The data constructors of Union are not exported
Strong Sum (Existential with the evidence) is an open union t is can be a GADT and hence not necessarily a Functor. Int is the index of t in the list r; that is, the index of t in the universe r
Moduleextensible-effects-5.0.0.1Haskell2010
Data.OpenUnion
Open unions (type-indexed co-products) for extensible effects All operations are constant-time, and there is no Typeable constraint
This is a variation of OpenUion5.hs, which relies on overlapping instances instead of closed type families. Closed type families have their problems: overlapping instances can resolve even for unground types, but closed type families are subject to a strict apartness condition.
This implementation is very similar to OpenUnion1.hs, but without the annoying Typeable constraint. We sort of emulate it:
Our list r of open union components is a small Universe. Therefore, we can use the Typeable-like evidence in that universe. We hence can define
data Union r v where
Union :: t v -> TRep t r -> Union r v -- t is existential
where
data TRep t r where
T0 :: TRep t (t ': r)
TS :: TRep t r -> TRep (any ': r)
Then Member is a type class that produces TRep Taken literally it doesn't seem much better than OpenUinion41.hs. However, we can cheat and use the index of the type t in the list r as the TRep. (We will need UnsafeCoerce then).
The interface is the same as of other OpenUnion*.hs
- 1 type
- 2 classes
- 2 values
- UndecidableInstances
- MonoLocalBinds
- ScopedTypeVariables
- TypeFamilies
- ViewPatterns
- GADTs
- GADTSyntax
- ConstraintKinds
- PolyKinds
- DataKinds
- TypeSynonymInstances
- FlexibleContexts
- FlexibleInstances
- ConstrainedClassMethods
- MultiParamTypeClasses
- FunctionalDependencies
- KindSignatures
- RankNTypes
- TypeOperators
- ExplicitNamespaces
- ExplicitForAll
- PatternSynonyms
- Packageextensible-effects-5.0.0.1
- Exports11
- LanguageHaskell2010
- LicenceMIT
- SourceOpenUnion.hs
Pattern synonym to project the union onto the effect t.
Orthogonal decomposition of the union: head and the rest.
Some helpful pattern synonyms. U0 : the first element of the union
U1 : everything excluding the first element of the union.
Typeclass that asserts that effect t is contained inside the effect-list
r.
The FindElem typeclass is an implementation detail and not required for
using the effect list or implementing custom effects.
Instances2Member
FindElem t r => Member t rDefined in extensible-effects-5.0.0.1 · Data.OpenUniont ~ s => Member t '[s]Defined in extensible-effects-5.0.0.1 · Data.OpenUnionExplicit type-level equality condition is a dirty hack to eliminate the type annotation in the trivial case, such as
run (runReader () get).There is no ambiguity when finding instances for
Member t (a ': b ': r), which the second instance is selected.The only case we have to concerned about is
Member t '[s]. But, in this case, values of definition is the same (if present), and the first one is chosen according to GHC User Manual, since the latter one is incoherent. This is the optimal choice.
class
class Member t r => SetMember (tag :: k -> Type -> Type) (t :: Type -> Type) (r :: [Type -> Type]) | tag r -> tThis class is used for emulating monad transformers
Instances1SetMember
(EQU t1 t2 p, MemberU' p tag t1 (t2 ': r)) => SetMember tag t1 (t2 ': r)Defined in extensible-effects-5.0.0.1 · Data.OpenUnion