| Safe Haskell | None |
|---|
Data.HList.HOccurs
Description
The HList library
(C) 2004, Oleg Kiselyov, Ralf Laemmel, Keean Schupke
Result-type-driven operations on typeful heterogeneous lists.
- class HOccurrence e1 l l' | e1 l -> l' where
- hOccurrence :: e1 -> HList l -> HList l'
- class HOccurrence' b e1 l l' | b e1 l -> l' where
- hOccurrence' :: Proxy b -> e1 -> HList l -> HList l'
- class HOccursMany e l where
- hOccursMany :: HList l -> [e]
- class HOccursMany' e l where
- hOccursMany' :: HList l -> [e]
- hOccursMany1 :: forall e l l'. (HOccurrence e l (e : l'), HOccursMany e l') => HList l -> (e, [e])
- hOccursFst :: forall e l l'. HOccurrence e l (e : l') => HList l -> e
- data TypeNotFound e
- class HOccurs' e l where
- hOccursOpt :: forall e l l'. (HOccurrence e l l', HOccursOpt' e l') => HList l -> Maybe e
- class HOccursOpt' e l where
- hOccursOpt' :: HList l -> Maybe e
- data TypeFound e
- class HOccursNot' b e l
Documentation
class HOccurrence e1 l l' | e1 l -> l' whereSource
Methods
hOccurrence :: e1 -> HList l -> HList l'Source
Instances
| HOccurrence e1 ([] *) ([] *) | |
| (HEq * e1 e b, HOccurrence' b e1 (: * e l) l') => HOccurrence e1 (: * e l) l' |
class HOccurrence' b e1 l l' | b e1 l -> l' whereSource
Methods
hOccurrence' :: Proxy b -> e1 -> HList l -> HList l'Source
Instances
| HOccurrence e1 l l' => HOccurrence' False e1 (: * e l) l' | |
| HOccurrence' True e1 (: * e l) (: * e l) |
class HOccursMany e l whereSource
Methods
hOccursMany :: HList l -> [e]Source
Instances
| (HOccurrence e l l', HOccursMany' e l') => HOccursMany e l |
class HOccursMany' e l whereSource
Methods
hOccursMany' :: HList l -> [e]Source
Instances
| HOccursMany' e ([] *) | |
| (~ * e e1, HOccursMany e l) => HOccursMany' e (: * e1 l) |
hOccursMany1 :: forall e l l'. (HOccurrence e l (e : l'), HOccursMany e l') => HList l -> (e, [e])Source
hOccursFst :: forall e l l'. HOccurrence e l (e : l') => HList l -> eSource
data TypeNotFound e Source
class HOccurs' e l whereSource
Instances
| Fail * (TypeNotFound * e) => HOccurs' e ([] *) | |
| (~ * e e1, HOccursNot * [*] e l) => HOccurs' e (: * e l) |
hOccursOpt :: forall e l l'. (HOccurrence e l l', HOccursOpt' e l') => HList l -> Maybe eSource
class HOccursOpt' e l whereSource
Methods
hOccursOpt' :: HList l -> Maybe eSource
Instances
| HOccursOpt' e ([] *) | |
| ~ * e e1 => HOccursOpt' e (: * e1 l) |
class HOccursNot' b e l Source
Instances
| HOccursNot k [*] e l => HOccursNot' k False e l | |
| Fail * (TypeFound k e) => HOccursNot' k True 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