extra-1.4.2: Extra functions I use.

Safe Haskell | None
Language | Haskell2010

Extra

Contents

- Control.Concurrent.Extra
- Control.Exception.Extra
- Control.Monad.Extra
- Data.Either.Extra
- Data.IORef.Extra
- Data.List.Extra
- Data.Tuple.Extra
- Numeric.Extra
- System.Directory.Extra
- System.Environment.Extra
- System.Info.Extra
- System.IO.Extra
- System.Process.Extra
- System.Time.Extra

Description

This module documents all the functions available in this package.

Most users should import the specific modules (e.g. `Data.List.Extra`), which also reexport their non-`Extra` modules (e.g. `Data.List`).

Synopsis

- getNumCapabilities :: IO Int
- setNumCapabilities :: Int -> IO ()
- withNumCapabilities :: Int -> IO a -> IO a
- forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId
- once :: IO a -> IO (IO a)
- onceFork :: IO a -> IO (IO a)
- data Lock
- newLock :: IO Lock
- withLock :: Lock -> IO a -> IO a
- withLockTry :: Lock -> IO a -> IO (Maybe a)
- data Var a
- newVar :: a -> IO (Var a)
- readVar :: Var a -> IO a
- modifyVar :: Var a -> (a -> IO (a, b)) -> IO b
- modifyVar_ :: Var a -> (a -> IO a) -> IO ()
- withVar :: Var a -> (a -> IO b) -> IO b
- data Barrier a
- newBarrier :: IO (Barrier a)
- signalBarrier :: Barrier a -> a -> IO ()
- waitBarrier :: Barrier a -> IO a
- waitBarrierMaybe :: Barrier a -> IO (Maybe a)
- retry :: Int -> IO a -> IO a
- retryBool :: Exception e => (e -> Bool) -> Int -> IO a -> IO a
- showException :: Show e => e -> IO String
- stringException :: String -> IO String
- errorIO :: String -> IO a
- ignore :: IO () -> IO ()
- catch_ :: IO a -> (SomeException -> IO a) -> IO a
- handle_ :: (SomeException -> IO a) -> IO a -> IO a
- try_ :: IO a -> IO (Either SomeException a)
- catchJust_ :: (SomeException -> Maybe b) -> IO a -> (b -> IO a) -> IO a
- handleJust_ :: (SomeException -> Maybe b) -> (b -> IO a) -> IO a -> IO a
- tryJust_ :: (SomeException -> Maybe b) -> IO a -> IO (Either b a)
- catchBool :: Exception e => (e -> Bool) -> IO a -> (e -> IO a) -> IO a
- handleBool :: Exception e => (e -> Bool) -> (e -> IO a) -> IO a -> IO a
- tryBool :: Exception e => (e -> Bool) -> IO a -> IO (Either e a)
- whenJust :: Applicative m => Maybe a -> (a -> m ()) -> m ()
- whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()
- unit :: m () -> m ()
- loopM :: Monad m => (a -> m (Either a b)) -> a -> m b
- whileM :: Monad m => m Bool -> m ()
- partitionM :: Monad m => (a -> m Bool) -> [a] -> m ([a], [a])
- concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b]
- mapMaybeM :: Monad m => (a -> m (Maybe b)) -> [a] -> m [b]
- findM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
- firstJustM :: Monad m => (a -> m (Maybe b)) -> [a] -> m (Maybe b)
- whenM :: Monad m => m Bool -> m () -> m ()
- unlessM :: Monad m => m Bool -> m () -> m ()
- ifM :: Monad m => m Bool -> m a -> m a -> m a
- notM :: Functor m => m Bool -> m Bool
- (||^) :: Monad m => m Bool -> m Bool -> m Bool
- (&&^) :: Monad m => m Bool -> m Bool -> m Bool
- orM :: Monad m => [m Bool] -> m Bool
- andM :: Monad m => [m Bool] -> m Bool
- anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool
- allM :: Monad m => (a -> m Bool) -> [a] -> m Bool
- isLeft :: Either a b -> Bool
- isRight :: Either a b -> Bool
- fromLeft :: Either l r -> l
- fromRight :: Either l r -> r
- fromEither :: Either a a -> a
- modifyIORef' :: IORef a -> (a -> a) -> IO ()
- writeIORef' :: IORef a -> a -> IO ()
- atomicModifyIORef' :: IORef a -> (a -> (a, b)) -> IO b
- atomicWriteIORef :: IORef a -> a -> IO ()
- atomicWriteIORef' :: IORef a -> a -> IO ()
- lower :: String -> String
- upper :: String -> String
- trim :: String -> String
- trimStart :: String -> String
- trimEnd :: String -> String
- word1 :: String -> (String, String)
- dropEnd :: Int -> [a] -> [a]
- takeEnd :: Int -> [a] -> [a]
- splitAtEnd :: Int -> [a] -> ([a], [a])
- breakEnd :: (a -> Bool) -> [a] -> ([a], [a])
- spanEnd :: (a -> Bool) -> [a] -> ([a], [a])
- dropWhileEnd :: (a -> Bool) -> [a] -> [a]
- dropWhileEnd' :: (a -> Bool) -> [a] -> [a]
- takeWhileEnd :: (a -> Bool) -> [a] -> [a]
- stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]
- stripInfix :: Eq a => [a] -> [a] -> Maybe ([a], [a])
- stripInfixEnd :: Eq a => [a] -> [a] -> Maybe ([a], [a])
- wordsBy :: (a -> Bool) -> [a] -> [[a]]
- linesBy :: (a -> Bool) -> [a] -> [[a]]
- breakOn :: Eq a => [a] -> [a] -> ([a], [a])
- breakOnEnd :: Eq a => [a] -> [a] -> ([a], [a])
- splitOn :: Eq a => [a] -> [a] -> [[a]]
- split :: (a -> Bool) -> [a] -> [[a]]
- chunksOf :: Int -> [a] -> [[a]]
- list :: b -> (a -> [a] -> b) -> [a] -> b
- uncons :: [a] -> Maybe (a, [a])
- unsnoc :: [a] -> Maybe ([a], a)
- cons :: a -> [a] -> [a]
- snoc :: [a] -> a -> [a]
- drop1 :: [a] -> [a]
- groupSort :: Ord k => [(k, v)] -> [(k, [v])]
- groupSortOn :: Ord b => (a -> b) -> [a] -> [[a]]
- groupSortBy :: (a -> a -> Ordering) -> [a] -> [[a]]
- nubOrd :: Ord a => [a] -> [a]
- nubOrdBy :: (a -> a -> Ordering) -> [a] -> [a]
- nubOrdOn :: Ord b => (a -> b) -> [a] -> [a]
- nubOn :: Eq b => (a -> b) -> [a] -> [a]
- groupOn :: Eq b => (a -> b) -> [a] -> [[a]]
- sortOn :: Ord b => (a -> b) -> [a] -> [a]
- disjoint :: Eq a => [a] -> [a] -> Bool
- allSame :: Eq a => [a] -> Bool
- anySame :: Eq a => [a] -> Bool
- repeatedly :: ([a] -> (b, [a])) -> [a] -> [b]
- for :: [a] -> (a -> b) -> [b]
- firstJust :: (a -> Maybe b) -> [a] -> Maybe b
- concatUnzip :: [([a], [b])] -> ([a], [b])
- concatUnzip3 :: [([a], [b], [c])] -> ([a], [b], [c])
- replace :: Eq a => [a] -> [a] -> [a] -> [a]
- merge :: Ord a => [a] -> [a] -> [a]
- mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
- first :: (a -> a') -> (a, b) -> (a', b)
- second :: (b -> b') -> (a, b) -> (a, b')
- (***) :: (a -> a') -> (b -> b') -> (a, b) -> (a', b')
- (&&&) :: (a -> b) -> (a -> c) -> a -> (b, c)
- dupe :: a -> (a, a)
- both :: (a -> b) -> (a, a) -> (b, b)
- fst3 :: (a, b, c) -> a
- snd3 :: (a, b, c) -> b
- thd3 :: (a, b, c) -> c
- showDP :: RealFloat a => Int -> a -> String
- intToDouble :: Int -> Double
- intToFloat :: Int -> Float
- floatToDouble :: Float -> Double
- doubleToFloat :: Double -> Float
- withCurrentDirectory :: FilePath -> IO a -> IO a
- createDirectoryPrivate :: String -> IO ()
- listContents :: FilePath -> IO [FilePath]
- listFiles :: FilePath -> IO [FilePath]
- listFilesInside :: (FilePath -> IO Bool) -> FilePath -> IO [FilePath]
- listFilesRecursive :: FilePath -> IO [FilePath]
- getExecutablePath :: IO FilePath
- lookupEnv :: String -> IO (Maybe String)
- isWindows :: Bool
- captureOutput :: IO a -> IO (String, a)
- withBuffering :: Handle -> BufferMode -> IO a -> IO a
- readFileEncoding :: TextEncoding -> FilePath -> IO String
- readFileUTF8 :: FilePath -> IO String
- readFileBinary :: FilePath -> IO String
- readFile' :: FilePath -> IO String
- readFileEncoding' :: TextEncoding -> FilePath -> IO String
- readFileUTF8' :: FilePath -> IO String
- readFileBinary' :: FilePath -> IO String
- writeFileEncoding :: TextEncoding -> FilePath -> String -> IO ()
- writeFileUTF8 :: FilePath -> String -> IO ()
- writeFileBinary :: FilePath -> String -> IO ()
- withTempFile :: (FilePath -> IO a) -> IO a
- withTempDir :: (FilePath -> IO a) -> IO a
- newTempFile :: IO (FilePath, IO ())
- newTempDir :: IO (FilePath, IO ())
- system_ :: String -> IO ()
- systemOutput :: String -> IO (ExitCode, String)
- systemOutput_ :: String -> IO String
- type Seconds = Double
- sleep :: Seconds -> IO ()
- timeout :: Seconds -> IO a -> IO (Maybe a)
- subtractTime :: UTCTime -> UTCTime -> Seconds
- showDuration :: Seconds -> String
- offsetTime :: IO (IO Seconds)
- offsetTimeIncrease :: IO (IO Seconds)
- duration :: IO a -> IO (Seconds, a)

