Data.HList.Label3

The HList library

(C) 2004-2006, Oleg Kiselyov, Ralf Laemmel, Keean Schupke

A model of labels as needed for extensible records. As before, all the information about labels is recorded in their type, so the labels of records may be purely phantom. In general, Labels are exclusively type-level entities and have no run-time representation.

Record labels are triplets of type-level naturals, namespace, and description. The namespace part helps avoid confusions between labels from different Haskell modules. The description is an arbitrary nullary type constructor.

For the sake of printing, the description is required to be the instance of Show. One must make sure that the show functions does not examine the value, as descr is purely phantom. Here's an example of the good Label description:

    data MyLabelDescr; instance Show MyLabelDescr where show _ = "descr"

which are automated by makeLabels3 from Data.HList.MakeLabel.

This model even allows the labels in a record to belong to different namespaces. To this end, the model employs the predicate for type equality.

Documentation

>>> let label3 = Label :: Label (Lbl HZero () ())
>>> let label6 = Label :: Label "6"

data Lbl (x :: HNat) (ns :: *) (desc :: *)

Instances

(HEqBy HLeFn n m b, ns ~ ns') => HEqBy HLeFn (Lbl n ns desc :: Type) (Lbl m ns' desc' :: Type) b

Data.HList.Label3 labels can only be compared if they belong to the same namespace.

Defined in Data.HList.HSort

Label t ~ Label (Lbl ix ns n) => SameLabels (Label t :: Type) (Lbl ix ns n :: Type)

Defined in Data.HList.Label3

Show desc => ShowLabel (Lbl x ns desc :: Type)

Equality on labels (descriptions are ignored) Use generic instance

Show label

Defined in Data.HList.Label3

Methods

showLabel :: Label (Lbl x ns desc) -> String

Show desc => Show (Label (Lbl x ns desc))

Defined in Data.HList.Label3

Methods

showsPrec :: Int -> Label (Lbl x ns desc) -> ShowS

show :: Label (Lbl x ns desc) -> String

showList :: [Label (Lbl x ns desc)] -> ShowS

HExtend (Label (Lbl n ns desc)) (Proxy (x ': xs))

Mixing two label kinds means we have to include Label:

>>> let r = label3 .*. label6 .*. emptyProxy
>>> :t r
r :: Proxy '[Label (Lbl 'HZero () ()), Label "6"]

Defined in Data.HList.Label3

Associated Types

type HExtendR (Label (Lbl n ns desc)) (Proxy (x ': xs))

Methods

(.*.) :: Label (Lbl n ns desc) -> Proxy (x ': xs) -> HExtendR (Label (Lbl n ns desc)) (Proxy (x ': xs))

HExtend (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs))

If possible, Label is left off:

>>> let q = label3 .*. label3 .*. emptyProxy
>>> :t q
q :: Proxy '[Lbl 'HZero () (), Lbl 'HZero () ()]

Defined in Data.HList.Label3

Associated Types

type HExtendR (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs))

Methods

(.*.) :: Label (Lbl n ns desc) -> Proxy (Lbl n' ns' desc' ': xs) -> HExtendR (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs))

(to ~ LabeledTo x, ToSym (to p q) x) => HExtend (to p q) (Proxy (Lbl n ns desc ': xs))

if the proxy has Data.HList.Label3.Lbl, then everything has to be wrapped in Label to make the kinds match up.

Defined in Data.HList.Labelable

Associated Types

type HExtendR (to p q) (Proxy (Lbl n ns desc ': xs))

Methods

(.*.) :: to p q -> Proxy (Lbl n ns desc ': xs) -> HExtendR (to p q) (Proxy (Lbl n ns desc ': xs))

type ZipTagged (Lbl ix ns n ': ts :: [Type]) (v ': vs)

Defined in Data.HList.Label3

type ZipTagged (Lbl ix ns n ': ts :: [Type]) (v ': vs) = Tagged (Lbl ix ns n) v ': ZipTagged ts vs

type HExtendR (Label (Lbl n ns desc)) (Proxy (x ': xs))

Defined in Data.HList.Label3

type HExtendR (Label (Lbl n ns desc)) (Proxy (x ': xs)) = Proxy (Label (Lbl n ns desc) ': MapLabel (x ': xs))

type HExtendR (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs))

Defined in Data.HList.Label3

type HExtendR (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs)) = Proxy (Lbl n ns desc ': (Lbl n' ns' desc' ': xs))

type HExtendR (to p q) (Proxy (Lbl n ns desc ': xs))

Defined in Data.HList.Labelable

type HExtendR (to p q) (Proxy (Lbl n ns desc ': xs))

Public constructors for labels

firstLabel :: ns -> desc -> Label (Lbl HZero ns desc)

Construct the first label

nextLabel :: Label (Lbl x ns desc) -> desc' -> Label (Lbl (HSucc x) ns desc')

Construct the next label

type family MapLabel (xs :: [k]) :: [*]

similar to Data.HList.Record.Labels1, but avoids producing Label (Label x)

Instances

type MapLabel ('[] :: [k])

Defined in Data.HList.Label3

type MapLabel ('[] :: [k]) = '[] :: [Type]

type MapLabel (x ': xs :: [k])

Defined in Data.HList.Label3

type MapLabel (x ': xs :: [k]) = AddLabel x ': MapLabel xs

type family AddLabel (x :: k) :: * where ...

Equations

AddLabel (Label x) = Label x

AddLabel x = Label x

Orphan instances

HExtend (Label y) (Proxy (x ': xs))

Associated Types

type HExtendR (Label y) (Proxy (x ': xs))

Methods

(.*.) :: Label y -> Proxy (x ': xs) -> HExtendR (Label y) (Proxy (x ': xs))

HExtend (Label y) (Proxy (x ': xs))

Associated Types

type HExtendR (Label y) (Proxy (x ': xs))

Methods

(.*.) :: Label y -> Proxy (x ': xs) -> HExtendR (Label y) (Proxy (x ': xs))

HExtend (Label y) (Proxy (x ': xs))

Associated Types

type HExtendR (Label y) (Proxy (x ': xs))

Methods

(.*.) :: Label y -> Proxy (x ': xs) -> HExtendR (Label y) (Proxy (x ': xs))

HExtend (Label y) (Proxy (x ': xs))

Mixing two label kinds means we have to include Label:

>>> let s = label6 .*. label3 .*. emptyProxy
>>> :t s
s :: Proxy '[Label "6", Label (Lbl 'HZero () ())]

Associated Types

type HExtendR (Label y) (Proxy (x ': xs))

Methods

(.*.) :: Label y -> Proxy (x ': xs) -> HExtendR (Label y) (Proxy (x ': xs))