ghcup-hs/lib/GHCup/Utils/Prelude.hs

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{-# LANGUAGE CPP #-}
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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
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{-|
Module : GHCup.Utils.Prelude
Description : MegaParsec utilities
Copyright : (c) Julian Ospald, 2020
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License : LGPL-3.0
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Maintainer : hasufell@hasufell.de
Stability : experimental
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Portability : portable
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GHCup specific prelude. Lots of Excepts functionality.
-}
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module GHCup.Utils.Prelude
(module GHCup.Utils.Prelude,
#if defined(IS_WINDOWS)
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module GHCup.Utils.Prelude.Windows
#else
module GHCup.Utils.Prelude.Posix
#endif
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)
where
import GHCup.Types
import GHCup.Errors
import GHCup.Types.Optics
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import {-# SOURCE #-} GHCup.Utils.Logger
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#if defined(IS_WINDOWS)
import GHCup.Utils.Prelude.Windows
#else
import GHCup.Utils.Prelude.Posix
#endif
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import Control.Applicative
import Control.Exception.Safe
import Control.Monad
import Control.Monad.IO.Class
import Control.Monad.Reader
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import Data.Bifunctor
import Data.ByteString ( ByteString )
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import Data.List ( nub, intercalate, stripPrefix, isPrefixOf, dropWhileEnd, intersperse )
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import Data.Maybe
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import Data.Foldable
import Data.List.NonEmpty ( NonEmpty( (:|) ))
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import Data.String
import Data.Text ( Text )
import Data.Versions
import Data.Word8 hiding ( isDigit )
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import Haskus.Utils.Types.List
import Haskus.Utils.Variant.Excepts
import Text.PrettyPrint.HughesPJClass ( prettyShow, Pretty )
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import System.IO.Error
import System.IO.Temp
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import System.IO.Unsafe
import System.Directory
import System.FilePath
import Control.Retry
import GHC.IO.Exception
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import qualified Data.ByteString as B
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import qualified Data.ByteString.Lazy as L
import qualified Data.Strict.Maybe as S
import qualified Data.List.Split as Split
import qualified Data.List.NonEmpty as NE
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import qualified Data.Text as T
import qualified Data.Text.Encoding as E
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import qualified Data.Text.Encoding.Error as E
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import qualified Data.Text.Lazy as TL
import qualified Data.Text.Lazy.Builder as B
import qualified Data.Text.Lazy.Builder.Int as B
import qualified Data.Text.Lazy.Encoding as TLE
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-- $setup
-- >>> import Data.ByteString.Internal (c2w, w2c)
-- >>> import Test.QuickCheck
-- >>> import Data.Word8
-- >>> import qualified Data.Text as T
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-- >>> import qualified Data.Char as C
-- >>> import Data.List
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-- >>> instance Arbitrary T.Text where arbitrary = T.pack <$> arbitrary
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fS :: IsString a => String -> a
fS = fromString
fromStrictMaybe :: S.Maybe a -> Maybe a
fromStrictMaybe = S.maybe Nothing Just
fSM :: S.Maybe a -> Maybe a
fSM = fromStrictMaybe
toStrictMaybe :: Maybe a -> S.Maybe a
toStrictMaybe = maybe S.Nothing S.Just
tSM :: Maybe a -> S.Maybe a
tSM = toStrictMaybe
internalError :: String -> IO a
internalError = fail . ("Internal error: " <>)
iE :: String -> IO a
iE = internalError
showT :: Show a => a -> Text
showT = fS . show
-- | Like 'when', but where the test can be monadic.
whenM :: Monad m => m Bool -> m () -> m ()
whenM ~b ~t = ifM b t (return ())
-- | Like 'unless', but where the test can be monadic.
unlessM :: Monad m => m Bool -> m () -> m ()
unlessM ~b ~f = ifM b (return ()) f
-- | Like @if@, but where the test can be monadic.