# Control.Concurrent.Extra

Extra functions available in `Control.Concurrent.Extra`.

getNumCapabilities :: IO Int

Returns the number of Haskell threads that can run truly simultaneously (on separate physical processors) at any given time. To change this value, use `setNumCapabilities`.

Since: 4.4.0.0

setNumCapabilities :: Int -> IO ()

Set the number of Haskell threads that can run truly simultaneously (on separate physical processors) at any given time. The number passed to `forkOn` is interpreted modulo this value. The initial value is given by the `+RTS -N` runtime flag.

This is also the number of threads that will participate in parallel garbage collection. It is strongly recommended that the number of capabilities is not set larger than the number of physical processor cores, and it may often be beneficial to leave one or more cores free to avoid contention with other processes in the machine.

Since: 4.5.0.0

withNumCapabilities :: Int -> IO a -> IO a

On GHC 7.6 and above with the `-threaded` flag, brackets a call to `setNumCapabilities`. On lower versions (which lack `setNumCapabilities`) this function just runs the argument action.

forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId

fork a thread and call the supplied function when the thread is about to terminate, with an exception or a returned value. The function is called with asynchronous exceptions masked.

    forkFinally action and_then =
      mask $ \restore ->
        forkIO $ try (restore action) >>= and_then

This function is useful for informing the parent when a child terminates, for example.

Since: 4.6.0.0

once :: IO a -> IO (IO a)

Given an action, produce a wrapped action that runs at most once. If the function raises an exception, the same exception will be reraised each time.

    let x ||| y = do t1 <- onceFork x; t2 <- onceFork y; t1; t2
    \(x :: IO Int) -> void (once x) == return ()
    \(x :: IO Int) -> join (once x) == x
    \(x :: IO Int) -> (do y <- once x; y; y) == x
    \(x :: IO Int) -> (do y <- once x; y ||| y) == x

onceFork :: IO a -> IO (IO a)

Like `once`, but immediately starts running the computation on a background thread.

