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Make Builtins.unsafeUncons use builtin list casing #7467

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Make Builtins.unsafeUncons use builtin list casing #7467

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e7f631c
Make `Builtins.unsafeUncons` use builtin list casing instead of head,…
sohang3112 Nov 28, 2025
f6458db
correct according to PR feedback by @SeungheonOh
sohang3112 Dec 5, 2025
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2 changes: 1 addition & 1 deletion plutus-tx/src/PlutusTx/Builtins.hs
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Expand Up @@ -491,7 +491,7 @@ uncons = BI.caseList' Nothing (\h t -> Just (h, t))

-- | Uncons a builtin list, failing if the list is empty, useful in patterns.
unsafeUncons :: BI.BuiltinList a -> (a, BI.BuiltinList a)
unsafeUncons l = (BI.head l, BI.tail l)
unsafeUncons l = BI.unsafeCaseList' (,)
{-# INLINE unsafeUncons #-}

-- | Turn a builtin pair into a normal pair, useful in patterns.
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5 changes: 5 additions & 0 deletions plutus-tx/src/PlutusTx/Builtins/Internal.hs
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Expand Up @@ -488,6 +488,11 @@ caseList' nilCase _ (BuiltinList []) = nilCase
caseList' _ consCase (BuiltinList (x : xs)) = consCase x (BuiltinList xs)
{-# OPAQUE caseList' #-}

-- | Similar to caseList', but empty list case is omitted so passing empty list will raise error
unsafeCaseList' :: forall a r. (a -> BuiltinList a -> r) -> BuiltinList a -> r
unsafeCaseList' f (BuiltinList (x : xs)) = f x (BuiltinList xs)
{-# OPAQUE unsafeCaseList' #-}

-- | Drops first n elements from the given list and never fails.
drop :: Integer -> BuiltinList a -> BuiltinList a
drop i (BuiltinList xs) = BuiltinList (Haskell.genericDrop i xs)
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