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Implement all HList functions in terms of single fold

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This commit is contained in:
George Thomas 2026-01-04 02:39:33 +00:00
parent 784e6f2fb2
commit 8e6179f103
1 changed files with 9 additions and 9 deletions
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18
haskell/Pre.hs
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@ -42,6 +42,7 @@ module Pre (
adjacentPairs,
sortPair,
HListF (..),
foldHListF,
mapHListF,
fromHListF,
unHListF,
@ -149,20 +150,19 @@ data HListF (f :: Type -> Type) (as :: List Type) :: Type where
HListF f as ->
HListF f (a ': as)
foldHListF :: (forall x xs. f x -> r xs -> r (x ': xs)) -> r '[] -> HListF f as -> r as
foldHListF f e = \case
HNilF -> e
HConsF x xs -> f x $ foldHListF f e xs
mapHListF :: (forall a. f a -> g a) -> HListF f as -> HListF g as
mapHListF t = \case
HNilF -> HNilF
HConsF x xs -> HConsF (t x) $ mapHListF t xs
mapHListF t = foldHListF (\x r -> HConsF (t x) $ r) HNilF
fromHListF :: HListF Identity as -> HList as
fromHListF = \case
HNilF -> HNil
HConsF (Identity x) xs -> HCons x $ fromHListF xs
fromHListF = foldHListF (\(Identity x) r -> HCons x r) HNil
unHListF :: HListF (Const b) as -> List b
unHListF = \case
HNilF -> []
HConsF (Const x) xs -> x : unHListF xs
unHListF = getConst . foldHListF (\(Const x) (Const r) -> Const $ x : r) (Const [])
instance Semigroup (TestDefM a b ()) where
(<>) = (>>)