ifM :: Monad m => m Bool -> m a -> m a -> m a
ifM ~b ~t ~f = do
b' <- b
if b' then t else f
whileM :: Monad m => m a -> (a -> m Bool) -> m a
whileM ~action ~f = do
a <- action
b' <- f a
if b' then whileM action f else pure a
whileM_ :: Monad m => m a -> (a -> m Bool) -> m ()
whileM_ ~action = void . whileM action
guardM :: (Monad m, Alternative m) => m Bool -> m ()
guardM ~f = guard =<< f
handleIO' :: (MonadIO m, MonadCatch m)
=> IOErrorType
-> (IOException -> m a)
-> m a
-> m a
handleIO' err handler = handleIO
(\e -> if err == ioeGetErrorType e then handler e else liftIO $ ioError e)
(??) :: forall e es a m . (Monad m, e :< es) => Maybe a -> e -> Excepts es m a
(??) m e = maybe (throwE e) pure m
(!?) :: forall e es a m
. (Monad m, e :< es)
=> m (Maybe a)
-> e
-> Excepts es m a
(!?) em e = lift em >>= (?? e)
lE :: forall e es a m . (Monad m, e :< es) => Either e a -> Excepts es m a
lE = liftE . veitherToExcepts . fromEither
lE' :: forall e' e es a m
. (Monad m, e :< es)
=> (e' -> e)
-> Either e' a
-> Excepts es m a
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lE' f = liftE . veitherToExcepts . fromEither . first f
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lEM :: forall e es a m . (Monad m, e :< es) => m (Either e a) -> Excepts es m a
lEM em = lift em >>= lE
lEM' :: forall e' e es a m
. (Monad m, e :< es)
=> (e' -> e)
-> m (Either e' a)
-> Excepts es m a
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lEM' f em = lift em >>= lE . first f
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-- for some obscure reason... this won't type-check if we move it to a different module
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catchWarn :: forall es m env . ( Pretty (V es)
, MonadReader env m
, HasLog env
, MonadIO m
, Monad m) => Excepts es m () -> Excepts '[] m ()
catchWarn = catchAllE @_ @es (\v -> lift $ logWarn (T.pack . prettyShow $ v))
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fromEither :: Either a b -> VEither '[a] b
fromEither = either (VLeft . V) VRight
liftIOException' :: ( MonadCatch m
, MonadIO m
, Monad m
, e :< es'
, LiftVariant es es'
)
=> IOErrorType
-> e
-> Excepts es m a
-> Excepts es' m a
liftIOException' errType ex =
handleIO
(\e ->
if errType == ioeGetErrorType e then throwE ex else liftIO $ ioError e
)
. liftE
liftIOException :: (MonadCatch m, MonadIO m, Monad m, e :< es')
=> IOErrorType
-> e
-> m a
-> Excepts es' m a
liftIOException errType ex =
handleIO
(\e ->
if errType == ioeGetErrorType e then throwE ex else liftIO $ ioError e
)
. lift
-- | Uses safe-exceptions.
hideError :: (MonadIO m, MonadCatch m) => IOErrorType -> m () -> m ()
hideError err = handleIO (\e -> if err == ioeGetErrorType e then pure () else liftIO . ioError $ e)
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hideErrorDef :: (MonadIO m, MonadCatch m) => [IOErrorType] -> a -> m a -> m a
hideErrorDef errs def =
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handleIO (\e -> if ioeGetErrorType e `elem` errs then pure def else liftIO $ ioError e)
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hideErrorDefM :: (MonadIO m, MonadCatch m) => [IOErrorType] -> m a -> m a -> m a
hideErrorDefM errs def =
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handleIO (\e -> if ioeGetErrorType e `elem` errs then def else liftIO $ ioError e)
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-- TODO: does this work?
