Copyright	(c) Fumiaki Kinoshita 2018
License	BSD3
Maintainer	Fumiaki Kinoshita <fumiexcel@gmail.com>
Safe Haskell	None
Language	Haskell2010

Data.Extensible.Field

Contents

Records and variants
Matching
Key / value
Internal

Description

Flexible records and variants

Synopsis

newtype Field (h :: v -> Type) (kv :: Assoc k v) = Field {
- getField :: h (AssocValue kv)
}
(@=) :: Wrapper h => FieldName k -> Repr h v -> Field h (k :> v)
(<@=>) :: (Functor f, Wrapper h) => FieldName k -> f (Repr h v) -> Comp f (Field h) (k :> v)
(@:>) :: FieldName k -> h v -> Field h (k :> v)
(@==) :: FieldName (k :: Symbol) -> v -> Field Identity (k :> v)
type FieldOptic k = forall kind. forall f p t xs (h :: kind -> Type) (v :: kind). (Extensible f p t, ExtensibleConstr t (Field h) xs (k :> v), Associate k v xs, Labelling k p, Wrapper h) => Optic' p f (t (Field h) xs) (Repr h v)
type FieldName k = Optic' (LabelPhantom k) Proxy (Inextensible (Field Proxy) '[k :> ()]) ()
liftField :: (g (AssocValue kv) -> h (AssocValue kv)) -> Field g kv -> Field h kv
liftField2 :: (f (AssocValue kv) -> g (AssocValue kv) -> h (AssocValue kv)) -> Field f kv -> Field g kv -> Field h kv
type RecordOf h = (:*) (Field h)
type Record = RecordOf Identity
emptyRecord :: Record '[]
type VariantOf h = (:|) (Field h)
type Variant = VariantOf Identity
matchWithField :: (forall x. f x -> g x -> r) -> RecordOf f xs -> VariantOf g xs -> r
matchField :: RecordOf (Match h r) xs -> VariantOf h xs -> r
type family AssocKey (kv :: Assoc k v) :: k where ...
type family AssocValue (kv :: Assoc k v) :: v where ...
class (pk (AssocKey kv), pv (AssocValue kv)) => KeyValue pk pv kv
proxyAssocKey :: proxy kv -> Proxy (AssocKey kv)
proxyAssocValue :: proxy kv -> Proxy (AssocValue kv)
stringAssocKey :: (IsString a, KnownSymbol (AssocKey kv)) => proxy kv -> a
class pk (AssocKey kv) => KeyIs pk kv
class pv (AssocValue kv) => ValueIs pv kv
data LabelPhantom s a b
type family Labelling s p :: Constraint where ...
data Inextensible (h :: k -> Type) (xs :: [k])

Documentation

newtype Field (h :: v -> Type) (kv :: Assoc k v) Source #

A Field h (k ':> v) is h v annotated with the field name k.

Field :: (v -> *) -> Assoc k v -> *

Constructors

Field
Fields getField :: h (AssocValue kv)