    \(x :: IO Int) -> join (onceFork x) == x
    \(x :: IO Int) -> (do a <- onceFork x; a; a) == x

data Lock

Like an MVar, but has no value. Used to guarantees single-threaded access, typically to some system resource. As an example:

    lock <- newLock
    let output = withLock . putStrLn
    forkIO $ do ...; output "hello"
    forkIO $ do ...; output "world"

Here we are creating a lock to ensure that when writing output our messages do not get interleaved. This use of MVar never blocks on a put. It is permissible, but rare, that a withLock contains a withLock inside it - but if so, watch out for deadlocks.

newLock :: IO Lock

Create a new `Lock`.

withLock :: Lock -> IO a -> IO a

Perform some operation while holding `Lock`. Will prevent all other operations from using the `Lock` while the action is ongoing.

withLockTry :: Lock -> IO a -> IO (Maybe a)

Like `withLock` but will never block. If the operation cannot be executed immediately it will return `Nothing`.

data Var a

Like an MVar, but must always be full. Used to on a mutable variable in a thread-safe way. As an example:

    hits <- newVar 0
    forkIO $ do ...; modifyVar_ hits (+1); ...
    i <- readVar hits
    print (HITS,i)

Here we have a variable which we modify atomically, so modifications are not interleaved. This use of MVar never blocks on a put. No modifyVar operation should ever block, and they should always complete in a reasonable timeframe. A Var should not be used to protect some external resource, only the variable contained within. Information from a readVar should not be subsequently inserted back into the Var.

newVar :: a -> IO (Var a)

Create a new `Var` with a value.

readVar :: Var a -> IO a

Read the current value of the `Var`.

modifyVar :: Var a -> (a -> IO (a, b)) -> IO b

Modify a `Var` producing a new value and a return result.

modifyVar_ :: Var a -> (a -> IO a) -> IO ()

Modify a `Var`, a restricted version of `modifyVar`.

withVar :: Var a -> (a -> IO b) -> IO b

Perform some operation using the value in the `Var`, a restricted version of `modifyVar`.

data Barrier a

Starts out empty, then is filled exactly once. As an example:

    bar <- newBarrier
    forkIO $ do ...; val <- ...; signalBarrier bar val
    print =<< waitBarrier bar

Here we create a barrier which will contain some computed value. A thread is forked to fill the barrier, while the main thread waits for it to complete. A barrier has similarities to a future or promise from other languages, has been known as an IVar in other Haskell work, and in some ways is like a manually managed thunk.

newBarrier :: IO (Barrier a)

Create a new `Barrier`.

signalBarrier :: Barrier a -> a -> IO ()

Write a value into the Barrier, releasing anyone at `waitBarrier`. Any subsequent attempts to signal the `Barrier` will throw an exception.

waitBarrier :: Barrier a -> IO a

Wait until a barrier has been signaled with `signalBarrier`.

waitBarrierMaybe :: Barrier a -> IO (Maybe a)

A version of `waitBarrier` that never blocks, returning `Nothing` if the barrier has not yet been signaled.

# Control.Exception.Extra

Extra functions available in `Control.Exception.Extra`.

retry :: Int -> IO a -> IO a

Retry an operation at most n times (n must be positive). If the operation fails the nth time it will throw that final exception.

    retry 1 (print "x")  == print "x"
    retry 3 (fail "die") == fail "die"

retryBool :: Exception e => (e -> Bool) -> Int -> IO a -> IO a

Retry an operation at most n times (n must be positive), while the exception value and type match a predicate. If the operation fails the nth time it will throw that final exception.

showException :: Show e => e -> IO String

Show a value, but if the result contains exceptions, produce `<Exception>`. Defined as ``stringException` . show`. Particularly useful for printing exceptions to users, remembering that exceptions can themselves contain undefined values.

stringException :: String -> IO String

Fully evaluate an input String. If the String contains embedded exceptions it will produce `<Exception>`.

    stringException "test"                           == return "test"
    stringException ("test" ++ undefined)            == return "test<Exception>"
    stringException ("test" ++ undefined ++ "hello") == return "test<Exception>"
    stringException ['t','e','s','t',undefined]      == return "test<Exception>"

errorIO :: String -> IO a

Like error, but in the `IO` monad. Note that while `fail` in `IO` raises an `IOException`, this function raises an `ErrorCall` exception.

    try (errorIO "Hello") == return (Left (ErrorCall "Hello"))

ignore :: IO () -> IO ()

Ignore any exceptions thrown by the action.

    ignore (print 1)    == print 1
    ignore (fail "die") == return ()

catch_ :: IO a -> (SomeException -> IO a) -> IO a

A version of `catch` without the `Exception` context, restricted to `SomeException`, so catches all exceptions.

handle_ :: (SomeException -> IO a) -> IO a -> IO a

Like `catch_` but for `handle`

try_ :: IO a -> IO (Either SomeException a)

Like `catch_` but for `try`

catchJust_ :: (SomeException -> Maybe b) -> IO a -> (b -> IO a) -> IO a

Like `catch_` but for `catchJust`

handleJust_ :: (SomeException -> Maybe b) -> (b -> IO a) -> IO a -> IO a

Like `catch_` but for `handleJust`

tryJust_ :: (SomeException -> Maybe b) -> IO a -> IO (Either b a)

Like `catch_` but for `tryJust`

catchBool :: Exception e => (e -> Bool) -> IO a -> (e -> IO a) -> IO a

Catch an exception if the predicate passes, then call the handler with the original exception. As an example:

    readFileExists x == catchBool isDoesNotExistError (readFile "myfile") (const $ return "")

handleBool :: Exception e => (e -> Bool) -> (e -> IO a) -> IO a -> IO a

Like `catchBool` but for `handle`.

tryBool :: Exception e => (e -> Bool) -> IO a -> IO (Either e a)

Like `catchBool` but for `try`.

# Control.Monad.Extra

Extra functions available in `Control.Monad.Extra`.

whenJust :: Applicative m => Maybe a -> (a -> m ()) -> m ()

Perform some operation on `Just`, given the field inside the `Just`.

    whenJust Nothing  print == return ()
    whenJust (Just 1) print == print 1

whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()

Like `whenJust`, but where the test can be monadic.

unit :: m () -> m ()

The identity function which requires the inner argument to be `()`. Useful for functions with overloaded return types.

