From 017e9d4ed91e8c891938f5b4a3dec8a7768ac147 Mon Sep 17 00:00:00 2001
From: effectfully <effectfully@gmail.com>
Date: Wed, 11 Sep 2024 20:03:11 +0200
Subject: [PATCH] [Builtins] Add the 'dropList' builtin

---
 .../src/PlutusCore/Default/Builtins.hs        | 25 +++++++++++++++++++
 .../Golden/DefaultFun/DropList.plc.golden     |  1 +
 2 files changed, 26 insertions(+)
 create mode 100644 plutus-core/plutus-core/test/TypeSynthesis/Golden/DefaultFun/DropList.plc.golden

diff --git a/plutus-core/plutus-core/src/PlutusCore/Default/Builtins.hs b/plutus-core/plutus-core/src/PlutusCore/Default/Builtins.hs
index ff2a4164a31..44e23803a38 100644
--- a/plutus-core/plutus-core/src/PlutusCore/Default/Builtins.hs
+++ b/plutus-core/plutus-core/src/PlutusCore/Default/Builtins.hs
@@ -103,6 +103,7 @@ data DefaultFun
     | HeadList
     | TailList
     | NullList
+    | DropList
     -- Data
     -- See Note [Pattern matching on built-in types].
     -- It is convenient to have a "choosing" function for a data type that has more than two
@@ -1543,6 +1544,27 @@ instance uni ~ DefaultUni => ToBuiltinMeaning uni DefaultFun where
             nullListDenotation
             (runCostingFunOneArgument . paramNullList)
 
+    toBuiltinMeaning _semvar DropList =
+        let dropListDenotation :: Integer -> SomeConstant uni [a] -> BuiltinResult (Opaque val a)
+            dropListDenotation n0 (SomeConstant (Some (ValueOf uniListA xs0))) = do
+                -- See Note [Operational vs structural errors within builtins].
+                case uniListA of
+                    DefaultUniList _ -> do
+                        let -- We don't want to restrict ourselves to 'Int, hence we can't use
+                            -- 'drop'. And 'genericDrop' is likely less efficient due to it being
+                            -- recursive and therefore not inlinable, so there's probably a lot of
+                            -- dictionary passing going on. And we don't care about fusion. So we
+                            -- just implement the straightforward thing ourselves.
+                            go n xs | n <= 0  = xs
+                            go _ []           = []
+                            go n (_ : xs)     = go (n - 1) xs
+                        pure . fromValueOf uniListA $ go n0 xs0
+                    _ -> throwing _StructuralUnliftingError "Expected a list but got something else"
+            {-# INLINE dropListDenotation #-}
+        in makeBuiltinMeaning
+            dropListDenotation
+            (runCostingFunTwoArguments . unimplementedCostingFun)
+
     -- Data
     toBuiltinMeaning _semvar ChooseData =
         let chooseDataDenotation :: Data -> a -> a -> a -> a -> a -> a
@@ -2144,6 +2166,8 @@ instance Flat DefaultFun where
 
               ExpModInteger           -> 87
 
+              DropList -> 88
+
     decode = go =<< decodeBuiltin
         where go 0  = pure AddInteger
               go 1  = pure SubtractInteger
@@ -2233,6 +2257,7 @@ instance Flat DefaultFun where
               go 85 = pure FindFirstSetBit
               go 86 = pure Ripemd_160
               go 87 = pure ExpModInteger
+              go 88 = pure DropList
               go t  = fail $ "Failed to decode builtin tag, got: " ++ show t
 
     size _ n = n + builtinTagWidth
diff --git a/plutus-core/plutus-core/test/TypeSynthesis/Golden/DefaultFun/DropList.plc.golden b/plutus-core/plutus-core/test/TypeSynthesis/Golden/DefaultFun/DropList.plc.golden
new file mode 100644
index 00000000000..b7b380c65b6
--- /dev/null
+++ b/plutus-core/plutus-core/test/TypeSynthesis/Golden/DefaultFun/DropList.plc.golden
@@ -0,0 +1 @@
+all a. integer -> list a -> a
\ No newline at end of file