Instances

Unbox (h (AssocValue x)) => Vector Vector (Field h x) Source #
Instance details Defined in Data.Extensible.Field Methods basicUnsafeFreeze :: PrimMonad m => Mutable Vector (PrimState m) (Field h x) -> m (Vector (Field h x)) # basicUnsafeThaw :: PrimMonad m => Vector (Field h x) -> m (Mutable Vector (PrimState m) (Field h x)) # basicLength :: Vector (Field h x) -> Int # basicUnsafeSlice :: Int -> Int -> Vector (Field h x) -> Vector (Field h x) # basicUnsafeIndexM :: Monad m => Vector (Field h x) -> Int -> m (Field h x) # basicUnsafeCopy :: PrimMonad m => Mutable Vector (PrimState m) (Field h x) -> Vector (Field h x) -> m () # elemseq :: Vector (Field h x) -> Field h x -> b -> b #
Unbox (h (AssocValue x)) => MVector MVector (Field h x) Source #
Instance details Defined in Data.Extensible.Field Methods basicLength :: MVector s (Field h x) -> Int # basicUnsafeSlice :: Int -> Int -> MVector s (Field h x) -> MVector s (Field h x) # basicOverlaps :: MVector s (Field h x) -> MVector s (Field h x) -> Bool # basicUnsafeNew :: PrimMonad m => Int -> m (MVector (PrimState m) (Field h x)) # basicInitialize :: PrimMonad m => MVector (PrimState m) (Field h x) -> m () # basicUnsafeReplicate :: PrimMonad m => Int -> Field h x -> m (MVector (PrimState m) (Field h x)) # basicUnsafeRead :: PrimMonad m => MVector (PrimState m) (Field h x) -> Int -> m (Field h x) # basicUnsafeWrite :: PrimMonad m => MVector (PrimState m) (Field h x) -> Int -> Field h x -> m () # basicClear :: PrimMonad m => MVector (PrimState m) (Field h x) -> m () # basicSet :: PrimMonad m => MVector (PrimState m) (Field h x) -> Field h x -> m () # basicUnsafeCopy :: PrimMonad m => MVector (PrimState m) (Field h x) -> MVector (PrimState m) (Field h x) -> m () # basicUnsafeMove :: PrimMonad m => MVector (PrimState m) (Field h x) -> MVector (PrimState m) (Field h x) -> m () # basicUnsafeGrow :: PrimMonad m => MVector (PrimState m) (Field h x) -> Int -> m (MVector (PrimState m) (Field h x)) #
(Bits r, FromBits r (h (AssocValue x))) => FromBits r (Field h x) Source #
Instance details Defined in Data.Extensible.Bits Associated Types type BitWidth (Field h x) :: Nat Source # Methods fromBits :: r -> Field h x Source # toBits :: Field h x -> r Source #
Wrapper h => Wrapper (Field h :: Assoc k v -> Type) Source #
Instance details Defined in Data.Extensible.Field Associated Types type Repr (Field h) v :: Type Source # Methods _Wrapper :: (Functor f, Profunctor p) => Optic' p f (Field h v0) (Repr (Field h) v0) Source # wrap :: Repr (Field h) v0 -> Field h v0 Source # unwrap :: Field h v0 -> Repr (Field h) v0 Source #
Forall (KeyValue KnownSymbol (Instance1 ToJSON h)) xs => ToJSON (Nullable (Field h :: Assoc Symbol v -> Type) :* xs) Source #
Instance details Defined in Data.Extensible.Dictionary Methods toJSON :: (Nullable (Field h) :* xs) -> Value # toEncoding :: (Nullable (Field h) :* xs) -> Encoding # toJSONList :: [Nullable (Field h) :* xs] -> Value # toEncodingList :: [Nullable (Field h) :* xs] -> Encoding #
Forall (KeyValue KnownSymbol (Instance1 ToJSON h)) xs => ToJSON ((Field h :: Assoc Symbol v -> Type) :* xs) Source #
Instance details Defined in Data.Extensible.Dictionary Methods toJSON :: (Field h :* xs) -> Value # toEncoding :: (Field h :* xs) -> Encoding # toJSONList :: [Field h :* xs] -> Value # toEncodingList :: [Field h :* xs] -> Encoding #
Forall (KeyValue KnownSymbol (Instance1 FromJSON h)) xs => FromJSON (Nullable (Field h :: Assoc Symbol v -> Type) :* xs) Source #
Instance details Defined in Data.Extensible.Dictionary Methods parseJSON :: Value -> Parser (Nullable (Field h) :* xs) # parseJSONList :: Value -> Parser [Nullable (Field h) :* xs] #
Forall (KeyValue KnownSymbol (Instance1 FromJSON h)) xs => FromJSON ((Field h :: Assoc Symbol v -> Type) :* xs) Source #	`parseJSON Null` is called for missing fields.
Instance details Defined in Data.Extensible.Dictionary Methods parseJSON :: Value -> Parser (Field h :* xs) # parseJSONList :: Value -> Parser [Field h :* xs] #
Forall (KeyValue KnownSymbol (Instance1 FromField h)) xs => FromNamedRecord ((Field h :: Assoc Symbol v -> Type) :* xs) Source #
Instance details Defined in Data.Extensible.Dictionary Methods parseNamedRecord :: NamedRecord -> Parser (Field h :* xs) #
