From f33b6fb3f5a5caf0d06148131fff0a22f8a18a39 Mon Sep 17 00:00:00 2001
From: "Michal J. Gajda" <mjgajda@gmail.com>
Date: Sun, 19 Apr 2026 21:12:34 +0200
Subject: [PATCH] Add containers group for redundant toList rewrites (fixes
 #1570)

New hint group `containers` with rewrites that bypass the intermediate
list produced by `Data.IntMap.toList`, `Data.IntSet.toList`,
`Data.Map.toList` and `Data.Set.toList` in favour of the container's
own fold and predicate primitives.

Adds a matching `package: containers` declaration so the group's
`imports:` can reference it. All rewrite targets are fully qualified
(`Data.Map.null`, `Data.IntSet.member`, etc.), so the group is safe to
enable by default: it only fires when the qualified containers API is
already in use.
---
 data/hlint.yaml | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/data/hlint.yaml b/data/hlint.yaml
index f007f2b2..dd750ca3 100644
--- a/data/hlint.yaml
+++ b/data/hlint.yaml
@@ -65,6 +65,18 @@
     modules:
     - import CodeWorld
 
+- package:
+    name: containers
+    modules:
+    - import Data.IntMap
+    - import Data.IntMap.Lazy
+    - import Data.IntMap.Strict
+    - import Data.IntSet
+    - import Data.Map
+    - import Data.Map.Lazy
+    - import Data.Map.Strict
+    - import Data.Set
+
 - group:
     name: default
     enabled: true
@@ -1133,6 +1145,43 @@
     rules:
     - hint: {lhs: "TH.varE 'a", rhs: "[|a|]", name: Use TH quotation brackets}
 
+- group:
+    name: containers
+    enabled: true
+    imports:
+    - package base
+    - package containers
+    rules:
+    - warn: {lhs: foldMap f (Data.IntMap.toList x), rhs: Data.IntMap.foldMapWithKey (curry f) x}
+    - warn: {lhs: foldr f z (Data.IntMap.toList x), rhs: Data.IntMap.foldrWithKey (curry f) z x}
+    - warn: {lhs: foldr' f z (Data.IntMap.toList x), rhs: Data.IntMap.foldrWithKey' (curry f) z x}
+    - warn: {lhs: foldl f z (Data.IntMap.toList x), rhs: Data.IntMap.foldlWithKey (curry . f) z x}
+    - warn: {lhs: foldl' f z (Data.IntMap.toList x), rhs: Data.IntMap.foldlWithKey' (curry . f) z x}
+    - warn: {lhs: null (Data.IntMap.toList x), rhs: Data.IntMap.null x}
+    - warn: {lhs: length (Data.IntMap.toList x), rhs: Data.IntMap.size x}
+
+    - warn: {lhs: foldr f z (Data.IntSet.toList x), rhs: Data.IntSet.foldr f z x}
+    - warn: {lhs: foldr' f z (Data.IntSet.toList x), rhs: Data.IntSet.foldr' f z x}
+    - warn: {lhs: foldl f z (Data.IntSet.toList x), rhs: Data.IntSet.foldl f z x}
+    - warn: {lhs: foldl' f z (Data.IntSet.toList x), rhs: Data.IntSet.foldl' f z x}
+    - warn: {lhs: null (Data.IntSet.toList x), rhs: Data.IntSet.null x}
+    - warn: {lhs: length (Data.IntSet.toList x), rhs: Data.IntSet.size x}
+    - warn: {lhs: elem y (Data.IntSet.toList x), rhs: Data.IntSet.member y x}
+    - warn: {lhs: notElem y (Data.IntSet.toList x), rhs: Data.IntSet.notMember y x}
+
+    - warn: {lhs: foldMap f (Data.Map.toList x), rhs: Data.Map.foldMapWithKey (curry f) x}
+    - warn: {lhs: foldr f z (Data.Map.toList x), rhs: Data.Map.foldrWithKey (curry f) z x}
+    - warn: {lhs: foldr' f z (Data.Map.toList x), rhs: Data.Map.foldrWithKey' (curry f) z x}
+    - warn: {lhs: foldl f z (Data.Map.toList x), rhs: Data.Map.foldlWithKey (curry . f) z x}
+    - warn: {lhs: foldl' f z (Data.Map.toList x), rhs: Data.Map.foldlWithKey' (curry . f) z x}
+    - warn: {lhs: null (Data.Map.toList x), rhs: Data.Map.null x}
+    - warn: {lhs: length (Data.Map.toList x), rhs: Data.Map.size x}
+
+    - warn: {lhs: null (Data.Set.toList x), rhs: Data.Set.null x}
+    - warn: {lhs: length (Data.Set.toList x), rhs: Data.Set.size x}
+    - warn: {lhs: elem y (Data.Set.toList x), rhs: Data.Set.member y x}
+    - warn: {lhs: notElem y (Data.Set.toList x), rhs: Data.Set.notMember y x}
+
 - group:
     name: attoparsec
     enabled: true