Opaleye.MaybeFields

MaybeFields is Opaleye's analogue to Maybe. You probably won't want to create values of type MaybeFields directly; instead they will appear as the result of left/right/outer join-like operations, such as optionalRestrict and optional.

MaybeFields type

data MaybeFields fields

The Opaleye analogue of Maybe. A value of type MaybeFields a either contains a value of type a, or it contains nothing.

Instances

Monad MaybeFields

Defined in Opaleye.Internal.MaybeFields

Methods

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

(>>) :: MaybeFields a -> MaybeFields b -> MaybeFields b

return :: a -> MaybeFields a

Functor MaybeFields

Defined in Opaleye.Internal.MaybeFields

Methods

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

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

Applicative MaybeFields

Defined in Opaleye.Internal.MaybeFields

Methods

pure :: a -> MaybeFields a

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

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

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

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

Default Unpackspec a b => Default Unpackspec (MaybeFields a) (MaybeFields b)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: Unpackspec (MaybeFields a) (MaybeFields b)

Default (WithNulls Binaryspec) a b => Default Binaryspec (MaybeFields a) (MaybeFields b)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: Binaryspec (MaybeFields a) (MaybeFields b)

Default IfPP a b => Default IfPP (MaybeFields a) (MaybeFields b)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: IfPP (MaybeFields a) (MaybeFields b)

Default EqPP a b => Default EqPP (MaybeFields a) (MaybeFields b)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: EqPP (MaybeFields a) (MaybeFields b)

Default FromFields fields haskells => Default FromFields (MaybeFields fields) (Maybe haskells)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: FromFields (MaybeFields fields) (Maybe haskells)

Default Valuesspec a b => Default Valuesspec (MaybeFields a) (MaybeFields b)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: Valuesspec (MaybeFields a) (MaybeFields b)

(Default ToFields a b, Default Nullspec a b) => Default ToFields (Maybe a) (MaybeFields b)

Defined in Opaleye.Internal.MaybeFields

Methods

def :: ToFields (Maybe a) (MaybeFields b)

Default (WithNulls Distinctspec) a b => Default Distinctspec (MaybeFields a) (MaybeFields b)

Defined in Opaleye.Internal.Distinct

Methods

def :: Distinctspec (MaybeFields a) (MaybeFields b)

(Default (Inferrable FromFields) fields haskells, Maybe haskells ~ maybe_haskells) => Default (Inferrable FromFields) (MaybeFields fields) maybe_haskells

Defined in Opaleye.Internal.MaybeFields

Methods

def :: Inferrable FromFields (MaybeFields fields) maybe_haskells

Creating a MaybeFields

nothingFields :: Default Nullspec a a => MaybeFields a

The Opaleye analogue of Nothing.

nothingFieldsOfTypeOf :: a -> MaybeFields a

The Opaleye analogue of const Nothing. Can be useful to avoid type inference problems, because it doesn't pick up a type class constraint.

justFields :: a -> MaybeFields a

The Opaleye analogue of Just. Equivalent to pure.

nullableToMaybeFields :: FieldNullable a -> MaybeFields (Field a)

Convert NULL to nothingFields and non-NULL to a justFields

Using a MaybeFields

matchMaybe :: Default IfPP b b => MaybeFields a -> (Maybe a -> b) -> b

Use a Haskell \case expression to pattern match on a MaybeFields.

example :: MaybeFields (Field SqlInt4) -> Field SqlInt4
example mf = matchMaybe mf $ \case
  Nothing -> 0
  Just x  -> x * 100

fromMaybeFields :: Default IfPP b b => b -> MaybeFields b -> b

The Opaleye analogue of fromMaybe

maybeFields :: Default IfPP b b => b -> (a -> b) -> MaybeFields a -> b

The Opaleye analogue of maybe

maybeFieldsToNullable :: MaybeFields (Field a) -> FieldNullable a

Convert nothingFields to NULL to a justFields to non-NULL

Creating a Select which returns MaybeFields

optional

Arguments

:: Default Unpackspec a a

=> SelectArr i a

Input query

-> SelectArr i (MaybeFields a)

The rows of the input query wrapped in "Just", unless the input query has no rows in which case a single row of "Nothing"

Convenient access to lateral left/right join functionality. Performs a LATERAL LEFT JOIN under the hood and has behaviour equivalent to the following Haskell function:

optional :: [a] -> [Maybe a]
optional q = case q of
    [] -> [Nothing]
    xs -> map Just xs

That is, if q :: SelectArr i a returns no rows, optional q :: SelectArr i (MaybeFields a) returns exactly one "Nothing" row. Otherwise, optional q returns exactly the rows of q wrapped in "Just". For example,

> let l1 = ["one", "two", "three"] :: [Field SqlText]
> runSelectI conn (optional (values l1))
[Just "one", Just "two", Just "three"]