hideExcept :: forall e es es' a m
. (Monad m, e :< es, LiftVariant (Remove e es) es')
=> e
-> a
-> Excepts es m a
-> Excepts es' m a
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hideExcept _ a =
catchLiftLeft ((\_ -> pure a) :: (e -> Excepts es' m a))
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hideExcept' :: forall e es es' m
. (Monad m, e :< es, LiftVariant (Remove e es) es')
=> e
-> Excepts es m ()
-> Excepts es' m ()
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hideExcept' _ =
catchLiftLeft ((\_ -> pure ()) :: (e -> Excepts es' m ()))
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reThrowAll :: forall e es es' a m
. (Monad m, e :< es')
=> (V es -> e)
-> Excepts es m a
-> Excepts es' m a
reThrowAll f = catchAllE (throwE . f)
reThrowAllIO :: forall e es es' a m
. (MonadCatch m, Monad m, MonadIO m, e :< es')
=> (V es -> e)
-> (IOException -> e)
-> Excepts es m a
-> Excepts es' m a
reThrowAllIO f g = handleIO (throwE . g) . catchAllE (throwE . f)
throwEither :: (Exception a, MonadThrow m) => Either a b -> m b
throwEither a = case a of
Left e -> throwM e
Right r -> pure r
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throwEither' :: (Exception a, MonadThrow m) => a -> Either x b -> m b
throwEither' e eth = case eth of
Left _ -> throwM e
Right r -> pure r
throwMaybe :: (Exception a, MonadThrow m) => a -> Maybe b -> m b
throwMaybe a m = case m of
Nothing -> throwM a
Just r -> pure r
throwMaybeM :: (Exception a, MonadThrow m) => a -> m (Maybe b) -> m b
throwMaybeM a am = do
m <- am
throwMaybe a m
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verToBS :: Version -> ByteString
verToBS = E.encodeUtf8 . prettyVer
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verToS :: Version -> String
verToS = T.unpack . prettyVer
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intToText :: Integral a => a -> T.Text
intToText = TL.toStrict . B.toLazyText . B.decimal
removeLensFieldLabel :: String -> String
removeLensFieldLabel str' =
maybe str' T.unpack . T.stripPrefix (T.pack "_") . T.pack $ str'
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pvpToVersion :: MonadThrow m => PVP -> Text -> m Version
pvpToVersion pvp_ rest =
either (\_ -> throwM $ ParseError "Couldn't convert PVP to Version") pure . version . (<> rest) . prettyPVP $ pvp_
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-- | Convert a version to a PVP and unparsable rest.
--
-- -- prop> \v -> let (Just (pvp', r)) = versionToPVP v in pvpToVersion pvp' r === Just v
versionToPVP :: MonadThrow m => Version -> m (PVP, Text)
versionToPVP (Version (Just _) _ _ _) = throwM $ ParseError "Unexpected epoch"
versionToPVP v = either (\_ -> (, rest v) <$> alternative v) (pure . (, mempty)) . pvp . prettyVer $ v
where
alternative :: MonadThrow m => Version -> m PVP
alternative v' = case NE.takeWhile isDigit (_vChunks v') of
[] -> throwM $ ParseError "Couldn't convert Version to PVP"
xs -> pure $ pvpFromList (unsafeDigit <$> xs)
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rest :: Version -> Text
rest (Version _ cs pr me) =
let chunks = NE.dropWhile isDigit cs
ver = intersperse (T.pack ".") . chunksAsT $ chunks
me' = maybe [] (\m -> [T.pack "+",m]) me
pr' = foldable [] (T.pack "-" :) $ intersperse (T.pack ".") (chunksAsT pr)
prefix = case (ver, pr', me') of
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(_:_, _, _) -> T.pack "."