Forall (KeyValue KnownSymbol (Instance1 ToField h)) xs => ToNamedRecord ((Field h :: Assoc Symbol v -> Type) :* xs) Source #
Instance details Defined in Data.Extensible.Dictionary Methods toNamedRecord :: (Field h :* xs) -> NamedRecord #
Bounded (h (AssocValue kv)) => Bounded (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods minBound :: Field h kv # maxBound :: Field h kv #
Enum (h (AssocValue kv)) => Enum (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods succ :: Field h kv -> Field h kv # pred :: Field h kv -> Field h kv # toEnum :: Int -> Field h kv # fromEnum :: Field h kv -> Int # enumFrom :: Field h kv -> [Field h kv] # enumFromThen :: Field h kv -> Field h kv -> [Field h kv] # enumFromTo :: Field h kv -> Field h kv -> [Field h kv] # enumFromThenTo :: Field h kv -> Field h kv -> Field h kv -> [Field h kv] #
Eq (h (AssocValue kv)) => Eq (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods (==) :: Field h kv -> Field h kv -> Bool # (/=) :: Field h kv -> Field h kv -> Bool #
Floating (h (AssocValue kv)) => Floating (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods pi :: Field h kv # exp :: Field h kv -> Field h kv # log :: Field h kv -> Field h kv # sqrt :: Field h kv -> Field h kv # (**) :: Field h kv -> Field h kv -> Field h kv # logBase :: Field h kv -> Field h kv -> Field h kv # sin :: Field h kv -> Field h kv # cos :: Field h kv -> Field h kv # tan :: Field h kv -> Field h kv # asin :: Field h kv -> Field h kv # acos :: Field h kv -> Field h kv # atan :: Field h kv -> Field h kv # sinh :: Field h kv -> Field h kv # cosh :: Field h kv -> Field h kv # tanh :: Field h kv -> Field h kv # asinh :: Field h kv -> Field h kv # acosh :: Field h kv -> Field h kv # atanh :: Field h kv -> Field h kv # log1p :: Field h kv -> Field h kv # expm1 :: Field h kv -> Field h kv # log1pexp :: Field h kv -> Field h kv # log1mexp :: Field h kv -> Field h kv #
Fractional (h (AssocValue kv)) => Fractional (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods (/) :: Field h kv -> Field h kv -> Field h kv # recip :: Field h kv -> Field h kv # fromRational :: Rational -> Field h kv #
Integral (h (AssocValue kv)) => Integral (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods quot :: Field h kv -> Field h kv -> Field h kv # rem :: Field h kv -> Field h kv -> Field h kv # div :: Field h kv -> Field h kv -> Field h kv # mod :: Field h kv -> Field h kv -> Field h kv # quotRem :: Field h kv -> Field h kv -> (Field h kv, Field h kv) # divMod :: Field h kv -> Field h kv -> (Field h kv, Field h kv) # toInteger :: Field h kv -> Integer #
Num (h (AssocValue kv)) => Num (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods (+) :: Field h kv -> Field h kv -> Field h kv # (-) :: Field h kv -> Field h kv -> Field h kv # (*) :: Field h kv -> Field h kv -> Field h kv # negate :: Field h kv -> Field h kv # abs :: Field h kv -> Field h kv # signum :: Field h kv -> Field h kv # fromInteger :: Integer -> Field h kv #
Ord (h (AssocValue kv)) => Ord (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods compare :: Field h kv -> Field h kv -> Ordering # (<) :: Field h kv -> Field h kv -> Bool # (<=) :: Field h kv -> Field h kv -> Bool # (>) :: Field h kv -> Field h kv -> Bool # (>=) :: Field h kv -> Field h kv -> Bool # max :: Field h kv -> Field h kv -> Field h kv # min :: Field h kv -> Field h kv -> Field h kv #
Real (h (AssocValue kv)) => Real (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods toRational :: Field h kv -> Rational #
RealFloat (h (AssocValue kv)) => RealFloat (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods floatRadix :: Field h kv -> Integer # floatDigits :: Field h kv -> Int # floatRange :: Field h kv -> (Int, Int) # decodeFloat :: Field h kv -> (Integer, Int) # encodeFloat :: Integer -> Int -> Field h kv # exponent :: Field h kv -> Int # significand :: Field h kv -> Field h kv # scaleFloat :: Int -> Field h kv -> Field h kv # isNaN :: Field h kv -> Bool # isInfinite :: Field h kv -> Bool # isDenormalized :: Field h kv -> Bool # isNegativeZero :: Field h kv -> Bool # isIEEE :: Field h kv -> Bool # atan2 :: Field h kv -> Field h kv -> Field h kv #
RealFrac (h (AssocValue kv)) => RealFrac (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods properFraction :: Integral b => Field h kv -> (b, Field h kv) # truncate :: Integral b => Field h kv -> b # round :: Integral b => Field h kv -> b # ceiling :: Integral b => Field h kv -> b # floor :: Integral b => Field h kv -> b #
