Z-IO-2.0.0.0: Simple and high performance IO toolkit for Haskell
Copyright(c) Dong Han 2017-2018
LicenseBSD
Maintainerwinterland1989@gmail.com
Stabilityexperimental
Portabilitynon-portable
Safe HaskellSafe-Inferred
LanguageHaskell2010

Z.IO.Exception

Description

This module implemented extensible io exception following approach described in /An Extensible Dynamically-Typed Hierarchy of Exceptions/ by Simon Marlow. The implementation in this module has simplified to meet common need. User who want to catch certain type of exceptions can directly use exception types this module provide, which are modeled after IOErrorType from GHC.IO.Exception.

Functions from this package will throw exceptions from this module only instead of the old IOError on IO exceptions. Exceptions from this module contain IOEInfo which is pretty detailed, but this also require user of this module do some extra work to keep error message's quality(provide CallStack, device informations, etc.). New defined IO exceptions are encouraged to include a IOEInfo, since it helps a lot when debugging.

Example for library author defining new io exception:

  data MyNetworkException = MyNetworkException IOEInfo ... deriving Show
  instance Exception MyNetworkException where
        toException = ioExceptionToException
        fromException = ioExceptionFromException

If you're dealing with OS's errno directly, you should convert the errno to libuv's errno in C side with uv_translate_sys_error from hs_uv.h, then use 'throwUVIfMinus/throwUVError' from this module.

Synopsis

The SomeIOException type

data SomeIOException Source #

The root type of all io exceptions, you can catch all io exception by catching this root type.

Constructors

forall e.Exception e => SomeIOException e 

Builtin io exception types

data IOEInfo Source #

IO exceptions informations.

Constructors

IOEInfo 

Fields

Instances

Instances details
Print IOEInfo Source # 
Instance details

Defined in Z.IO.Exception

Methods

toUTF8BuilderP :: Int -> IOEInfo -> Builder () #

Show IOEInfo Source # 
Instance details

Defined in Z.IO.Exception

Throw io exceptions

throwOOMIfNull Source #

Arguments

:: HasCallStack 
=> IO (Ptr a)

the allocation action

-> IO (Ptr a) 

Throw ResourceExhausted if allocation return a nullPtr.

throwUVIfMinus Source #

Arguments

:: (HasCallStack, Integral a) 
=> IO a

the IO action

-> IO a 

Throw appropriate IO exception if return value < 0 (libuv's convention).

throwUVIfMinus_ Source #

Arguments

:: (HasCallStack, Integral a) 
=> IO a

the IO action

-> IO () 

Throw appropriate IO exception if return value < 0, otherwise ignore the result.

throwUVIf :: (HasCallStack, Integral a) => IO a -> (a -> Bool) -> IO a Source #

Throw appropriate IO exception if condition is true.

throwUVIf_ :: (HasCallStack, Integral a) => IO a -> (a -> Bool) -> IO () Source #

Throw appropriate IO exception if condition is true, otherwise ignore the result.

throwUV :: (Integral a, HasCallStack) => a -> IO b Source #

Throw a UV Exception with given libuv's errno.

throwECLOSED :: HasCallStack => IO a Source #

Throw ResourceVanished with name ECLOSED and description 'resource is closed'.

throwOtherError :: HasCallStack => Text -> Text -> IO a Source #

Throw OtherError with custom name and description.

unwrap :: (HasCallStack, Print e) => Text -> Either e a -> IO a Source #

Try to unwrap a value from Right, throw OtherError name desc with desc == toText e if 'Left e'.

unwrap' :: HasCallStack => Text -> Text -> Maybe a -> IO a Source #

Try to unwrap a value from Just, throw OtherError name desc if Nothing.

Sync exception tools

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

Same as upstream catch, but will not catch asynchronous exceptions

ignoreSync :: IO a -> IO () Source #

Ingore all synchronous exceptions.

Re-exports

assert :: Bool -> a -> a #

If the first argument evaluates to True, then the result is the second argument. Otherwise an AssertionFailed exception is raised, containing a String with the source file and line number of the call to assert.

