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-rw-r--r--src/Inky/OnlyWhen.idr44
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diff --git a/src/Inky/OnlyWhen.idr b/src/Inky/OnlyWhen.idr
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--- a/src/Inky/OnlyWhen.idr
+++ /dev/null
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-module Inky.OnlyWhen
-
-import Data.DPair
-
-public export
-data OnlyWhen : Maybe a -> (a -> Type) -> Type where
- Yes : forall x. (px : p x) -> Just x `OnlyWhen` p
- No : ((x : a) -> Not (p x)) -> Nothing `OnlyWhen` p
-
-public export
-decDPair : (v ** v `OnlyWhen` p) <=> Dec (x : a ** p x)
-decDPair =
- MkEquivalence
- { leftToRight = \case
- (Just x ** Yes px) => Yes (x ** px)
- (Nothing ** No prf) => No (\(x ** px) => prf x px)
- , rightToLeft = \case
- Yes (x ** px) => (Just x ** Yes px)
- No prf => (Nothing ** No (\x, px => prf (x ** px)))
- }
-
-public export
-decExists : Exists (`OnlyWhen` p) <=> Dec (Exists p)
-decExists =
- MkEquivalence
- { leftToRight = \case
- Evidence .(Just x) (Yes px) => Yes (Evidence x px)
- Evidence .(Nothing) (No prf) => No (\(Evidence x px) => void (prf x px))
- , rightToLeft = \case
- Yes (Evidence x px) => Evidence (Just x) (Yes px)
- No prf => Evidence Nothing (No (\x, px => prf (Evidence x px)))
- }
-
-public export
-decSubset : Subset (Maybe a) (`OnlyWhen` p) <=> Dec (Subset a p)
-decSubset =
- MkEquivalence
- { leftToRight = \case
- Element (Just x) prf => Yes (Element x (case prf of Yes px => px))
- Element Nothing prf => No (\(Element x px) => void (case prf of No prf => prf x px))
- , rightToLeft = \case
- Yes (Element x px) => Element (Just x) (Yes px)
- No prf => Element Nothing (No (\x, px => prf (Element x px)))
- }