_ -> T.pack ""
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in prefix <> mconcat (ver <> pr' <> me')
where
chunksAsT :: Functor t => t VChunk -> t Text
chunksAsT = fmap (foldMap f)
where
f :: VUnit -> Text
f (Digits i) = T.pack $ show i
f (Str s) = s
foldable :: Foldable f => f b -> (f a -> f b) -> f a -> f b
foldable d g f | null f = d
| otherwise = g f
isDigit :: VChunk -> Bool
isDigit (Digits _ :| []) = True
isDigit _ = False
unsafeDigit :: VChunk -> Int
unsafeDigit (Digits x :| []) = fromIntegral x
unsafeDigit _ = error "unsafeDigit: wrong input"
pvpFromList :: [Int] -> PVP
pvpFromList = PVP . NE.fromList . fmap fromIntegral
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-- | Safe 'decodeUtf8With'. Replaces an invalid input byte with
-- the Unicode replacement character U+FFFD.
decUTF8Safe :: ByteString -> Text
decUTF8Safe = E.decodeUtf8With E.lenientDecode
decUTF8Safe' :: L.ByteString -> Text
decUTF8Safe' = TL.toStrict . TLE.decodeUtf8With E.lenientDecode
-- | Escape a version for use in regex
escapeVerRex :: Version -> ByteString
escapeVerRex = B.pack . go . B.unpack . verToBS
where
go [] = []
go (x : xs) | x == _period = [_backslash, _period] ++ go xs
| otherwise = x : go xs
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-- | More permissive version of 'createDirRecursive'. This doesn't
-- error when the destination is a symlink to a directory.
createDirRecursive' :: FilePath -> IO ()
createDirRecursive' p =
handleIO (\e -> if isAlreadyExistsError e then isSymlinkDir e else throwIO e)
. createDirectoryIfMissing True
$ p
where
isSymlinkDir e = do
ft <- pathIsSymbolicLink p
case ft of
True -> do
rp <- canonicalizePath p
rft <- doesDirectoryExist rp
case rft of
True -> pure ()
_ -> throwIO e
_ -> throwIO e
-- | Recursively copy the contents of one directory to another path.
--
-- This is a rip-off of Cabal library.
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copyDirectoryRecursive :: FilePath -> FilePath -> (FilePath -> FilePath -> IO ()) -> IO ()
copyDirectoryRecursive srcDir destDir doCopy = do
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srcFiles <- getDirectoryContentsRecursive srcDir
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copyFilesWith destDir [ (srcDir, f)
| f <- srcFiles ]
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where
-- | Common implementation of 'copyFiles', 'installOrdinaryFiles',
-- 'installExecutableFiles' and 'installMaybeExecutableFiles'.
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copyFilesWith :: FilePath -> [(FilePath, FilePath)] -> IO ()
copyFilesWith targetDir srcFiles = do
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-- Create parent directories for everything
let dirs = map (targetDir </>) . nub . map (takeDirectory . snd) $ srcFiles
traverse_ (createDirectoryIfMissing True) dirs
-- Copy all the files
sequence_ [ let src = srcBase </> srcFile
dest = targetDir </> srcFile
in doCopy src dest
| (srcBase, srcFile) <- srcFiles ]
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-- | List all the files in a directory and all subdirectories.
--
-- The order places files in sub-directories after all the files in their
-- parent directories. The list is generated lazily so is not well defined if
-- the source directory structure changes before the list is used.
