extra-1.7.3: Extra functions I use.

Control.Exception.Extra

Contents

Description

Extra functions for Control.Exception. These functions provide retrying, showing in the presence of exceptions, and functions to catch/ignore exceptions, including monomorphic (no Exception context) versions.

If you want to use a safer set of exceptions see the safe-exceptions package.

Synopsis

# Documentation

A constraint which documents that a function is partial, and on GHC 8.0 and above produces a stack trace on failure. For example:

myHead :: Partial => [a] -> a


When using Partial with GHC 7.8 or below you need to enable the language feature ConstraintKinds, e.g. {-# LANGUAGE ConstraintKinds #-} at the top of the file.

retry :: Int -> IO a -> IO a Source #

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 Source #

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.

errorWithoutStackTrace :: [Char] -> a #

A variant of error that does not produce a stack trace.

Since: base-4.9.0.0

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

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.

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

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

An IO action that when evaluated calls error, in the IO monad. Note that while fail in IO raises an IOException, this function raises an ErrorCall exception with a call stack.

catch (errorIO "Hello") (\(ErrorCall x) -> pure x) == pure "Hello"
seq (errorIO "foo") (print 1) == print 1

# Exception catching/ignoring

ignore :: IO () -> IO () Source #

Ignore any exceptions thrown by the action.

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

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

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

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

Like catch_ but for handle

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

Like catch_ but for try

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

Like catch_ but for catchJust

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

Like catch_ but for handleJust

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

Like catch_ but for tryJust

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

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 \$ pure "")


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

Like catchBool but for handle.

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

Like catchBool but for try.