Assertions can normally be turned on or off with a compiler flag (for GHC, assertions are normally on unless optimisation is turned on with -O or the -fignore-asserts option is given). When assertions are turned off, the first argument to assert is ignored, and the second argument is returned as the result.

data ArrayException #

Exceptions generated by array operations

Constructors

IndexOutOfBounds String

An attempt was made to index an array outside its declared bounds.

UndefinedElement String

An attempt was made to evaluate an element of an array that had not been initialized.

data Handler a #

You need this when using catches.

Constructors

Exception e => Handler (e -> IO a) 

Instances

Instances details
Functor Handler

Since: base-4.6.0.0

Instance details

Defined in Control.Exception

Methods

fmap :: (a -> b) -> Handler a -> Handler b #

(<$) :: a -> Handler b -> Handler a #

catches :: IO a -> [Handler a] -> IO a #

Sometimes you want to catch two different sorts of exception. You could do something like

f = expr `catch` \ (ex :: ArithException) -> handleArith ex
         `catch` \ (ex :: IOException)    -> handleIO    ex

However, there are a couple of problems with this approach. The first is that having two exception handlers is inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the first, e.g. in the example above, if handleArith throws an IOException then the second exception handler will catch it.

Instead, we provide a function catches, which would be used thus:

f = expr `catches` [Handler (\ (ex :: ArithException) -> handleArith ex),
                    Handler (\ (ex :: IOException)    -> handleIO    ex)]

allowInterrupt :: IO () #

When invoked inside mask, this function allows a masked asynchronous exception to be raised, if one exists. It is equivalent to performing an interruptible operation (see #interruptible), but does not involve any actual blocking.

When called outside mask, or inside uninterruptibleMask, this function has no effect.

Since: base-4.4.0.0

newtype TypeError #

An expression that didn't typecheck during compile time was called. This is only possible with -fdefer-type-errors. The String gives details about the failed type check.

Since: base-4.9.0.0

Constructors

TypeError String 

Instances

Instances details
Exception TypeError

Since: base-4.9.0.0

Instance details

Defined in Control.Exception.Base

Show TypeError

Since: base-4.9.0.0

Instance details

Defined in Control.Exception.Base

newtype RecUpdError #

A record update was performed on a constructor without the appropriate field. This can only happen with a datatype with multiple constructors, where some fields are in one constructor but not another. The String gives information about the source location of the record update.

Constructors

RecUpdError String 

Instances

Instances details
Exception RecUpdError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Show RecUpdError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

newtype RecSelError #

A record selector was applied to a constructor without the appropriate field. This can only happen with a datatype with multiple constructors, where some fields are in one constructor but not another. The String gives information about the source location of the record selector.

Constructors

RecSelError String 

Instances

Instances details
Exception RecSelError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Show RecSelError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

newtype RecConError #

An uninitialised record field was used. The String gives information about the source location where the record was constructed.

Constructors

RecConError String 

Instances

Instances details
Exception RecConError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Show RecConError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

newtype PatternMatchFail #

A pattern match failed. The String gives information about the source location of the pattern.

Constructors

PatternMatchFail String 

data NonTermination #

Thrown when the runtime system detects that the computation is guaranteed not to terminate. Note that there is no guarantee that the runtime system will notice whether any given computation is guaranteed to terminate or not.

Constructors

NonTermination 

newtype NoMethodError #

A class method without a definition (neither a default definition, nor a definition in the appropriate instance) was called. The String gives information about which method it was.

Constructors

NoMethodError String 

Instances

Instances details
Exception NoMethodError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Show NoMethodError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

data NestedAtomically #

Thrown when the program attempts to call atomically, from the stm package, inside another call to atomically.

