From d64c53d0517d59db51543d78c9148a5260ceccfe Mon Sep 17 00:00:00 2001
From: Profpatsch <mail@profpatsch.de>
Date: Thu, 6 Mar 2025 23:58:12 +0100
Subject: [PATCH] feat(users/Profpatsch/my-prelude): add RevList

Change-Id: Icb98c6ec77a305f44149f344dccbf3969a83951a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13210
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
---
 users/Profpatsch/my-prelude/default.nix      |  1 +
 users/Profpatsch/my-prelude/my-prelude.cabal |  1 +
 users/Profpatsch/my-prelude/src/RevList.hs   | 28 ++++++++++++++++++++
 3 files changed, 30 insertions(+)
 create mode 100644 users/Profpatsch/my-prelude/src/RevList.hs

diff --git a/users/Profpatsch/my-prelude/default.nix b/users/Profpatsch/my-prelude/default.nix
index c739676c1..7755943c4 100644
--- a/users/Profpatsch/my-prelude/default.nix
+++ b/users/Profpatsch/my-prelude/default.nix
@@ -19,6 +19,7 @@ pkgs.haskellPackages.mkDerivation {
     ./src/Test.hs
     ./src/Parse.hs
     ./src/Pretty.hs
+    ./src/RevList.hs
     ./src/Seconds.hs
     ./src/Tool.hs
     ./src/ValidationParseT.hs
diff --git a/users/Profpatsch/my-prelude/my-prelude.cabal b/users/Profpatsch/my-prelude/my-prelude.cabal
index dba092fda..b4dd64668 100644
--- a/users/Profpatsch/my-prelude/my-prelude.cabal
+++ b/users/Profpatsch/my-prelude/my-prelude.cabal
@@ -70,6 +70,7 @@ library
       Test
       Postgres.Decoder
       Postgres.MonadPostgres
+      RevList
       ValidationParseT
       Parse
       Pretty
diff --git a/users/Profpatsch/my-prelude/src/RevList.hs b/users/Profpatsch/my-prelude/src/RevList.hs
new file mode 100644
index 000000000..0f002c07e
--- /dev/null
+++ b/users/Profpatsch/my-prelude/src/RevList.hs
@@ -0,0 +1,28 @@
+module RevList where
+
+import Data.Semigroup qualified as Semigroup
+import PossehlAnalyticsPrelude
+
+-- | A reversed list; `:` adds to the end of the list, and '(<>)' is reversed (i.e. @longList <> [oneElement]@ will be O(1) instead of O(n))
+--
+-- Invariant: the inner list is already reversed.
+newtype RevList a = RevList [a]
+  deriving stock (Eq)
+  deriving (Semigroup, Monoid) via (Semigroup.Dual [a])
+
+empty :: RevList a
+empty = RevList []
+
+singleton :: a -> RevList a
+singleton a = RevList [a]
+
+-- | (@O(n)@) Turn the list into a reversed list (by reversing)
+revList :: [a] -> RevList a
+revList xs = RevList $ reverse xs
+
+-- | (@O(n)@) Turn the reversed list into a list (by reversing)
+revListToList :: RevList a -> [a]
+revListToList (RevList rev) = reverse rev
+
+instance (Show a) => Show (RevList a) where
+  show (RevList rev) = rev & show