Data.HList.HOccurs

The HList library

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

Result-type-driven operations on typeful heterogeneous lists.

Documentation

class HOccursMany e l where

Methods

hOccursMany :: l -> [e]

Instances

HOccursMany e HNil

HOccursMany e l => HOccursMany e (TIP l)

(HOccursMany e l, HList l) => HOccursMany e (HCons e' l)

(HOccursMany e l, HList l) => HOccursMany e (HCons e l)

class HOccursMany1 e l where

Methods

hOccursMany1 :: l -> (e, [e])

Instances

HOccursMany1 e l => HOccursMany1 e (TIP l)

(HOccursMany1 e l, HList l) => HOccursMany1 e (HCons e' l)

(HOccursMany e l, HList l) => HOccursMany1 e (HCons e l)

class HOccursFst e l where

Methods

hOccursFst :: l -> e

Instances

HOccursFst e l => HOccursFst e (TIP l)

(HOccursFst e l, HList l) => HOccursFst e (HCons e' l)

HList l => HOccursFst e (HCons e l)

class HOccurs e l where

Methods

hOccurs :: l -> e

Instances

Fail (TypeNotFound e) => HOccurs e HNil

HOccurs e (HCons x (HCons y l)) => HOccurs e (TIP (HCons x (HCons y l)))

TypeCast e' e => HOccurs e (TIP (HCons e' HNil))

(HOccurs e l, HList l) => HOccurs e (HCons e' l)

(HList l, HOccursNot e l) => HOccurs e (HCons e l)

data TypeNotFound e

class HOccurs' e l where

Methods

hOccurs' :: l -> e

Instances

(TypeEq e e' b, HOccursBool b e (HCons e' l)) => HOccurs' e (HCons e' l)

class HOccursBool b e l where

Methods

hOccursBool :: b -> l -> e

Instances

(HOccurs' e l, HList l) => HOccursBool HFalse e (HCons e' l)

(HList l, HOccursNot e l) => HOccursBool HTrue e (HCons e l)

class HOccursOpt e l where

Methods

hOccursOpt :: l -> Maybe e

Instances

HOccursOpt e HNil

HOccursOpt e l => HOccursOpt e (TIP l)

HOccursOpt e l => HOccursOpt e (HCons e' l)

HOccursOpt e (HCons e l)

data TypeFound e

class HOccursNot e l

Instances

HOccursNot e HNil

HOccursNot e l => HOccursNot e (TIP l)

HOccursNot e l => HOccursNot e (HCons e' l)

Fail (TypeFound e) => HOccursNot e (HCons e l)

class HProject l l' where

Methods

hProject :: l -> l'

Instances

HProject l HNil

(HList l', HOccurs e l, HProject l l') => HProject l (HCons e l')

Illustration of typical test scenarios

Retrieve the Breed of an animal.

 ghci-or-hugs> hOccurs myAnimal :: Breed
 Cow

Normal hOccurs requires specification of the result type even if the result type is determined by the fact that we are faced with a singleton list.

 ghci-or-hugs> hOccurs (HCons 1 HNil)

 <interactive>:1:
     No instance for (HOccurs e1 (HCons e HNil))

However, hOccurs can be elaborated as improved as follows:

 ghci-or-hugs> hLookup (HCons 1 HNil)
 1