Bodigrim pushed to branch wip/warning-for-last-and-init at Glasgow Haskell Compiler / GHC Commits: 5ee11c69 by Mike Pilgrem at 2025-11-22T15:10:19+00:00 Re CLC issue 292 Warn GHC.Internal.List.{init,last} are partial Also corrects the warning for `tail` to refer to `Data.List.uncons` (like the existing warning for `head`). In module `Settings.Warnings`, applies `-Wno-x-partial` to the `filepath`, and `parsec` packages (outside GHC's repository). Also bumps submodules. - - - - - 25 changed files: - compiler/GHC/Core/Utils.hs - compiler/GHC/Driver/Session/Units.hs - compiler/GHC/Prelude/Basic.hs - ghc/GHCi/UI.hs - ghc/Main.hs - hadrian/src/Settings/Warnings.hs - libraries/base/changelog.md - libraries/ghc-boot-th/GHC/Boot/TH/Ppr.hs - libraries/ghc-internal/src/GHC/Internal/Float.hs - libraries/ghc-internal/src/GHC/Internal/List.hs - libraries/ghc-internal/src/GHC/Internal/System/IO.hs - libraries/ghc-internal/src/GHC/Internal/TH/Syntax.hs - libraries/template-haskell/vendored-filepath/System/FilePath/Posix.hs - libraries/template-haskell/vendored-filepath/System/FilePath/Windows.hs - testsuite/tests/driver/j-space/jspace.hs - testsuite/tests/rts/KeepCafsBase.hs - utils/check-exact/Main.hs - utils/check-exact/Transform.hs - utils/check-exact/Utils.hs - utils/ghc-pkg/Main.hs - utils/ghc-toolchain/src/GHC/Toolchain/Utils.hs - utils/haddock/haddock-api/src/Haddock/Backends/Xhtml.hs - utils/haddock/haddock-library/src/Documentation/Haddock/Parser.hs - utils/hpc - utils/hsc2hs Changes: ===================================== compiler/GHC/Core/Utils.hs ===================================== @@ -115,7 +115,6 @@ import GHC.Utils.Misc import Data.ByteString ( ByteString ) import Data.Function ( on ) import Data.List ( sort, sortBy, partition, zipWith4, mapAccumL ) -import qualified Data.List as Partial ( init, last ) import Data.Ord ( comparing ) import Control.Monad ( guard ) import qualified Data.Set as Set @@ -1896,10 +1895,10 @@ app_ok fun_ok primop_ok fun args PrimOpId op _ | primOpIsDiv op - , Lit divisor <- Partial.last args + , Lit divisor <- last args -- there can be 2 args (most div primops) or 3 args -- (WordQuotRem2Op), hence the use of last/init - -> not (isZeroLit divisor) && all (expr_ok fun_ok primop_ok) (Partial.init args) + -> not (isZeroLit divisor) && all (expr_ok fun_ok primop_ok) (init args) -- Special case for dividing operations that fail -- In general they are NOT ok-for-speculation -- (which primop_ok will catch), but they ARE OK ===================================== compiler/GHC/Driver/Session/Units.hs ===================================== @@ -39,7 +39,7 @@ import System.FilePath import Control.Monad import Data.List ( partition, (\\) ) import qualified Data.Set as Set -import Prelude +import GHC.Prelude import GHC.ResponseFile (expandResponse) import Data.Bifunctor import GHC.Data.Graph.Directed ===================================== compiler/GHC/Prelude/Basic.hs ===================================== @@ -2,8 +2,8 @@ {-# OPTIONS_HADDOCK not-home #-} {-# OPTIONS_GHC -O2 #-} -- See Note [-O2 Prelude] --- See Note [Proxies for head and tail] -{-# OPTIONS_GHC -Wno-unrecognised-warning-flags -Wno-x-partial #-} +-- See Note [Proxies for partial list functions] +{-# OPTIONS_GHC -Wno-x-partial #-} -- | Custom minimal GHC "Prelude" -- @@ -24,7 +24,7 @@ module GHC.Prelude.Basic , bit , shiftL, shiftR , setBit, clearBit - , head, tail, unzip + , head, tail, init, last, unzip , strictGenericLength ) where @@ -59,7 +59,7 @@ NoImplicitPrelude. There are two motivations for this: -} import qualified Prelude -import Prelude as X hiding ((<>), Applicative(..), Foldable(..), head, tail, unzip) +import Prelude as X hiding ((<>), Applicative(..), Foldable(..), head, tail, init, last, unzip) import Control.Applicative (Applicative(..)) import Data.Foldable as X (Foldable (elem, foldMap, foldl, foldl', foldr, length, null, product, sum)) import Data.Foldable1 as X hiding (head, last) @@ -118,24 +118,35 @@ setBit = \ x i -> x Bits..