    \(x :: Maybe ()) -> unit x == x

loopM :: Monad m => (a -> m (Either a b)) -> a -> m b

A looping operation, where the predicate returns `Left` as a seed for the next loop or `Right` to abort the loop.

whileM :: Monad m => m Bool -> m ()

Keep running an operation until it becomes `False`. As an example:

    whileM $ do sleep 0.1; notM $ doesFileExist "foo.txt"
    readFile "foo.txt"

If you need some state persisted between each test, use `loopM`.

partitionM :: Monad m => (a -> m Bool) -> [a] -> m ([a], [a])

A version of `partition` that works with a monadic predicate.

    partitionM (Just . even) [1,2,3] == Just ([2], [1,3])
    partitionM (const Nothing) [1,2,3] == Nothing

concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b]

A version of `concatMap` that works with a monadic predicate.

mapMaybeM :: Monad m => (a -> m (Maybe b)) -> [a] -> m [b]

A version of `mapMaybe` that works with a monadic predicate.

findM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)

Like `find`, but where the test can be monadic.

    findM (Just . isUpper) "teST"             == Just (Just 'S')
    findM (Just . isUpper) "test"             == Just Nothing
    findM (Just . const True) ["x",undefined] == Just (Just "x")

firstJustM :: Monad m => (a -> m (Maybe b)) -> [a] -> m (Maybe b)

Like `findM`, but also allows you to compute some additional information in the predicate.

whenM :: Monad m => m Bool -> m () -> m ()

Like `when`, but where the test can be monadic.

unlessM :: Monad m => m Bool -> m () -> m ()

Like `unless`, but where the test can be monadic.

ifM :: Monad m => m Bool -> m a -> m a -> m a

Like `if`, but where the test can be monadic.

notM :: Functor m => m Bool -> m Bool

Like `not`, but where the test can be monadic.

(||^) :: Monad m => m Bool -> m Bool -> m Bool

The lazy `||` operator lifted to a monad. If the first argument evaluates to `True` the second argument will not be evaluated.

    Just True  ||^ undefined  == Just True
    Just False ||^ Just True  == Just True
    Just False ||^ Just False == Just False

(&&^) :: Monad m => m Bool -> m Bool -> m Bool

The lazy `&&` operator lifted to a monad. If the first argument evaluates to `False` the second argument will not be evaluated.

    Just False &&^ undefined  == Just False
    Just True  &&^ Just True  == Just True
    Just True  &&^ Just False == Just False

orM :: Monad m => [m Bool] -> m Bool

A version of `or` lifted to a monad. Retains the short-circuiting behaviour.

    orM [Just False,Just True ,undefined] == Just True
    orM [Just False,Just False,undefined] == undefined
    \xs -> Just (or xs) == orM (map Just xs)

andM :: Monad m => [m Bool] -> m Bool

A version of `and` lifted to a monad. Retains the short-circuiting behaviour.

    andM [Just True,Just False,undefined] == Just False
    andM [Just True,Just True ,undefined] == undefined
    \xs -> Just (and xs) == andM (map Just xs)

anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool

A version of `any` lifted to a monad. Retains the short-circuiting behaviour.

    anyM Just [False,True ,undefined] == Just True
    anyM Just [False,False,undefined] == undefined
    \(f :: Int -> Maybe Bool) xs -> anyM f xs == orM (map f xs)

allM :: Monad m => (a -> m Bool) -> [a] -> m Bool

A version of `all` lifted to a monad. Retains the short-circuiting behaviour.

    allM Just [True,False,undefined] == Just False
    allM Just [True,True ,undefined] == undefined
    \(f :: Int -> Maybe Bool) xs -> anyM f xs == orM (map f xs)

# Data.Either.Extra

Extra functions available in `Data.Either.Extra`.

isLeft :: Either a b -> Bool

Return `True` if the given value is a `Left`-value, `False` otherwise.

#### Examples

Basic usage:

    >>> isLeft (Left "foo")
    True
    >>> isLeft (Right 3)
    False

Assuming a `Left` value signifies some sort of error, we can use `isLeft` to write a very simple error-reporting function that does absolutely nothing in the case of success, and outputs "ERROR" if any error occurred.

This example shows how `isLeft` might be used to avoid pattern matching when one does not care about the value contained in the constructor:

    >>> import Control.Monad ( when )
    >>> let report e = when (isLeft e) $ putStrLn "ERROR"
    >>> report (Right 1)
    >>> report (Left "parse error")
    ERROR

Since: 4.7.0.0

isRight :: Either a b -> Bool

Return `True` if the given value is a `Right`-value, `False` otherwise.

#### Examples

Basic usage:

    >>> isRight (Left "foo")
    False
    >>> isRight (Right 3)
    True

Assuming a `Left` value signifies some sort of error, we can use `isRight` to write a very simple reporting function that only outputs "SUCCESS" when a computation has succeeded.

This example shows how `isRight` might be used to avoid pattern matching when one does not care about the value contained in the constructor:

    >>> import Control.Monad ( when )
    >>> let report e = when (isRight e) $ putStrLn "SUCCESS"
    >>> report (Left "parse error")
    >>> report (Right 1)
    SUCCESS

Since: 4.7.0.0

fromLeft :: Either l r -> l

The `fromLeft` function extracts the element out of a `Left` and throws an error if its argument is `Right`. Much like `fromJust`, using this function in polished code is usually a bad idea.

    \x -> fromLeft (Left  x) == x
    \x -> fromLeft (Right x) == undefined

fromRight :: Either l r -> r

The `fromRight` function extracts the element out of a `Right` and throws an error if its argument is `Left`. Much like `fromJust`, using this function in polished code is usually a bad idea.

    \x -> fromRight (Right x) == x
    \x -> fromRight (Left  x) == undefined

fromEither :: Either a a -> a

Pull the value out of an `Either` where both alternatives have the same type.

    \x -> fromEither (Left x ) == x
    \x -> fromEither (Right x) == x

# Data.IORef.Extra

Extra functions available in `Data.IORef.Extra`.

modifyIORef' :: IORef a -> (a -> a) -> IO ()

Strict version of `modifyIORef`

Since: 4.6.0.0

writeIORef' :: IORef a -> a -> IO ()

Evaluates the value before calling `writeIORef`.

atomicModifyIORef' :: IORef a -> (a -> (a, b)) -> IO b

Strict version of `atomicModifyIORef`. This forces both the value stored in the `IORef` as well as the value returned.

