ndmitchell/hlint

Incorrect "Use map" when folding over a `Foldable`

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#1,601 opened on 2024年5月29日

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good first issue

説明

The code

triggersUseMap :: Foldable f => f a -> [a]
triggersUseMap = foldr (\k acc -> identity k : acc) []

-- to get rid of "Avoid lambda" and "Redundant id"
identity :: a -> a
identity = id

triggers the hint

                        Warning: Use map
Found:
  foldr (\ k acc -> identity k : acc) []
Perhaps:
  map (\ k -> identity k)

but obviously triggersUseMap = map (\k -> identity k) does not compile, since map wants a list and the argument is a general Foldable.

The same warning is not emitted with the eta reduced version:

noHint :: Foldable f => f a -> [a]
noHint = foldr ((:) . identity) [] -- no hint even with foldr (:) []

This is using GHC 9.6.5 with HLint 3.6.1 (apologies if this was already fixed in 3.8, but I didn't see anything of the sort in the changelog)

コントリビューターガイド

技術スタック
なし
領域
tooling
Issue 種別
bug
難度新しい貢献者にとっての実装難度です。1 は非常に小さな変更、5 は専門的な作業です。
4
推定時間経験ある貢献者が調査、実装、テスト、pull request 準備にかけるおおよその時間範囲です。
1-3 hours
活動状況Issue が今どれだけ取り組みやすいかを示します。新しい、活発、古い、ブロック中、またはメンテナー入力待ちなどです。
fresh
明確さ期待される変更、受け入れ条件、次の手順がどれだけ明確かを示します。
clear
前提条件
Haskell programmingType class FoldableHLint architecture
初心者向け度初回貢献者にどれだけ取り組みやすいかを 1-100 で推定したスコアです。
25
調査方針
Investigate the 'Use map' hint logic in hlint, specifically in the file 'Hint/ListMap.hs' or similar, where the foldr to map conversion is defined. The hint incorrectly suggests map for any Foldable, but map only works on lists. The fix likely involves checking that the argument is a list type, not a general Foldable. Also examine the existing tests for this hint to ensure the fix doesn't break other cases. Refer to the GHC version and HLint version mentioned.