Constructors

NestedAtomically 

tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a) #

A variant of try that takes an exception predicate to select which exceptions are caught (c.f. catchJust). If the exception does not match the predicate, it is re-thrown.

try :: Exception e => IO a -> IO (Either e a) #

Similar to catch, but returns an Either result which is (Right a) if no exception of type e was raised, or (Left ex) if an exception of type e was raised and its value is ex. If any other type of exception is raised then it will be propagated up to the next enclosing exception handler.

 try a = catch (Right `liftM` a) (return . Left)

onException :: IO a -> IO b -> IO a #

Like finally, but only performs the final action if there was an exception raised by the computation.

mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a #

This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".

handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a #

A version of catchJust with the arguments swapped around (see handle).

handle :: Exception e => (e -> IO a) -> IO a -> IO a #

A version of catch with the arguments swapped around; useful in situations where the code for the handler is shorter. For example:

  do handle (\NonTermination -> exitWith (ExitFailure 1)) $
     ...

finally #

Arguments

:: IO a

computation to run first

-> IO b

computation to run afterward (even if an exception was raised)

-> IO a 

A specialised variant of bracket with just a computation to run afterward.

catchJust #

Arguments

:: Exception e 
=> (e -> Maybe b)

Predicate to select exceptions

-> IO a

Computation to run

-> (b -> IO a)

Handler

-> IO a 

The function catchJust is like catch, but it takes an extra argument which is an exception predicate, a function which selects which type of exceptions we're interested in.

catchJust (\e -> if isDoesNotExistErrorType (ioeGetErrorType e) then Just () else Nothing)
          (readFile f)
          (\_ -> do hPutStrLn stderr ("No such file: " ++ show f)
                    return "")

Any other exceptions which are not matched by the predicate are re-raised, and may be caught by an enclosing catch, catchJust, etc.

bracket_ :: IO a -> IO b -> IO c -> IO c #

A variant of bracket where the return value from the first computation is not required.

bracketOnError #

Arguments

:: IO a

computation to run first ("acquire resource")

-> (a -> IO b)

computation to run last ("release resource")

-> (a -> IO c)

computation to run in-between

-> IO c 

Like bracket, but only performs the final action if there was an exception raised by the in-between computation.

bracket #

Arguments

:: IO a

computation to run first ("acquire resource")

-> (a -> IO b)

computation to run last ("release resource")

-> (a -> IO c)

computation to run in-between

-> IO c 

When you want to acquire a resource, do some work with it, and then release the resource, it is a good idea to use bracket, because bracket will install the necessary exception handler to release the resource in the event that an exception is raised during the computation. If an exception is raised, then bracket will re-raise the exception (after performing the release).

A common example is opening a file:

bracket
  (openFile "filename" ReadMode)
  (hClose)
  (\fileHandle -> do { ... })

The arguments to bracket are in this order so that we can partially apply it, e.g.:

withFile name mode = bracket (openFile name mode) hClose

Bracket wraps the release action with mask, which is sufficient to ensure that the release action executes to completion when it does not invoke any interruptible actions, even in the presence of asynchronous exceptions. For example, hClose is uninterruptible when it is not racing other uses of the handle. Similarly, closing a socket (from "network" package) is also uninterruptible under similar conditions. An example of an interruptible action is killThread. Completion of interruptible release actions can be ensured by wrapping them in in uninterruptibleMask_, but this risks making the program non-responsive to Control-C, or timeouts. Another option is to run the release action asynchronously in its own thread:

void $ uninterruptibleMask_ $ forkIO $ do { ... }

The resource will be released as soon as possible, but the thread that invoked bracket will not block in an uninterruptible state.

throwTo :: Exception e => ThreadId -> e -> IO () #

throwTo raises an arbitrary exception in the target thread (GHC only).

Exception delivery synchronizes between the source and target thread: throwTo does not return until the exception has been raised in the target thread. The calling thread can thus be certain that the target thread has received the exception. Exception delivery is also atomic with respect to other exceptions. Atomicity is a useful property to have when dealing with race conditions: e.g. if there are two threads that can kill each other, it is guaranteed that only one of the threads will get to kill the other.

Whatever work the target thread was doing when the exception was raised is not lost: the computation is suspended until required by another thread.

If the target thread is currently making a foreign call, then the exception will not be raised (and hence throwTo will not return) until the call has completed. This is the case regardless of whether the call is inside a mask or not. However, in GHC a foreign call can be annotated as interruptible, in which case a throwTo will cause the RTS to attempt to cause the call to return; see the GHC documentation for more details.