> let l2 = [] :: [Field SqlText]
> runSelectI conn (optional (values l2))
[Nothing]

optionalRestrict is a special case of optional and could be written in terms of optional as follows (except that optionalRestrict doesn't use LATERAL under the hood and optional does).

optionalRestrict q = optional $ proc cond -> do
  a <- q -< ()
  restrict -< cond a
  returnA -< a

traverseMaybeFields

Arguments

:: (Default Unpackspec a a, Default Unpackspec b b)

=> SelectArr a b

-> SelectArr (MaybeFields a) (MaybeFields b)

traverseMaybeFields is analogous to Haskell's traverse :: (a -> [b]) -> Maybe a -> [Maybe b]. In particular, traverse has the following definition that generalises to traverseMaybeFields:

• traverse _ Nothing = pure Nothing

• traverse f (Just x) = fmap Just (f x)

Using a Select which returns MaybeFields

catMaybeFields :: SelectArr i (MaybeFields a) -> SelectArr i a

The Opaleye analogue of catMaybes. Most commonly you will want to use this at type

catMaybeFields :: Select (MaybeFields a) -> Select a

maybeFieldsToSelect :: SelectArr (MaybeFields a) a

The Opaleye analogue of maybeToList. Unless you are using arrow notation you'll probably find catMaybeFields easier to use.

Adaptors

data Nullspec fields fields'

Instances

Profunctor Nullspec

Defined in Opaleye.Internal.Values

Methods

dimap :: (a -> b) -> (c -> d) -> Nullspec b c -> Nullspec a d

lmap :: (a -> b) -> Nullspec b c -> Nullspec a c

rmap :: (b -> c) -> Nullspec a b -> Nullspec a c

(#.) :: forall a b c q. Coercible c b => q b c -> Nullspec a b -> Nullspec a c

(.#) :: forall a b c q. Coercible b a => Nullspec b c -> q a b -> Nullspec a c

ProductProfunctor Nullspec

Defined in Opaleye.Internal.Values

Methods

purePP :: b -> Nullspec a b

(****) :: Nullspec a (b -> c) -> Nullspec a b -> Nullspec a c

empty :: Nullspec () ()

(***!) :: Nullspec a b -> Nullspec a' b' -> Nullspec (a, a') (b, b')

IsSqlType b => Default Nullspec a (Field_ n b)

Defined in Opaleye.Internal.Values

Methods

def :: Nullspec a (Field_ n b)

Functor (Nullspec a)

Defined in Opaleye.Internal.Values

Methods

fmap :: (a0 -> b) -> Nullspec a a0 -> Nullspec a b

(<$) :: a0 -> Nullspec a b -> Nullspec a a0

Applicative (Nullspec a)

Defined in Opaleye.Internal.Values

Methods

pure :: a0 -> Nullspec a a0

(<*>) :: Nullspec a (a0 -> b) -> Nullspec a a0 -> Nullspec a b

liftA2 :: (a0 -> b -> c) -> Nullspec a a0 -> Nullspec a b -> Nullspec a c

(*>) :: Nullspec a a0 -> Nullspec a b -> Nullspec a b

(<*) :: Nullspec a a0 -> Nullspec a b -> Nullspec a a0

nullspecField :: IsSqlType b => Nullspec a (Field_ n b)

nullspecMaybeFields :: Nullspec a b -> Nullspec (MaybeFields a) (MaybeFields b)

nullspecList :: Nullspec a [b]

nullspecEitherLeft :: Nullspec a b -> Nullspec a (Either b b')

nullspecEitherRight :: Nullspec a b' -> Nullspec a (Either b b')

binaryspecMaybeFields :: WithNulls Binaryspec a b -> Binaryspec (MaybeFields a) (MaybeFields b)

distinctspecMaybeFields :: WithNulls Distinctspec a b -> Distinctspec (MaybeFields a) (MaybeFields b)

fromFieldsMaybeFields :: FromFields fields haskells -> FromFields (MaybeFields fields) (Maybe haskells)

toFieldsMaybeFields :: Nullspec a b -> ToFields a b -> ToFields (Maybe a) (MaybeFields b)

unpackspecMaybeFields :: Unpackspec a b -> Unpackspec (MaybeFields a) (MaybeFields b)

valuesspecMaybeFields :: Valuesspec a b -> Valuesspec (MaybeFields a) (MaybeFields b)

Explicit versions

nothingFieldsExplicit :: Nullspec a b -> MaybeFields b

fromMaybeFieldsExplicit :: IfPP b b -> b -> MaybeFields b -> b

maybeFieldsExplicit :: IfPP b b' -> b -> (a -> b) -> MaybeFields a -> b'

optionalExplicit :: Unpackspec a a -> SelectArr i a -> SelectArr i (MaybeFields a)

traverseMaybeFieldsExplicit :: Unpackspec a a -> Unpackspec b b -> SelectArr a b -> SelectArr (MaybeFields a) (MaybeFields b)