in Basement.Monad type Failure Maybe = ()
type Rep1 Maybe	Since: base-4.6.0.0
Instance details 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))
type StM Maybe a
Instance details Defined in Control.Monad.Trans.Control type StM Maybe a = a
type DemoteRep (Maybe a)
Instance details Defined in GHC.Generics type DemoteRep (Maybe a) = Maybe (DemoteRep a)
type Rep (Maybe a)	Since: base-4.6.0.0
Instance details 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)))
data Sing (b :: Maybe a)
Instance details 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 ToMaybeT (SqlExpr (Maybe a))
Instance details Defined in Database.Esqueleto.Experimental.ToMaybe type ToMaybeT (SqlExpr (Maybe a)) = SqlExpr (Maybe a)
type Code (Maybe a)
Instance details Defined in Generics.SOP.Instances type Code (Maybe a) = '['[] :: [Type], '[a]]
type DatatypeInfoOf (Maybe a)
Instance details Defined in Generics.SOP.Instances type DatatypeInfoOf (Maybe a) = 'ADT "GHC.Maybe" "Maybe" '['Constructor "Nothing", 'Constructor "Just"] '['[] :: [StrictnessInfo], '['StrictnessInfo 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy]]
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
type Index (Maybe a)
Instance details Defined in Optics.At.Core type Index (Maybe a) = ()
type IxKind (Maybe a)
Instance details Defined in Optics.At.Core type IxKind (Maybe a) = An_AffineTraversal
type IxValue (Maybe a)
Instance details Defined in Optics.At.Core type IxValue (Maybe a) = a

maybeToList :: Maybe a -> [a] #

The maybeToList function returns an empty list when given Nothing or a singleton list when given Just.

Examples

Expand

Basic usage:

>>> maybeToList (Just 7)
[7]

>>> maybeToList Nothing
[]

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import Text.Read ( readMaybe )
>>> sum $ maybeToList (readMaybe "3")
3
>>> sum $ maybeToList (readMaybe "")
0

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

The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.

Examples

Expand

Basic usage:

>>> maybe False odd (Just 3)
True

>>> maybe False odd Nothing
False

Read an integer from a string using readMaybe. If we succeed, return twice the integer; that is, apply (*2) to it. If instead we fail to parse an integer, return 0 by default:

>>> import Text.Read ( readMaybe )
>>> maybe 0 (*2) (readMaybe "5")
10
>>> maybe 0 (*2) (readMaybe "")
0

Apply show to a Maybe Int. If we have Just n, we want to show the underlying Int n. But if we have Nothing, we return the empty string instead of (for example) "Nothing":

>>> maybe "" show (Just 5)
"5"
>>> maybe "" show Nothing
""

mapMaybe :: (a -> Maybe b) -> [a] -> [b] #

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

Examples

Expand

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import Text.Read ( readMaybe )
>>> let readMaybeInt = readMaybe :: String -> Maybe Int
>>> mapMaybe readMaybeInt ["1", "Foo", "3"]
[1,3]
>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
[1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3]
[1,2,3]

listToMaybe :: [a] -> Maybe a #

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Examples

Expand

Basic usage:

>>> listToMaybe []
Nothing

>>> listToMaybe [9]
Just 9

>>> listToMaybe [1,2,3]
Just 1

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5]
[5]
>>> maybeToList $ listToMaybe []
[]

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3]
[1]

isNothing :: Maybe a -> Bool #

The isNothing function returns True iff its argument is Nothing.

Examples

Expand

Basic usage:

>>> isNothing (Just 3)
False

>>> isNothing (Just ())
False

>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

isJust :: Maybe a -> Bool #

The isJust function returns True iff its argument is of the form Just _.

Examples

Expand

Basic usage:

>>> isJust (Just 3)
True

>>> isJust (Just ())
True

>>> isJust Nothing
False

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)
True

fromMaybe :: a -> Maybe a -> a #

The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

Examples

Expand

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )
>>> fromMaybe 0 (readMaybe "5")
5
>>> fromMaybe 0 (readMaybe "")
0

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

The catMaybes function takes a list of Maybes and returns a list of all the Just values.

Examples

Expand

Basic usage:

>>> catMaybes [Just 1, Nothing, Just 3]
[1,3]

When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

>>> import Text.Read ( readMaybe )
>>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[Just 1,Nothing,Just 3]
>>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[1,3]

note :: a -> Maybe b -> Either a b Source #

Tag the Nothing value of a Maybe

hush :: Either a b -> Maybe b Source #

Suppress the Left value of an Either