--
getDirectoryContentsRecursive :: FilePath -> IO [FilePath]
getDirectoryContentsRecursive topdir = recurseDirectories [""]
where
recurseDirectories :: [FilePath] -> IO [FilePath]
recurseDirectories [] = return []
recurseDirectories (dir:dirs) = unsafeInterleaveIO $ do
(files, dirs') <- collect [] [] =<< getDirectoryContents (topdir </> dir)
files' <- recurseDirectories (dirs' ++ dirs)
return (files ++ files')
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where
collect files dirs' [] = return (reverse files
,reverse dirs')
collect files dirs' (entry:entries) | ignore entry
= collect files dirs' entries
collect files dirs' (entry:entries) = do
let dirEntry = dir </> entry
isDirectory <- doesDirectoryExist (topdir </> dirEntry)
if isDirectory
then collect files (dirEntry:dirs') entries
else collect (dirEntry:files) dirs' entries
ignore ['.'] = True
ignore ['.', '.'] = True
ignore _ = False
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-- https://github.com/haskell/directory/issues/110
-- https://github.com/haskell/directory/issues/96
-- https://www.sqlite.org/src/info/89f1848d7f
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recyclePathForcibly :: ( MonadIO m
, MonadReader env m
, HasDirs env
, MonadMask m
)
=> FilePath
-> m ()
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recyclePathForcibly fp
| isWindows = do
Dirs { recycleDir } <- getDirs
tmp <- liftIO $ createTempDirectory recycleDir "recyclePathForcibly"
let dest = tmp </> takeFileName fp
liftIO (moveFile fp dest)
`catch`
(\e -> if isPermissionError e {- EXDEV on windows -} then recover (liftIO $ removePathForcibly fp) else throwIO e)
`finally`
liftIO (handleIO (\_ -> pure ()) $ removePathForcibly tmp)
| otherwise = liftIO $ removePathForcibly fp
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rmPathForcibly :: ( MonadIO m
, MonadMask m
)
=> FilePath
-> m ()
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rmPathForcibly fp
| isWindows = recover (liftIO $ removePathForcibly fp)
| otherwise = liftIO $ removePathForcibly fp
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rmDirectory :: (MonadIO m, MonadMask m)
=> FilePath
-> m ()
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rmDirectory fp
| isWindows = recover (liftIO $ removeDirectory fp)
| otherwise = liftIO $ removeDirectory fp
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-- https://www.sqlite.org/src/info/89f1848d7f
-- https://github.com/haskell/directory/issues/96
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recycleFile :: ( MonadIO m
, MonadMask m
, MonadReader env m
, HasDirs env
)
=> FilePath
-> m ()
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recycleFile fp
| isWindows = do
Dirs { recycleDir } <- getDirs
liftIO $ whenM (doesDirectoryExist fp) $ ioError (IOError Nothing InappropriateType "recycleFile" "" Nothing (Just fp))
tmp <- liftIO $ createTempDirectory recycleDir "recycleFile"
let dest = tmp </> takeFileName fp
liftIO (moveFile fp dest)
`catch`
(\e -> if isPermissionError e {- EXDEV on windows -} then recover (liftIO $ removePathForcibly fp) else throwIO e)
`finally`
liftIO (handleIO (\_ -> pure ()) $ removePathForcibly tmp)
| otherwise = liftIO $ removeFile fp
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rmFile :: ( MonadIO m
, MonadMask m
)
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=> FilePath
-> m ()
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rmFile fp
| isWindows = recover (liftIO $ removeFile fp)
| otherwise = liftIO $ removeFile fp
rmDirectoryLink :: (MonadIO m, MonadMask m, MonadReader env m, HasDirs env)
=> FilePath
-> m ()
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rmDirectoryLink fp
| isWindows = recover (liftIO $ removeDirectoryLink fp)
| otherwise = liftIO $ removeDirectoryLink fp
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recover :: (MonadIO m, MonadMask m) => m a -> m a
recover action =
recovering (fullJitterBackoff 25000 <> limitRetries 10)
[\_ -> Handler (\e -> pure $ isPermissionError e)
,\_ -> Handler (\e -> pure (ioeGetErrorType e == InappropriateType))
,\_ -> Handler (\e -> pure (ioeGetErrorType e == UnsatisfiedConstraints))