Since: 4.6.0.0

atomicWriteIORef :: IORef a -> a -> IO ()

Variant of `writeIORef` with the "barrier to reordering" property that `atomicModifyIORef` has.

Since: 4.6.0.0

atomicWriteIORef' :: IORef a -> a -> IO ()

Evaluates the value before calling `atomicWriteIORef`.

# Data.List.Extra

Extra functions available in `Data.List.Extra`.

lower :: String -> String

Convert a string to lower case.

    lower "This is A TEST" == "this is a test"
    lower "" == ""

upper :: String -> String

Convert a string to upper case.

    upper "This is A TEST" == "THIS IS A TEST"
    upper "" == ""

trim :: String -> String

Remove spaces from either side of a string. A combination of `trimEnd` and `trimStart`.

    trim      "  hello   " == "hello"
    trimStart "  hello   " == "hello   "
    trimEnd   "  hello   " == "  hello"
    \s -> trim s == trimEnd (trimStart s)

trimStart :: String -> String

Remove spaces from the start of a string, see `trim`.

trimEnd :: String -> String

Remove spaces from the end of a string, see `trim`.

word1 :: String -> (String, String)

Split the first word off a string. Useful for when starting to parse the beginning of a string, but you want to accurately perserve whitespace in the rest of the string.

    word1 "" == ("", "")
    word1 "keyword rest of string" == ("keyword","rest of string")
    word1 "  keyword\n  rest of string" == ("keyword","rest of string")
    \s -> fst (word1 s) == concat (take 1 $ words s)
    \s -> words (snd $ word1 s) == drop 1 (words s)

dropEnd :: Int -> [a] -> [a]

Drop a number of elements from the end of the list.

    dropEnd 3 "hello"  == "he"
    dropEnd 5 "bye"    == ""
    dropEnd (-1) "bye" == "bye"
    \i xs -> dropEnd i xs `isPrefixOf` xs
    \i xs -> length (dropEnd i xs) == max 0 (length xs - max 0 i)
    \i -> take 3 (dropEnd 5 [i..]) == take 3 [i..]

takeEnd :: Int -> [a] -> [a]

Take a number of elements from the end of the list.

    takeEnd 3 "hello"  == "llo"
    takeEnd 5 "bye"    == "bye"
    takeEnd (-1) "bye" == ""
    \i xs -> takeEnd i xs `isSuffixOf` xs
    \i xs -> length (takeEnd i xs) == min (max 0 i) (length xs)

splitAtEnd :: Int -> [a] -> ([a], [a])

``splitAtEnd` n xs` returns a split where the second element tries to contain `n` elements.

    splitAtEnd 3 "hello" == ("he","llo")
    splitAtEnd 3 "he"    == ("", "he")
    \i xs -> uncurry (++) (splitAt i xs) == xs
    \i xs -> splitAtEnd i xs == (dropEnd i xs, takeEnd i xs)

breakEnd :: (a -> Bool) -> [a] -> ([a], [a])

Break, but from the end.

    breakEnd isLower "youRE" == ("you","RE")
    breakEnd isLower "youre" == ("youre","")
    breakEnd isLower "YOURE" == ("","YOURE")
    \f xs -> breakEnd (not . f) xs == spanEnd f  xs

spanEnd :: (a -> Bool) -> [a] -> ([a], [a])

Span, but from the end.

    spanEnd isUpper "youRE" == ("you","RE")
    spanEnd (not . isSpace) "x y z" == ("x y ","z")
    \f xs -> uncurry (++) (spanEnd f xs) == xs
    \f xs -> spanEnd f xs == swap (both reverse (span f (reverse xs)))

dropWhileEnd :: (a -> Bool) -> [a] -> [a]

The `dropWhileEnd` function drops the largest suffix of a list in which the given predicate holds for all elements. For example:

    dropWhileEnd isSpace "foo\n" == "foo"
    dropWhileEnd isSpace "foo bar" == "foo bar"
    dropWhileEnd isSpace ("foo\n" ++ undefined) == "foo" ++ undefined

Since: 4.5.0.0

dropWhileEnd' :: (a -> Bool) -> [a] -> [a]

A version of `dropWhileEnd` but with different strictness properties. The function `dropWhileEnd` can be used on an infinite list and tests the property on each character. In contrast, `dropWhileEnd'` is strict in the spine of the list but only tests the trailing suffix. This version usually outperforms `dropWhileEnd` if the list is short or the test is expensive. Note the tests below cover both the prime and non-prime variants.

    dropWhileEnd  isSpace "ab cde  " == "ab cde"
    dropWhileEnd' isSpace "ab cde  " == "ab cde"
    last (dropWhileEnd  even [undefined,3]) == undefined
    last (dropWhileEnd' even [undefined,3]) == 3
    head (dropWhileEnd  even (3:undefined)) == 3
    head (dropWhileEnd' even (3:undefined)) == undefined

takeWhileEnd :: (a -> Bool) -> [a] -> [a]

A version of `takeWhile` operating from the end.

    takeWhileEnd even [2,3,4,6] == [4,6]

stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]

Return the prefix of the second string if its suffix matches the entire first string.

Examples:

    stripSuffix "bar" "foobar" == Just "foo"
    stripSuffix ""    "baz"    == Just "baz"
    stripSuffix "foo" "quux"   == Nothing

stripInfix :: Eq a => [a] -> [a] -> Maybe ([a], [a])

Return the the string before and after the search string, or `Nothing` if the search string is not present.

Examples:

    stripInfix "::" "a::b::c" == Just ("a", "b::c")
    stripInfix "/" "foobar"   == Nothing

stripInfixEnd :: Eq a => [a] -> [a] -> Maybe ([a], [a])

Similar to `stripInfix`, but searches from the end of the string.

    stripInfixEnd "::" "a::b::c" == Just ("a::b", "c")

wordsBy :: (a -> Bool) -> [a] -> [[a]]

A variant of `words` with a custom test. In particular, adjacent separators are discarded, as are leading or trailing separators.

    wordsBy (== ':') "::xyz:abc::123::" == ["xyz","abc","123"]
    \s -> wordsBy isSpace s == words s

linesBy :: (a -> Bool) -> [a] -> [[a]]

A variant of `lines` with a custom test. In particular, if there is a trailing separator it will be discarded.

    linesBy (== ':') "::xyz:abc::123::" == ["","","xyz","abc","","123",""]
    \s -> linesBy (== '\n') s == lines s
    linesBy (== ';') "my;list;here;" == ["my","list","here"]

breakOn :: Eq a => [a] -> [a] -> ([a], [a])

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 patch, use `stripInfix`.

    breakOn "::" "a::b::c" == ("a", "::b::c")
    breakOn "/" "foobar"   == ("foobar", "")
    \needle haystack -> let (prefix,match) = breakOn needle haystack in prefix ++ match == haystack

breakOnEnd :: Eq a => [a] -> [a] -> ([a], [a])

Similar to `breakOn`, but searches from the end of the string.

