ndmitchell/hlint

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

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#1,601 opened on May 29, 2024

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good first issue

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Description

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)

Contributor guide

Research direction
Look for the 'Use map' hint implementation in HLint source and understand how it currently matches. Consider adding a type check so that the hint only applies when the Foldable is specifically a list. The test file for this hint may need updating.
Tech stack
haskell
Domain
tooling
Issue type
Bug
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3
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1-3 hours
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Active
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Clear
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HaskellGit
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70