]
(\_ -> action)
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copyFileE :: (CopyError :< xs, MonadCatch m, MonadIO m) => FilePath -> FilePath -> Excepts xs m ()
copyFileE from = handleIO (throwE . CopyError . show) . liftIO . copyFile from
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-- | Gathering monoidal values
--
-- >>> traverseFold (pure . (:["0"])) ["1","2"]
-- ["1","0","2","0"]
-- >>> traverseFold Just ["1","2","3","4","5"]
-- Just "12345"
--
-- prop> \t -> traverseFold Just t === Just (mconcat t)
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traverseFold :: (Foldable t, Applicative m, Monoid b) => (a -> m b) -> t a -> m b
traverseFold f = foldl (\mb a -> (<>) <$> mb <*> f a) (pure mempty)
-- | Gathering monoidal values
forFold :: (Foldable t, Applicative m, Monoid b) => t a -> (a -> m b) -> m b
forFold = \t -> (`traverseFold` t)
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-- | Strip @\\r@ and @\\n@ from 'String's
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--
-- >>> stripNewline "foo\n\n\n"
-- "foo"
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-- >>> stripNewline "foo\n\n\nfoo"
-- "foofoo"
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-- >>> stripNewline "foo\r"
-- "foo"
-- >>> stripNewline "foo"
-- "foo"
--
-- prop> \t -> stripNewline (t <> "\n") === stripNewline t
-- prop> \t -> not (any (isNewLine . c2w) t) ==> stripNewline t == t
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stripNewline :: String -> String
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stripNewline = filter (`notElem` "\n\r")
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-- | Strip @\\r@ and @\\n@ from end of 'String'.
--
-- >>> stripNewlineEnd "foo\n\n\n"
-- "foo"
-- >>> stripNewlineEnd "foo\n\n\nfoo"
-- "foo\n\n\nfoo"
-- >>> stripNewlineEnd "foo\r"
-- "foo"
-- >>> stripNewlineEnd "foo"
-- "foo"
--
-- prop> \t -> stripNewlineEnd (t <> "\n") === stripNewlineEnd t
-- prop> \t -> not (any (isNewLine . c2w) t) ==> stripNewlineEnd t == t
stripNewlineEnd :: String -> String
stripNewlineEnd = dropWhileEnd (`elem` "\n\r")
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-- | Strip @\\r@ and @\\n@ from 'Text's
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--
-- >>> stripNewline' "foo\n\n\n"
-- "foo"
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-- >>> stripNewline' "foo\n\n\nfoo"
-- "foofoo"
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-- >>> stripNewline' "foo\r"
-- "foo"
-- >>> stripNewline' "foo"
-- "foo"
--
-- prop> \t -> stripNewline' (t <> "\n") === stripNewline' t
-- prop> \t -> not (T.any (isNewLine . c2w) t) ==> stripNewline' t == t
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stripNewline' :: T.Text -> T.Text
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stripNewline' = T.filter (`notElem` "\n\r")
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-- | Is the word8 a newline?
--
-- >>> isNewLine (c2w '\n')
-- True
-- >>> isNewLine (c2w '\r')
-- True
--
-- prop> \w -> w /= _lf && w /= _cr ==> not (isNewLine w)
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isNewLine :: Word8 -> Bool
isNewLine w
| w == _lf = True
| w == _cr = True
| otherwise = False
-- | Split on a PVP suffix.
--
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-- >>> splitOnPVP "-" "ghc-iserv-dyn-9.3.20210706"
-- ("ghc-iserv-dyn","9.3.20210706")
-- >>> splitOnPVP "-" "ghc-iserv-dyn"
-- ("ghc-iserv-dyn","")
splitOnPVP :: String -> String -> (String, String)
splitOnPVP c s = case Split.splitOn c s of
[] -> def
[_] -> def
xs
| let l = last xs
, (Right _) <- pvp (T.pack l) -> (intercalate c (init xs), l)
| otherwise -> def
where
def = (s, "")
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-- | Like 'find', but where the test can be monadic.
--
-- >>> findM (Just . C.isUpper) "teST"
-- Just (Just 'S')
-- >>> findM (Just . C.isUpper) "test"
-- Just Nothing
-- >>> findM (Just . const True) ["x",undefined]
-- Just (Just "x")
findM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
findM ~p = foldr (\x -> ifM (p x) (pure $ Just x)) (pure Nothing)
-- | Drops the given suffix from a list.