The first element of the returned tuple is the prefix of `haystack` up to and including the last match of `needle`. The second is the remainder of `haystack`, following the match.

    breakOnEnd "::" "a::b::c" == ("a::b::", "c")

splitOn :: Eq a => [a] -> [a] -> [[a]]

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"    ""                 == [""]
    \s x -> s /= "" ==> intercalate s (splitOn s x) == x
    \c x -> splitOn [c] x                           == split (==c) x

split :: (a -> Bool) -> [a] -> [[a]]

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"]

chunksOf :: Int -> [a] -> [[a]]

Split a list into chunks of a given size. The last chunk may contain fewer than n elements. The chunk size must be positive.

    chunksOf 3 "my test" == ["my ","tes","t"]
    chunksOf 3 "mytest"  == ["myt","est"]
    chunksOf 8 ""        == []
    chunksOf 0 "test"    == undefined

list :: b -> (a -> [a] -> b) -> [a] -> b

Non-recursive transform over a list, like `maybe`.

    list 1 (\v _ -> v - 2) [5,6,7] == 3
    list 1 (\v _ -> v - 2) []      == 1
    \nil cons xs -> maybe nil (uncurry cons) (uncons xs) == list nil cons xs

uncons :: [a] -> Maybe (a, [a])

Decompose a list into its head and tail. If the list is empty, returns `Nothing`. If the list is non-empty, returns ``Just` (x, xs)`, where `x` is the head of the list and `xs` its tail.

Since: 4.8.0.0

unsnoc :: [a] -> Maybe ([a], a)

If the list is empty returns `Nothing`, otherwise returns the `init` and the `last`.

    unsnoc "test" == Just ("tes",'t')
    unsnoc ""     == Nothing
    \xs -> unsnoc xs == if null xs then Nothing else Just (init xs, last xs)

cons :: a -> [a] -> [a]

Append an element to the start of a list, an alias for '(:)'.

    cons 't' "est" == "test"
    \x xs -> uncons (cons x xs) == Just (x,xs)

snoc :: [a] -> a -> [a]

Append an element to the end of a list, takes O(n) time.

    snoc "tes" 't' == "test"
    \xs x -> unsnoc (snoc xs x) == Just (xs,x)

drop1 :: [a] -> [a]

Equivalent to `drop 1`, but likely to be faster and a single lexeme.

    drop1 ""         == ""
    drop1 "test"     == "est"
    \xs -> drop 1 xs == drop1 xs

groupSort :: Ord k => [(k, v)] -> [(k, [v])]

A combination of `group` and `sort`.

    groupSort [(1,'t'),(3,'t'),(2,'e'),(2,'s')] == [(1,"t"),(2,"es"),(3,"t")]
    \xs -> map fst (groupSort xs) == sort (nub (map fst xs))
    \xs -> concatMap snd (groupSort xs) == map snd (sortOn fst xs)

groupSortOn :: Ord b => (a -> b) -> [a] -> [[a]]

A combination of `group` and `sort`, using a part of the value to compare on.

    groupSortOn length ["test","of","sized","item"] == [["of"],["test","item"],["sized"]]

groupSortBy :: (a -> a -> Ordering) -> [a] -> [[a]]

A combination of `group` and `sort`, using a predicate to compare on.

    groupSortBy (compare `on` length) ["test","of","sized","item"] == [["of"],["test","item"],["sized"]]

nubOrd :: Ord a => [a] -> [a]

O(n log n). The `nubOrd` function removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element. Unlike the standard `nub` operator, this version requires an `Ord` instance and consequently runs asymptotically faster.

    nubOrd "this is a test" == "this ae"
    nubOrd (take 4 ("this" ++ undefined)) == "this"
    \xs -> nubOrd xs == nub xs

nubOrdBy :: (a -> a -> Ordering) -> [a] -> [a]

A version of `nubOrd` with a custom predicate.

    nubOrdBy (compare `on` length) ["a","test","of","this"] == ["a","test","of"]

nubOrdOn :: Ord b => (a -> b) -> [a] -> [a]

A version of `nubOrd` which operates on a portion of the value.

    nubOrdOn length ["a","test","of","this"] == ["a","test","of"]

nubOn :: Eq b => (a -> b) -> [a] -> [a]

A version of `nub` where the equality is done on some extracted value. `nubOn f` is equivalent to `nubBy ((==) `on` f)`, but has the performance advantage of only evaluating `f` once for each element in the input list.

groupOn :: Eq b => (a -> b) -> [a] -> [[a]]

A version of `group` where the equality is done on some extracted value.

sortOn :: Ord b => (a -> b) -> [a] -> [a]

Sort a list by comparing the results of a key function applied to each element. `sortOn f` is equivalent to `sortBy . comparing f`, but has the performance advantage of only evaluating `f` once for each element in the input list. This is called the decorate-sort-undecorate paradigm, or Schwartzian transform.

Since: 4.8.0.0

disjoint :: Eq a => [a] -> [a] -> Bool

Are two lists disjoint, with no elements in common.

    disjoint [1,2,3] [4,5] == True
    disjoint [1,2,3] [4,1] == False

allSame :: Eq a => [a] -> Bool

Are all elements the same.

    allSame [1,1,2] == False
    allSame [1,1,1] == True
    allSame [1]     == True
    allSame []      == True
    allSame (1:1:2:undefined) == False
    \xs -> allSame xs == (length (nub xs) <= 1)

anySame :: Eq a => [a] -> Bool

Is there any element which occurs more than once.

    anySame [1,1,2] == True
    anySame [1,2,3] == False
    anySame (1:2:1:undefined) == True
    anySame [] == False
    \xs -> anySame xs == (length (nub xs) < length xs)

repeatedly :: ([a] -> (b, [a])) -> [a] -> [b]

Apply some operation repeatedly, producing an element of output and the remainder of the list.