|. bit i clearBit :: (Num a, Bits.Bits a) => a -> Int -> a clearBit = \ x i -> x Bits..&. Bits.complement (bit i) -{- Note [Proxies for head and tail] +{- Note [Proxies for partial list functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Prelude.head and Prelude.tail have recently acquired {-# WARNING in "x-partial" #-}, +Prelude.head, Prelude.tail, Prelude.init and Prelude.last +have recently acquired {-# WARNING in "x-partial" #-}, but the GHC codebase uses them fairly extensively and insists on building warning-free. Thus, instead of adding {-# OPTIONS_GHC -Wno-x-partial #-} to every module which employs them, we define warning-less proxies and export them from GHC.Prelude. -} --- See Note [Proxies for head and tail] +-- See Note [Proxies for partial list functions] head :: HasCallStack => [a] -> a head = Prelude.head {-# INLINE head #-} --- See Note [Proxies for head and tail] +-- See Note [Proxies for partial list functions] tail :: HasCallStack => [a] -> [a] tail = Prelude.tail {-# INLINE tail #-} +-- See Note [Proxies for partial list functions] +init :: HasCallStack => [a] -> [a] +init = Prelude.init +{-# INLINE init #-} + +-- See Note [Proxies for partial list functions] +last :: HasCallStack => [a] -> a +last = Prelude.last +{-# INLINE last #-} + {- | The 'genericLength' function defined in base can't be specialised due to the NOINLINE pragma. ===================================== ghc/GHCi/UI.hs ===================================== @@ -145,7 +145,7 @@ import Data.Time.LocalTime ( getZonedTime ) import Data.Time.Format ( formatTime, defaultTimeLocale ) import Data.Version ( showVersion ) import qualified Data.Semigroup as S -import Prelude hiding ((<>)) +import GHC.Prelude import GHC.Utils.Exception as Exception hiding (catch, mask, handle) import Foreign hiding (void) ===================================== ghc/Main.hs ===================================== @@ -2,7 +2,7 @@ {-# LANGUAGE LambdaCase #-} {-# LANGUAGE NondecreasingIndentation #-} {-# LANGUAGE TupleSections #-} -{-# OPTIONS -fno-warn-incomplete-patterns -optc-DNON_POSIX_SOURCE #-} +{-# OPTIONS_GHC -Wno-x-partial -Wno-incomplete-patterns -optc-DNON_POSIX_SOURCE #-} ----------------------------------------------------------------------------- -- ===================================== hadrian/src/Settings/Warnings.hs ===================================== @@ -72,7 +72,11 @@ ghcWarningsArgs = do , package terminfo ? pure [ "-Wno-unused-imports", "-Wno-deriving-typeable" ] , package stm ? pure [ "-Wno-deriving-typeable" ] , package osString ? pure [ "-Wno-deriving-typeable", "-Wno-unused-imports" ] - , package parsec ? pure [ "-Wno-deriving-typeable" ] + , package parsec ? pure [ "-Wno-deriving-typeable" + , "-Wno-x-partial" + ] + + , package filepath ? pure [ "-Wno-x-partial" ] , package cabal ? pure [ "-Wno-deriving-typeable", "-Wno-incomplete-record-selectors" ] -- The -Wno-incomplete-record-selectors is due to ===================================== libraries/base/changelog.md ===================================== @@ -1,6 +1,9 @@ # Changelog for [`base` package](http://hackage.haskell.org/package/base) ## 4.23.0.0 *TBA* + * Add `{-# WARNING in "x-partial" #-}` to `Data.List.{init,last}`. + Use `{-# OPTIONS_GHC -Wno-x-partial #-}` to disable it. + ([CLC proposal #87](https://github.com/haskell/core-libraries-committee/issues/292)) * Remove deprecated, unstable heap representation details from `GHC.Exts` ([CLC proposal #212](https://github.com/haskell/core-libraries-committee/issues/212)) * Add `Data.List.NonEmpty.mapMaybe`. ([CLC proposal #337](https://github.com/haskell/core-libraries-committee/issues/337)) * Fix issues with toRational for types capable to represent infinite and not-a-number values ([CLC proposal #338](https://github.com/haskell/core-libraries-committee/issues/338)) ===================================== libraries/ghc-boot-th/GHC/Boot/TH/Ppr.hs ===================================== @@ -1,5 +1,6 @@ {-# LANGUAGE Trustworthy #-} {-# LANGUAGE LambdaCase #-} +{-# OPTIONS_GHC -Wno-x-partial #-} -- | contains a prettyprinter for the -- Template Haskell datatypes ===================================== libraries/ghc-internal/src/GHC/Internal/Float.hs ===================================== @@ -13,6 +13,9 @@ {-# OPTIONS_HADDOCK not-home #-} {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-} +-- For init in formatRealFloatAlt +{-# OPTIONS_GHC -Wno-x-partial #-} + ----------------------------------------------------------------------------- -- | -- Module : GHC.Internal.Float ===================================== libraries/ghc-internal/src/GHC/Internal/List.hs ===================================== @@ -190,12 +190,18 @@ tail :: HasCallStack => [a] -> [a] tail (_:xs) = xs tail [] = errorEmptyList "tail" -{-# WARNING in "x-partial" tail "This is a partial function, it throws an error on empty lists. Replace it with 'drop' 1, or use pattern matching or 'GHC.Internal.Data.List.uncons' instead. Consider refactoring to use \"Data.List.NonEmpty\"." #-} +{-# WARNING in "x-partial" tail "This is a partial function, it throws an error on empty lists. Replace it with 'drop' 1, or use pattern matching or 'Data.List.uncons' instead. Consider refactoring to use \"Data.List.NonEmpty\"." #-} -- | \(\mathcal{O}(n)\). Extract the last element of a list, which must be -- finite and non-empty. -- --- WARNING: This function is partial. Consider using 'unsnoc' instead. +-- To disable the warning about partiality put +-- @{-# OPTIONS_GHC -Wno-x-partial -Wno-unrecognised-warning-flags #-}@ +-- at the top of the file. To disable it throughout a package put the same +-- options into @ghc-options@ section of Cabal file. To disable it in GHCi +-- put @:set -Wno-x-partial -Wno-unrecognised-warning-flags@ into @~/.ghci@ +-- config file. See also the +-- [migration guide](https://github.com/haskell/core-libraries-committee/blob/main/guides/warning...). -- -- ==== __Examples__ -- @@ -218,10 +224,18 @@ last xs = foldl (\_ x -> x) lastError xs lastError :: HasCallStack => a lastError = errorEmptyList "last" +{-# WARNING in "x-partial" last "This is a partial function, it throws an error on empty lists. Use 'Data.List.unsnoc' instead. Consider refactoring to use \"Data.List.NonEmpty\"." #-} + -- | \(\mathcal{O}(n)\). Return all the elements of a list except the last one. -- The list must be non-empty. -- --- WARNING: This function is partial. Consider using 'unsnoc' instead. +-- To disable the warning about partiality put +-- @{-# OPTIONS_GHC -Wno-x-partial -Wno-unrecognised-warning-flags #-}@ +-- at the top of the file. To disable it throughout a package put the same +-- options into @ghc-options@ section of Cabal file. To disable it in GHCi +-- put @:set -Wno-x-partial -Wno-unrecognised-warning-flags@ into @~/.ghci@ +-- config file. See also the +-- [migration guide](https://github.com/haskell/core-libraries-committee/blob/main/guides/warning...). -- -- ==== __Examples__ -- @@ -240,6 +254,8 @@ init (x:xs) = init' x xs where init' _ [] = [] init' y (z:zs) = y : init' z zs +{-# WARNING in "x-partial" init "This is a partial function, it throws an error on empty lists. Use 'Data.List.unsnoc' instead. Consider refactoring to use \"Data.List.NonEmpty\"." #-} + -- | \(\mathcal{O}(1)\). Test whether a list is empty. -- -- >>> null [] ===================================== libraries/ghc-internal/src/GHC/Internal/System/IO.hs ===================================== @@ -1,5 +1,6 @@ {-# LANGUAGE Trustworthy #-} {-# LANGUAGE CPP, NoImplicitPrelude, CApiFFI #-} +{-# OPTIONS_GHC -Wno-x-partial #-} ----------------------------------------------------------------------------- -- | ===================================== libraries/ghc-internal/src/GHC/Internal/TH/Syntax.hs ===================================== @@ -1,4 +1,5 @@ {-# OPTIONS_HADDOCK not-home #-} -- we want users to import Language.Haskell.TH.Syntax instead +{-# OPTIONS_GHC -Wno-x-partial #-} {-# LANGUAGE CPP #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveGeneric #-} ===================================== libraries/template-haskell/vendored-filepath/System/FilePath/Posix.hs ===================================== @@ -1,6 +1,7 @@ -- Vendored from filepath v1.4.2.2 {-# LANGUAGE PatternGuards #-} +{-# OPTIONS_GHC -Wno-x-partial #-} -- This template expects CPP definitions for: -- MODULE_NAME = Posix | Windows ===================================== libraries/template-haskell/vendored-filepath/System/FilePath/Windows.hs ===================================== @@ -1,6 +1,7 @@ -- Vendored from filepath v1.4.2.2 {-# LANGUAGE PatternGuards #-} +{-# OPTIONS_GHC -Wno-x-partial #-} -- This template expects CPP definitions for: -- MODULE_NAME = Posix | Windows ===================================== testsuite/tests/driver/j-space/jspace.hs ===================================== @@ -7,7 +7,7 @@ import System.Environment import GHC.Driver.Env.Types import GHC.Profiling import System.Mem -import Data.List (isPrefixOf) +import Data.List (isPrefixOf, unsnoc) import Control.Monad import System.Exit import GHC.Platform @@ -41,7 +41,9 @@ initGhcM xs = do requestHeapCensus performGC [ys] <- filter (isPrefixOf (ghcUnitId <> ":GHC.Unit.Module.ModDetails.ModDetails")) . lines <$> readFile "jspace.hp" - let (n :: Int) = read (last (words ys)) + let (n :: Int) = case unsnoc (words ys) of + Nothing -> error "input is unexpectedly empty" + Just (_, lst) -> read lst -- The output should be 50 * 8 * word_size (i.e. 3600, or 1600 on 32-bit architectures): -- the test contains DEPTH + WIDTH + 2 = 50 modules J, H_0, .., H_DEPTH, W_1, .., W_WIDTH, -- and each ModDetails contains 1 (info table) + 8 word-sized fields. ===================================== testsuite/tests/rts/KeepCafsBase.hs ===================================== @@ -1,3 +1,5 @@ +{-# OPTIONS_GHC -Wno-x-partial #-} + module KeepCafsBase (x) where x :: Int ===================================== utils/check-exact/Main.hs ===================================== @@ -6,6 +6,7 @@ {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-} {-# OPTIONS_GHC -Wno-incomplete-patterns #-} {-# OPTIONS_GHC -Wno-orphans #-} +{-# OPTIONS_GHC -Wno-x-partial #-} import Data.Data import Data.List (intercalate) ===================================== utils/check-exact/Transform.hs ===================================== @@ -96,6 +96,7 @@ import GHC.Data.FastString import GHC.Types.SrcLoc import Data.Data +import Data.List (unsnoc) import Data.List.NonEmpty (NonEmpty (..)) import qualified Data.List.NonEmpty as NE import Data.Maybe @@ -212,8 +213,9 @@ captureTypeSigSpacing (L l (SigD x (TypeSig (AnnSig (EpUniTok dca u) mp md) ns ( where -- we want DPs for the distance from the end of the ns to the -- AnnDColon, and to the start of the ty - rd = case last ns of - L (EpAnn anc' _ _) _ -> epaLocationRealSrcSpan anc' + rd = case unsnoc ns of + Nothing -> error "unexpected empty list in 'ns' variable" + Just (_, L (EpAnn anc' _ _) _) -> epaLocationRealSrcSpan anc' dca' = case dca of EpaSpan ss@(RealSrcSpan r _) -> (EpaDelta ss (ss2delta (ss2posEnd rd) r) []) _ -> dca @@ -294,7 +296,7 @@ setEntryDP (L (EpAnn (EpaSpan ss@(RealSrcSpan r _)) an cs) a) dp where cs'' = setPriorComments cs [] csd = L (EpaDelta ss dp NoComments) c:commentOrigDeltas cs' - lc = last $ (L ca c:cs') + lc = NE.last (L ca c :| cs') delta = case getLoc lc of EpaSpan (RealSrcSpan rr _) -> ss2delta (ss2pos rr) r EpaSpan _ -> (SameLine 0) ===================================== utils/check-exact/Utils.hs ===================================== @@ -37,7 +37,7 @@ import GHC.Base (NonEmpty(..)) import GHC.Parser.Lexer (allocateComments) import Data.Data hiding ( Fixity ) -import Data.List (sortBy, partition) +import Data.List (sortBy, partition, unsnoc) import qualified Data.Map.Strict as Map import Debug.Trace @@ -734,8 +734,9 @@ ghead info [] = error $ "ghead "++info++" []" ghead _info (h:_) = h glast :: String -> [a] -> a -glast info [] = error $ "glast " ++ info ++ " []" -glast _info h = last h +glast info xs = case unsnoc xs of + Nothing -> error $ "glast " ++ info ++ " []" + Just (_, lst) -> lst gtail :: String -> [a] -> [a] gtail info [] = error $ "gtail " ++ info ++ " []" ===================================== utils/ghc-pkg/Main.hs ===================================== @@ -8,7 +8,7 @@ {-# LANGUAGE DataKinds #-} {-# LANGUAGE TupleSections #-} {-# LANGUAGE ScopedTypeVariables #-} -{-# OPTIONS_GHC -fno-warn-orphans #-} +{-# OPTIONS_GHC -Wno-orphans -Wno-x-partial #-} -- Fine if this comes from make/Hadrian or the pre-built base. #include ===================================== utils/ghc-toolchain/src/GHC/Toolchain/Utils.hs ===================================== @@ -14,6 +14,7 @@ module GHC.Toolchain.Utils import Control.Exception import Control.Monad import Control.Monad.IO.Class +import Data.List (unsnoc) import System.Directory import System.FilePath import System.IO.Error @@ -67,5 +68,4 @@ isSuccess = \case ExitFailure _ -> False lastLine :: String -> String -lastLine "" = "" -lastLine s = last $ lines s +lastLine = maybe "" snd . unsnoc . lines ===================================== utils/haddock/haddock-api/src/Haddock/Backends/Xhtml.hs ===================================== @@ -755,7 +755,7 @@ ppHtmlIndex divAlphabet << unordList ( map (\str -> anchor ! [href (subIndexHtmlFile str)] << str) $ - [ [c] | c <- initialChars, any ((== c) . toUpper . head . fst) index + [ [c] | c <- initialChars, any (maybe False ((== c) . toUpper . fst) . List.uncons . fst) index ] ++ [merged_name] ) @@ -772,7 +772,7 @@ ppHtmlIndex writeUtf8File (joinPath [odir, subIndexHtmlFile [c]]) (renderToString debug html) where html = indexPage True (Just c) index_part - index_part = [(n, stuff) | (n, stuff) <- this_ix, toUpper (head n) == c] + index_part = [(n, stuff) | (n@(headN : _), stuff) <- this_ix, toUpper headN == c] index :: [(String, Map GHC.Name [(Module, Bool)])] index = sortBy cmp (Map.toAscList full_index) ===================================== utils/haddock/haddock-library/src/Documentation/Haddock/Parser.hs ===================================== @@ -30,7 +30,7 @@ import Control.Arrow (first) import Control.Monad import Data.Char (chr, isAlpha, isSpace, isUpper) import Data.Functor (($>)) -import Data.List (elemIndex, intercalate, intersperse, unfoldr) +import Data.List (elemIndex, intercalate, intersperse, unfoldr, unsnoc) import Data.Maybe (fromMaybe, mapMaybe) import Data.Monoid import qualified Data.Set as Set @@ -870,10 +870,10 @@ codeblock = DocCodeBlock . parseParagraph . dropSpaces <$> ("@" *> skipHorizontalSpace *> "\n" *> block' <* "@") where - dropSpaces xs = - case splitByNl xs of - [] -> xs - ys -> case T.uncons (last ys) of + dropSpaces xs = let ys = splitByNl xs in + case unsnoc ys of + Nothing -> xs + Just (_, lastYs) -> case T.uncons lastYs of Just (' ', _) -> case mapM dropSpace ys of Nothing -> xs Just zs -> T.intercalate "\n" zs ===================================== utils/hpc ===================================== @@ -1 +1 @@ -Subproject commit 5923da3fe77993b7afc15b5163cffcaa7da6ecf5 +Subproject commit dd43f7e139d7a4f4908d1e8af35a75939f763ef1 ===================================== utils/hsc2hs ===================================== @@ -1 +1 @@ -Subproject commit fe3990b9f35000427b016a79330d9f195587cad8 +Subproject commit 2059c961fc28bbfd0cafdbef96d5d21f1d911b53 View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/5ee11c6936ff74dee8a880c45c48f54b... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/5ee11c6936ff74dee8a880c45c48f54b... 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