Important note: the behaviour of throwTo differs from that described in the paper "Asynchronous exceptions in Haskell" (http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm). In the paper, throwTo is non-blocking; but the library implementation adopts a more synchronous design in which throwTo does not return until the exception is received by the target thread. The trade-off is discussed in Section 9 of the paper. Like any blocking operation, throwTo is therefore interruptible (see Section 5.3 of the paper). Unlike other interruptible operations, however, throwTo is always interruptible, even if it does not actually block.

There is no guarantee that the exception will be delivered promptly, although the runtime will endeavour to ensure that arbitrary delays don't occur. In GHC, an exception can only be raised when a thread reaches a safe point, where a safe point is where memory allocation occurs. Some loops do not perform any memory allocation inside the loop and therefore cannot be interrupted by a throwTo.

If the target of throwTo is the calling thread, then the behaviour is the same as throwIO, except that the exception is thrown as an asynchronous exception. This means that if there is an enclosing pure computation, which would be the case if the current IO operation is inside unsafePerformIO or unsafeInterleaveIO, that computation is not permanently replaced by the exception, but is suspended as if it had received an asynchronous exception.

Note that if throwTo is called with the current thread as the target, the exception will be thrown even if the thread is currently inside mask or uninterruptibleMask.

data SomeAsyncException #

Superclass for asynchronous exceptions.

Since: base-4.7.0.0

Constructors

Exception e => SomeAsyncException e 

data Deadlock #

There are no runnable threads, so the program is deadlocked. The Deadlock exception is raised in the main thread only.

Constructors

Deadlock 

Instances

Instances details
Exception Deadlock

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Show Deadlock

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

newtype CompactionFailed #

Compaction found an object that cannot be compacted. Functions cannot be compacted, nor can mutable objects or pinned objects. See compact.

Since: base-4.10.0.0

Constructors

CompactionFailed String 

Instances

Instances details
Exception CompactionFailed

Since: base-4.10.0.0

Instance details

Defined in GHC.IO.Exception

Show CompactionFailed

Since: base-4.10.0.0

Instance details

Defined in GHC.IO.Exception

data BlockedIndefinitelyOnSTM #

The thread is waiting to retry an STM transaction, but there are no other references to any TVars involved, so it can't ever continue.

data BlockedIndefinitelyOnMVar #

The thread is blocked on an MVar, but there are no other references to the MVar so it can't ever continue.

data AsyncException #

Asynchronous exceptions.

Constructors

StackOverflow

The current thread's stack exceeded its limit. Since an exception has been raised, the thread's stack will certainly be below its limit again, but the programmer should take remedial action immediately.

HeapOverflow

The program's heap is reaching its limit, and the program should take action to reduce the amount of live data it has. Notes:

  • It is undefined which thread receives this exception. GHC currently throws this to the same thread that receives UserInterrupt, but this may change in the future.
  • The GHC RTS currently can only recover from heap overflow if it detects that an explicit memory limit (set via RTS flags). has been exceeded. Currently, failure to allocate memory from the operating system results in immediate termination of the program.
ThreadKilled

This exception is raised by another thread calling killThread, or by the system if it needs to terminate the thread for some reason.

UserInterrupt

This exception is raised by default in the main thread of the program when the user requests to terminate the program via the usual mechanism(s) (e.g. Control-C in the console).

newtype AssertionFailed #

assert was applied to False.

Constructors

AssertionFailed String 

Instances

Instances details
Exception AssertionFailed

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Show AssertionFailed

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

data AllocationLimitExceeded #

This thread has exceeded its allocation limit. See setAllocationCounter and enableAllocationLimit.

Since: base-4.8.0.0

ioError :: IOError -> IO a #

Raise an IOException in the IO monad.

asyncExceptionToException :: Exception e => e -> SomeException #

Since: base-4.7.0.0

data MaskingState #

Describes the behaviour of a thread when an asynchronous exception is received.