(KnownSymbol k, Wrapper h, Show (Repr h v2)) => Show (Field h (k :> v2)) Source #	Shows in `field @= value` style instead of the derived one.
Instance details Defined in Data.Extensible.Field Methods showsPrec :: Int -> Field h (k :> v2) -> ShowS # show :: Field h (k :> v2) -> String # showList :: [Field h (k :> v2)] -> ShowS #
Generic (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Associated Types type Rep (Field h kv) :: Type -> Type # Methods from :: Field h kv -> Rep (Field h kv) x # to :: Rep (Field h kv) x -> Field h kv #
Semigroup (h (AssocValue kv)) => Semigroup (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods (<>) :: Field h kv -> Field h kv -> Field h kv # sconcat :: NonEmpty (Field h kv) -> Field h kv # stimes :: Integral b => b -> Field h kv -> Field h kv #
Monoid (h (AssocValue kv)) => Monoid (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods mempty :: Field h kv # mappend :: Field h kv -> Field h kv -> Field h kv # mconcat :: [Field h kv] -> Field h kv #
Lift (h (AssocValue x)) => Lift (Field h x) Source #
Instance details Defined in Data.Extensible.Field Methods lift :: Field h x -> Q Exp #
Arbitrary (h (AssocValue kv)) => Arbitrary (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods arbitrary :: Gen (Field h kv) # shrink :: Field h kv -> [Field h kv] #
Hashable (h (AssocValue kv)) => Hashable (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods hashWithSalt :: Int -> Field h kv -> Int # hash :: Field h kv -> Int #
ToJSON (h (AssocValue kv)) => ToJSON (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods toJSON :: Field h kv -> Value # toEncoding :: Field h kv -> Encoding # toJSONList :: [Field h kv] -> Value # toEncodingList :: [Field h kv] -> Encoding #
FromJSON (h (AssocValue kv)) => FromJSON (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods parseJSON :: Value -> Parser (Field h kv) # parseJSONList :: Value -> Parser [Field h kv] #
Storable (h (AssocValue kv)) => Storable (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods sizeOf :: Field h kv -> Int # alignment :: Field h kv -> Int # peekElemOff :: Ptr (Field h kv) -> Int -> IO (Field h kv) # pokeElemOff :: Ptr (Field h kv) -> Int -> Field h kv -> IO () # peekByteOff :: Ptr b -> Int -> IO (Field h kv) # pokeByteOff :: Ptr b -> Int -> Field h kv -> IO () # peek :: Ptr (Field h kv) -> IO (Field h kv) # poke :: Ptr (Field h kv) -> Field h kv -> IO () #
FromField (h (AssocValue kv)) => FromField (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods parseField :: Field0 -> Parser (Field h kv) #
ToField (h (AssocValue kv)) => ToField (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods toField :: Field h kv -> Field0 #
NFData (h (AssocValue kv)) => NFData (Field h kv) Source #
Instance details Defined in Data.Extensible.Field Methods rnf :: Field h kv -> () #
(KnownSymbol k, Pretty (h v2)) => Pretty (Field h (k :> v2)) Source #
Instance details Defined in Data.Extensible.Field Methods pretty :: Field h (k :> v2) -> Doc ann # prettyList :: [Field h (k :> v2)] -> Doc ann #
Unbox (h (AssocValue x)) => Unbox (Field h x) Source #
Instance details Defined in Data.Extensible.Field
data MVector s (Field h x) Source #
Instance details Defined in Data.Extensible.Field data MVector s (Field h x) = MV_Field (MVector s (h (AssocValue x)))
type Repr (Field h :: Assoc k v -> Type) (kv :: Assoc k v) Source #
Instance details Defined in Data.Extensible.Field type Repr (Field h :: Assoc k v -> Type) (kv :: Assoc k v) = Repr h (AssocValue kv)
type Rep (Field h kv) Source #
Instance details Defined in Data.Extensible.Field type Rep (Field h kv) = D1 (MetaData "Field" "Data.Extensible.Field" "extensible-0.5-HtYa3fOZlWC7KXWLoStAWc" True) (C1 (MetaCons "Field" PrefixI True) (S1 (MetaSel (Just "getField") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 (h (AssocValue kv)))))
data Vector (Field h x) Source #
Instance details Defined in Data.Extensible.Field data Vector (Field h x) = V_Field (Vector (h (AssocValue x)))
type BitWidth (Field h x) Source #
Instance details Defined in Data.Extensible.Bits type BitWidth (Field h x) = BitWidth (h (AssocValue x))