-- It returns the original sequence if the sequence doesn't end with the given suffix.
--
-- >>> dropSuffix "!" "Hello World!"
-- "Hello World"
-- >>> dropSuffix "!" "Hello World!!"
-- "Hello World!"
-- >>> dropSuffix "!" "Hello World."
-- "Hello World."
dropSuffix :: Eq a => [a] -> [a] -> [a]
dropSuffix a b = fromMaybe b $ stripSuffix a b
-- | Return the prefix of the second list if its suffix
-- matches the entire first list.
--
-- >>> stripSuffix "bar" "foobar"
-- Just "foo"
-- >>> stripSuffix "" "baz"
-- Just "baz"
-- >>> stripSuffix "foo" "quux"
-- Nothing
stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]
stripSuffix a b = reverse <$> stripPrefix (reverse a) (reverse b)
-- | Drops the given prefix from a list.
-- It returns the original sequence if the sequence doesn't start with the given prefix.
--
-- >>> dropPrefix "Mr. " "Mr. Men"
-- "Men"
-- >>> dropPrefix "Mr. " "Dr. Men"
-- "Dr. Men"
dropPrefix :: Eq a => [a] -> [a] -> [a]
dropPrefix a b = fromMaybe b $ stripPrefix a b
-- | Break a list into pieces separated by the first
-- list argument, consuming the delimiter. An empty delimiter is
-- invalid, and will cause an error to be raised.
--
-- >>> splitOn "\r\n" "a\r\nb\r\nd\r\ne"
-- ["a","b","d","e"]
-- >>> splitOn "aaa" "aaaXaaaXaaaXaaa"
-- ["","X","X","X",""]
-- >>> splitOn "x" "x"
-- ["",""]
-- >>> splitOn "x" ""
-- [""]
--
-- prop> \s x -> s /= "" ==> intercalate s (splitOn s x) == x
-- prop> \c x -> splitOn [c] x == split (==c) x
splitOn :: Eq a => [a] -> [a] -> [[a]]
splitOn [] _ = error "splitOn, needle may not be empty"
splitOn _ [] = [[]]
splitOn needle haystack = a : if null b then [] else splitOn needle $ drop (length needle) b
where (a,b) = breakOn needle haystack
-- | Splits a list into components delimited by separators,
-- where the predicate returns True for a separator element. The
-- resulting components do not contain the separators. Two adjacent
-- separators result in an empty component in the output.
--
-- >>> split (== 'a') "aabbaca"
-- ["","","bb","c",""]
-- >>> split (== 'a') ""
-- [""]
-- >>> split (== ':') "::xyz:abc::123::"
-- ["","","xyz","abc","","123","",""]
-- >>> split (== ',') "my,list,here"
-- ["my","list","here"]
split :: (a -> Bool) -> [a] -> [[a]]
split _ [] = [[]]
split f (x:xs)
| f x = [] : split f xs
| y:ys <- split f xs = (x:y) : ys
| otherwise = [[]]
-- | Find the first instance of @needle@ in @haystack@.
-- The first element of the returned tuple
-- is the prefix of @haystack@ before @needle@ is matched. The second
-- is the remainder of @haystack@, starting with the match.
-- If you want the remainder /without/ the match, use 'stripInfix'.
--
-- >>> breakOn "::" "a::b::c"
-- ("a","::b::c")
-- >>> breakOn "/" "foobar"
-- ("foobar","")
--
-- prop> \needle haystack -> let (prefix,match) = breakOn needle haystack in prefix ++ match == haystack
breakOn :: Eq a => [a] -> [a] -> ([a], [a])
breakOn needle haystack | needle `isPrefixOf` haystack = ([], haystack)
breakOn _ [] = ([], [])
breakOn needle (x:xs) = first (x:) $ breakOn needle xs