    \xs -> repeatedly (splitAt 3) xs  == chunksOf 3 xs
    \xs -> repeatedly word1 (trim xs) == words xs

for :: [a] -> (a -> b) -> [b]

Flipped version of `map`.

    for [1,2,3] (+1) == [2,3,4]

firstJust :: (a -> Maybe b) -> [a] -> Maybe b

Find the first element of a list for which the operation returns `Just`, along with the result of the operation. Like `find` but useful where the function also computes some expensive information that can be reused. Particular useful when the function is monadic, see `firstJustM`.

    firstJust id [Nothing,Just 3]  == Just 3
    firstJust id [Nothing,Nothing] == Nothing

concatUnzip :: [([a], [b])] -> ([a], [b])

A merging of `unzip` and `concat`.

    concatUnzip [("a","AB"),("bc","C")] == ("abc","ABC")

concatUnzip3 :: [([a], [b], [c])] -> ([a], [b], [c])

A merging of `unzip3` and `concat`.

    concatUnzip3 [("a","AB",""),("bc","C","123")] == ("abc","ABC","123")

replace :: Eq a => [a] -> [a] -> [a] -> [a]

Replace a subsequence everywhere it occurs. The first argument must not be the empty list.

    replace "el" "_" "Hello Bella" == "H_lo B_la"
    replace "el" "e" "Hello"       == "Helo"
    replace "" "e" "Hello"         == undefined
    \xs ys -> not (null xs) ==> replace xs xs ys == ys

merge :: Ord a => [a] -> [a] -> [a]

Merge two lists which are assumed to be ordered.

    merge "ace" "bd" == "abcde"
    \xs ys -> merge (sort xs) (sort ys) == sort (xs ++ ys)

mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]

Like `merge`, but with a custom ordering function.

# Data.Tuple.Extra

Extra functions available in `Data.Tuple.Extra`.

first :: (a -> a') -> (a, b) -> (a', b)

Update the first component of a pair.

    first succ (1,"test") == (2,"test")

second :: (b -> b') -> (a, b) -> (a, b')

Update the second component of a pair.

    second reverse (1,"test") == (1,"tset")

(***) :: (a -> a') -> (b -> b') -> (a, b) -> (a', b') infixr 3

Given two functions, apply one to the first component and one to the second. A specialised version of `***`.

    (succ *** reverse) (1,"test") == (2,"tset")

(&&&) :: (a -> b) -> (a -> c) -> a -> (b, c) infixr 3

Given two functions, apply both to a single argument to form a pair. A specialised version of `&&&`.

    (succ &&& pred) 1 == (2,0)

dupe :: a -> (a, a)

Duplicate a single value into a pair.

    dupe 12 == (12, 12)

both :: (a -> b) -> (a, a) -> (b, b)

Apply a single function to both components of a pair.

    both succ (1,2) == (2,3)

fst3 :: (a, b, c) -> a

Extract the `fst` of a triple.

snd3 :: (a, b, c) -> b

Extract the `snd` of a triple.

thd3 :: (a, b, c) -> c

Extract the final element of a triple.

# Numeric.Extra

Extra functions available in `Numeric.Extra`.

showDP :: RealFloat a => Int -> a -> String

Show a number to a fixed number of decimal places.

    showDP 4 pi == "3.1416"
    showDP 0 pi == "3"
    showDP 2 3  == "3.00"

intToDouble :: Int -> Double

Specialised numeric conversion, type restricted version of `fromIntegral`.

intToFloat :: Int -> Float

Specialised numeric conversion, type restricted version of `fromIntegral`.

floatToDouble :: Float -> Double

Specialised numeric conversion, type restricted version of `realToFrac`.

doubleToFloat :: Double -> Float

Specialised numeric conversion, type restricted version of `realToFrac`.

# System.Directory.Extra

Extra functions available in `System.Directory.Extra`.

withCurrentDirectory :: FilePath -> IO a -> IO a

Set the current directory, perform an operation, then change back. Remember that the current directory is a global variable, so calling this function multithreaded is almost certain to go wrong. Avoid changing the current directory if you can.

    withTempDir $ \dir -> do writeFile (dir </> "foo.txt") ""; withCurrentDirectory dir $ doesFileExist "foo.txt"

createDirectoryPrivate :: String -> IO ()

Create a directory with permissions so that only the current user can view it. On Windows this function is equivalent to `createDirectory`.

listContents :: FilePath -> IO [FilePath]

List the files and directories directly within a directory. Each result will be prefixed by the query directory, and the special directories `.` and `..` will be ignored. Intended as a cleaned up version of `getDirectoryContents`.

    withTempDir $ \dir -> do writeFile (dir </> "test.txt") ""; (== [dir </> "test.txt"]) <$> listContents dir
    let touch = mapM_ $ \x -> createDirectoryIfMissing True (takeDirectory x) >> writeFile x ""
    let listTest op as bs = withTempDir $ \dir -> do touch $ map (dir </>) as; res <- op dir; return $ map (drop (length dir + 1)) res == bs
    listTest listContents ["bar.txt","foo/baz.txt","zoo"] ["bar.txt","foo","zoo"]

listFiles :: FilePath -> IO [FilePath]

Like `listContents`, but only returns the files in a directory, not other directories. Each file will be prefixed by the query directory.

    listTest listFiles ["bar.txt","foo/baz.txt","zoo"] ["bar.txt","zoo"]

listFilesInside :: (FilePath -> IO Bool) -> FilePath -> IO [FilePath]

Like `listFilesRecursive`, but with a predicate to decide where to recurse into. Typically directories starting with `.` would be ignored. The initial argument directory will have the test applied to it.

    listTest (listFilesInside $ return . not . isPrefixOf "." . takeFileName)
        ["bar.txt","foo" </> "baz.txt",".foo" </> "baz2.txt", "zoo"] ["bar.txt","zoo","foo" </> "baz.txt"]
    listTest (listFilesInside $ const $ return False) ["bar.txt"] []

listFilesRecursive :: FilePath -> IO [FilePath]

Like `listFiles`, but goes recursively through all subdirectories.

    listTest listFilesRecursive ["bar.txt","zoo","foo" </> "baz.txt"] ["bar.txt","zoo","foo" </> "baz.txt"]

# System.Environment.Extra

Extra functions available in `System.Environment.Extra`.

getExecutablePath :: IO FilePath

Returns the absolute pathname of the current executable.