(@=) :: Wrapper h => FieldName k -> Repr h v -> Field h (k :> v) infix 1 Source #

Annotate a value by the field name.

foo :: Record '["num" >: Int, "str" >: String]
foo = #num @= 42
  <: #str @= "foo"
  <: nil

(<@=>) :: (Functor f, Wrapper h) => FieldName k -> f (Repr h v) -> Comp f (Field h) (k :> v) infix 1 Source #

Lifted (@=)

foo :: IO (Record '["num" >: Int, "str" >: String])
foo = hsequence
  $ #num <@=> readLn
  <: #str <@=> getLine
  <: nil

(@:>) :: FieldName k -> h v -> Field h (k :> v) infix 1 Source #

Annotate a value by the field name without Wrapper.

(@==) :: FieldName (k :: Symbol) -> v -> Field Identity (k :> v) infix 1 Source #

Kind-monomorphic, unwrapped version of @=

type FieldOptic k = forall kind. forall f p t xs (h :: kind -> Type) (v :: kind). (Extensible f p t, ExtensibleConstr t (Field h) xs (k :> v), Associate k v xs, Labelling k p, Wrapper h) => Optic' p f (t (Field h) xs) (Repr h v) Source #

FieldOptic s is a type of optics that points a field/constructor named s.

The yielding fields can be Lenses for Records and Prisms for Variants.

FieldOptic "foo" = Associate "foo" a xs => Lens' (Record xs) a
FieldOptic "foo" = Associate "foo" a xs => Prism' (Variant xs) a

FieldOptics can be generated using mkField defined in the Data.Extensible.TH module.

type FieldName k = Optic' (LabelPhantom k) Proxy (Inextensible (Field Proxy) '[k :> ()]) () Source #

When you see this type as an argument, it expects a FieldLens. This type is used to resolve the name of the field internally.

liftField :: (g (AssocValue kv) -> h (AssocValue kv)) -> Field g kv -> Field h kv Source #

Lift a function for the content.

liftField2 :: (f (AssocValue kv) -> g (AssocValue kv) -> h (AssocValue kv)) -> Field f kv -> Field g kv -> Field h kv Source #

Lift a function for the content.

Records and variants

type RecordOf h = (:*) (Field h) Source #

The type of records which contain several fields.

