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Fix: dont suggest "Use :" when OverloadedLists is enabled (fixes #1602) #1686

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Fix: dont suggest "Use :" when OverloadedLists is enabled (fixes #1602) #1686

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3 changes: 2 additions & 1 deletion data/hlint.yaml
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Original file line number Diff line number Diff line change
Expand Up @@ -211,7 +211,8 @@
- hint: {lhs: any (const True) x, rhs: not (null x), name: Use null}
- hint: {lhs: length x /= 0, rhs: not (null x), note: IncreasesLaziness, name: Use null}
- hint: {lhs: 0 /= length x, rhs: not (null x), note: IncreasesLaziness, name: Use null}
- hint: {lhs: "\\x -> [x]", rhs: "(:[])", name: "Use :"}
# Moved to code-based hint in Hint/List.hs to skip under OverloadedLists (issue #1602)
# - hint: {lhs: "\\x -> [x]", rhs: "(:[])", name: "Use :"}
- hint: {lhs: map f (zip x y), rhs: zipWith (curry f) x y, side: not (isApp f)}
- hint: {lhs: "map f (fromMaybe [] x)", rhs: "maybe [] (map f) x"}
- hint: {lhs: "concatMap f (fromMaybe [] x)", rhs: "maybe [] (concatMap f) x"}
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18 changes: 18 additions & 0 deletions src/Hint/List.hs
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Expand Up @@ -37,6 +37,9 @@ foo = [_ | x <- _, let _ = A{x}]
issue1039 = foo (map f [1 | _ <- []]) -- [f 1 | _ <- []]
{-# LANGUAGE OverloadedLists #-} \
issue114 = True:[]
{-# LANGUAGE OverloadedLists #-} \
no_issue1602 = \x -> [x]
yes_issue1602 = \x -> [x] -- (:[])
</TEST>
-}

Expand Down Expand Up @@ -188,6 +191,7 @@ checks overloadedListsOn = let (*) = (,) in drop1 -- see #174
, "Use :" * useCons
]
<> ["Use list literal" * useList | not overloadedListsOn ] -- see #114
<> ["Use :" * useConsLambda | not overloadedListsOn ] -- see #1602

pchecks :: [(String, LPat GhcPs -> Maybe (LPat GhcPs, [(String, R.SrcSpan)], String))]
pchecks = let (*) = (,) in drop1 -- see #174
Expand Down Expand Up @@ -263,6 +267,20 @@ useCons False (view -> App2 op x y) | varToStr op == "++"
gen x = noLocA . OpApp noExtField x (noLocA (HsVar noExtField (noLocA consDataCon_RDR)))
useCons _ _ = Nothing

-- Suggest `(: [])` for `\x -> [x]`. Gated on `not overloadedListsOn`
-- because under OverloadedLists the literal `[x]` can be polymorphic
-- (e.g. Vector a, Set a, NonEmpty a), whereas `(: [])` specialises
-- to the list type and the rewrite produces code that no longer
-- type-checks (issue #1602).
useConsLambda :: Bool -> LHsExpr GhcPs -> Maybe (LHsExpr GhcPs, [(String, R.SrcSpan)], String)
useConsLambda _ (SimpleLambda [view -> PVar_ x] (L _ (ExplicitList _ [L _ (HsVar _ (L _ y))])))
| occNameStr y == x =
let consSection = noLocA (HsPar noAnn (noLocA (SectionR noExtField
(noLocA (HsVar noExtField (noLocA consDataCon_RDR)))
(noLocA (ExplicitList noAnn [])))))
in Just (consSection, [], "(: [])")
useConsLambda _ _ = Nothing

typeListChar :: LHsType GhcPs
typeListChar =
noLocA $ HsListTy noAnn
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