Note that for scripts and interactive sessions, this is the path to the interpreter (e.g. ghci.)

Since: 4.6.0.0

lookupEnv :: String -> IO (Maybe String)

Return the value of the environment variable `var`, or `Nothing` if there is no such value.

For POSIX users, this is equivalent to `getEnv`.

Since: 4.6.0.0

# System.Info.Extra

Extra functions available in `System.Info.Extra`.

isWindows :: Bool

Return `True` on Windows and `False` otherwise. A runtime version of `#ifdef minw32_HOST_OS`. Equivalent to `os == "mingw32"`, but: more efficient; doesn't require typing an easily mistypeable string; actually asks about your OS not a library; doesn't bake in 32bit assumptions that are already false. </rant>

    isWindows == (os == "mingw32")

# System.IO.Extra

Extra functions available in `System.IO.Extra`.

captureOutput :: IO a -> IO (String, a)

Capture the `stdout` and `stderr` of a computation.

    captureOutput (print 1) == return ("1\n",())

withBuffering :: Handle -> BufferMode -> IO a -> IO a

Execute an action with a custom `BufferMode`, a wrapper around `hSetBuffering`.

readFileEncoding :: TextEncoding -> FilePath -> IO String

Like `readFile`, but setting an encoding.

readFileUTF8 :: FilePath -> IO String

Like `readFile`, but with the encoding `utf8`.

readFileBinary :: FilePath -> IO String

Like `readFile`, but for binary files.

readFile' :: FilePath -> IO String

A strict version of `readFile`. When the string is produced, the entire file will have been read into memory and the file handle will have been closed. Closing the file handle does not rely on the garbage collector.

    \(filter isHexDigit -> s) -> fmap (== s) $ withTempFile $ \file -> do writeFile file s; readFile' file

readFileEncoding' :: TextEncoding -> FilePath -> IO String

A strict version of `readFileEncoding`, see `readFile'` for details.

readFileUTF8' :: FilePath -> IO String

A strict version of `readFileUTF8`, see `readFile'` for details.

readFileBinary' :: FilePath -> IO String

A strict version of `readFileBinary`, see `readFile'` for details.

writeFileEncoding :: TextEncoding -> FilePath -> String -> IO ()

Write a file with a particular encoding.

writeFileUTF8 :: FilePath -> String -> IO ()

Write a file with the `utf8` encoding.

    \s -> withTempFile $ \file -> do writeFileUTF8 file s; fmap (== s) $ readFileUTF8' file

writeFileBinary :: FilePath -> String -> IO ()

Write a binary file.

    \s -> withTempFile $ \file -> do writeFileBinary file s; fmap (== s) $ readFileBinary' file

withTempFile :: (FilePath -> IO a) -> IO a

Create a temporary file in the temporary directory. The file will be deleted after the action completes (provided the file is not still open). The `FilePath` will not have any file extension, will exist, and will be zero bytes long. If you require a file with a specific name, use `withTempDir`.

    withTempFile doesFileExist == return True
    (doesFileExist =<< withTempFile return) == return False
    withTempFile readFile' == return ""

withTempDir :: (FilePath -> IO a) -> IO a

Create a temporary directory inside the system temporary directory. The directory will be deleted after the action completes.

    withTempDir doesDirectoryExist == return True
    (doesDirectoryExist =<< withTempDir return) == return False
    withTempDir listFiles == return []

newTempFile :: IO (FilePath, IO ())

Provide a function to create a temporary file, and a way to delete a temporary file. Most users should use `withTempFile` which combines these operations.

newTempDir :: IO (FilePath, IO ())

Provide a function to create a temporary directory, and a way to delete a temporary directory. Most users should use `withTempDir` which combines these operations.

# System.Process.Extra

Extra functions available in `System.Process.Extra`.

system_ :: String -> IO ()

A version of `system` that throws an error if the `ExitCode` is not `ExitSuccess`.

systemOutput :: String -> IO (ExitCode, String)

A version of `system` that also captures the output, both `stdout` and `stderr`. Returns a pair of the `ExitCode` and the output.

systemOutput_ :: String -> IO String

A version of `system` that captures the output (both `stdout` and `stderr`) and throws an error if the `ExitCode` is not `ExitSuccess`.

# System.Time.Extra

Extra functions available in `System.Time.Extra`.

type Seconds = Double

A type alias for seconds, which are stored as `Double`.

sleep :: Seconds -> IO ()

Sleep for a number of seconds.

    fmap (round . fst) (duration $ sleep 1) == return 1

timeout :: Seconds -> IO a -> IO (Maybe a)

A version of `timeout` that takes `Seconds` and never overflows the bounds of an `Int`. In addition, the bug that negative timeouts run for ever has been fixed.

    timeout (-3) (print 1) == return Nothing
    timeout 0.1  (print 1) == fmap Just (print 1)
    timeout 0.1  (sleep 2 >> print 1) == return Nothing
    do (t, _) <- duration $ timeout 0.1 $ sleep 1000; return $ t < 1

subtractTime :: UTCTime -> UTCTime -> Seconds

Calculate the difference between two times in seconds. Usually the first time will be the end of an event, and the second time will be the beginning.

    \a b -> a > b ==> subtractTime a b > 0

showDuration :: Seconds -> String

Show a number of seconds, typically a duration, in a suitable manner with responable precision for a human.

    showDuration 3.435   == "3.44s"
    showDuration 623.8   == "10m24s"
    showDuration 62003.8 == "17h13m"
    showDuration 1e8     == "27777h47m"

offsetTime :: IO (IO Seconds)

Call once to start, then call repeatedly to get the elapsed time since the first call. Values will usually increase, unless the system clock is updated (if you need the guarantee, see `offsetTimeIncrease`).

offsetTimeIncrease :: IO (IO Seconds)

Like `offsetTime`, but results will never decrease (though they may stay the same).

    do f <- offsetTimeIncrease; xs <- replicateM 10 f; return $ xs == sort xs

duration :: IO a -> IO (Seconds, a)

Record how long a computation takes in `Seconds`.