RecordOf :: (v -> *) -> [Assoc k v] -> *

type Record = RecordOf Identity Source #

Simple record

emptyRecord :: Record '[] Source #

An empty Record.

type VariantOf h = (:|) (Field h) Source #

The dual of RecordOf

VariantOf :: (v -> *) -> [Assoc k v] -> *

type Variant = VariantOf Identity Source #

Simple variant

Matching

matchWithField :: (forall x. f x -> g x -> r) -> RecordOf f xs -> VariantOf g xs -> r Source #

Select a corresponding field of a variant.

matchField :: RecordOf (Match h r) xs -> VariantOf h xs -> r Source #

Pattern matching on a Variant

Key / value

type family AssocKey (kv :: Assoc k v) :: k where ... Source #

Take the type of the key

Equations

AssocKey (k :> v) = k

type family AssocValue (kv :: Assoc k v) :: v where ... Source #

Take the type of the value

Equations

AssocValue (k :> v) = v

class (pk (AssocKey kv), pv (AssocValue kv)) => KeyValue pk pv kv Source #

Combined constraint for Assoc

Instances

(pk k2, pv v2) => KeyValue (pk :: k1 -> Constraint) (pv :: v1 -> Constraint) (k2 :> v2 :: Assoc k1 v1) Source #
Instance details Defined in Data.Extensible.Field

proxyAssocKey :: proxy kv -> Proxy (AssocKey kv) Source #

Proxy-level AssocKey. This is useful when using symbolVal.

proxyAssocValue :: proxy kv -> Proxy (AssocValue kv) Source #

Proxy-level AssocValue.

stringAssocKey :: (IsString a, KnownSymbol (AssocKey kv)) => proxy kv -> a Source #

Get a string from a proxy of Assoc Symbol v.

class pk (AssocKey kv) => KeyIs pk kv Source #

Constraint applied to AssocKey

Instances

pk k2 => KeyIs (pk :: k1 -> Constraint) (k2 :> v2 :: Assoc k1 v1) Source #
Instance details Defined in Data.Extensible.Field

class pv (AssocValue kv) => ValueIs pv kv Source #

Constraint applied to AssocValue

Instances

pv v2 => ValueIs (pv :: v1 -> Constraint) (k2 :> v2 :: Assoc k1 v1) Source #
Instance details Defined in Data.Extensible.Field

Internal

data LabelPhantom s a b Source #

A ghostly type which spells the field name

Instances

Profunctor (LabelPhantom s :: Type -> Type -> Type) Source #

Instance details

Defined in Data.Extensible.Field

Methods

dimap :: (a -> b) -> (c -> d) -> LabelPhantom s b c -> LabelPhantom s a d #

lmap :: (a -> b) -> LabelPhantom s b c -> LabelPhantom s a c #

rmap :: (b -> c) -> LabelPhantom s a b -> LabelPhantom s a c #

(#.) :: Coercible c b => q b c -> LabelPhantom s a b -> LabelPhantom s a c #

(.#) :: Coercible b a => LabelPhantom s b c -> q a b -> LabelPhantom s a c #

type family Labelling s p :: Constraint where ... Source #

Signifies a field name internally

Equations

Labelling s (LabelPhantom t) = s ~ t
Labelling s p = ()

data Inextensible (h :: k -> Type) (xs :: [k]) Source #

The trivial inextensible data type

Instances

(Functor f, Profunctor p) => Extensible f p (Inextensible :: (k -> Type) -> [k] -> Type) Source #
Instance details Defined in Data.Extensible.Field Associated Types type ExtensibleConstr Inextensible h xs x :: Constraint Source # Methods pieceAt :: ExtensibleConstr Inextensible h xs x => Membership xs x -> Optic' p f (Inextensible h xs) (h x) Source #
type ExtensibleConstr (Inextensible :: (k -> Type) -> [k] -> Type) (h :: k -> Type) (xs :: [k]) (x :: k) Source #
Instance details Defined in Data.Extensible.Field type ExtensibleConstr (Inextensible :: (k -> Type) -> [k] -> Type) (h :: k -> Type) (xs :: [k]) (x :: k) = ()