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Idris2/libs/base/Data/So.idr
2020-05-18 14:00:08 +01:00

26 lines
762 B
Idris

module Data.So
||| Ensure that some run-time Boolean test has been performed.
|||
||| This lifts a Boolean predicate to the type level. See the function `choose`
||| if you need to perform a Boolean test and convince the type checker of this
||| fact.
|||
||| If you find yourself using `So` for something other than primitive types,
||| it may be appropriate to define a type of evidence for the property that you
||| care about instead.
public export
data So : Bool -> Type where
Oh : So True
export
implementation Uninhabited (So False) where
uninhabited Oh impossible
||| Perform a case analysis on a Boolean, providing clients with a `So` proof
export
choose : (b : Bool) -> Either (So b) (So (not b))
choose True = Left Oh
choose False = Right Oh