Constructors

Unmasked

asynchronous exceptions are unmasked (the normal state)

MaskedInterruptible

the state during mask: asynchronous exceptions are masked, but blocking operations may still be interrupted

MaskedUninterruptible

the state during uninterruptibleMask: asynchronous exceptions are masked, and blocking operations may not be interrupted

Instances

Instances details
Show MaskingState

Since: base-4.3.0.0

Instance details

Defined in GHC.IO

Eq MaskingState

Since: base-4.3.0.0

Instance details

Defined in GHC.IO

uninterruptibleMask_ :: IO a -> IO a #

Like uninterruptibleMask, but does not pass a restore action to the argument.

uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b #

Like mask, but the masked computation is not interruptible (see Control.Exception). THIS SHOULD BE USED WITH GREAT CARE, because if a thread executing in uninterruptibleMask blocks for any reason, then the thread (and possibly the program, if this is the main thread) will be unresponsive and unkillable. This function should only be necessary if you need to mask exceptions around an interruptible operation, and you can guarantee that the interruptible operation will only block for a short period of time.

throwIO :: Exception e => e -> IO a #

A variant of throw that can only be used within the IO monad.

Although throwIO has a type that is an instance of the type of throw, the two functions are subtly different:

throw e   `seq` x  ===> throw e
throwIO e `seq` x  ===> x

The first example will cause the exception e to be raised, whereas the second one won't. In fact, throwIO will only cause an exception to be raised when it is used within the IO monad. The throwIO variant should be used in preference to throw to raise an exception within the IO monad because it guarantees ordering with respect to other IO operations, whereas throw does not.

mask_ :: IO a -> IO a #

Like mask, but does not pass a restore action to the argument.

mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b #

Executes an IO computation with asynchronous exceptions masked. That is, any thread which attempts to raise an exception in the current thread with throwTo will be blocked until asynchronous exceptions are unmasked again.

The argument passed to mask is a function that takes as its argument another function, which can be used to restore the prevailing masking state within the context of the masked computation. For example, a common way to use mask is to protect the acquisition of a resource:

mask $ \restore -> do
    x <- acquire
    restore (do_something_with x) `onException` release
    release

This code guarantees that acquire is paired with release, by masking asynchronous exceptions for the critical parts. (Rather than write this code yourself, it would be better to use bracket which abstracts the general pattern).

Note that the restore action passed to the argument to mask does not necessarily unmask asynchronous exceptions, it just restores the masking state to that of the enclosing context. Thus if asynchronous exceptions are already masked, mask cannot be used to unmask exceptions again. This is so that if you call a library function with exceptions masked, you can be sure that the library call will not be able to unmask exceptions again. If you are writing library code and need to use asynchronous exceptions, the only way is to create a new thread; see forkIOWithUnmask.

Asynchronous exceptions may still be received while in the masked state if the masked thread blocks in certain ways; see Control.Exception.

Threads created by forkIO inherit the MaskingState from the parent; that is, to start a thread in the MaskedInterruptible state, use mask_ $ forkIO .... This is particularly useful if you need to establish an exception handler in the forked thread before any asynchronous exceptions are received. To create a new thread in an unmasked state use forkIOWithUnmask.

interruptible :: IO a -> IO a #

Allow asynchronous exceptions to be raised even inside mask, making the operation interruptible (see the discussion of "Interruptible operations" in Exception).

When called outside mask, or inside uninterruptibleMask, this function has no effect.

Since: base-4.9.0.0

getMaskingState :: IO MaskingState #

Returns the MaskingState for the current thread.

evaluate :: a -> IO a #

Evaluate the argument to weak head normal form.

evaluate is typically used to uncover any exceptions that a lazy value may contain, and possibly handle them.

evaluate only evaluates to weak head normal form. If deeper evaluation is needed, the force function from Control.DeepSeq may be handy:

evaluate $ force x

There is a subtle difference between evaluate x and return $! x, analogous to the difference between throwIO and throw. If the lazy value x throws an exception, return $! x will fail to return an IO action and will throw an exception instead. evaluate x, on the other hand, always produces an IO action; that action will throw an exception upon execution iff x throws an exception upon evaluation.

The practical implication of this difference is that due to the imprecise exceptions semantics,

(return $! error "foo") >> error "bar"

may throw either "foo" or "bar", depending on the optimizations performed by the compiler. On the other hand,

evaluate (error "foo") >> error "bar"

is guaranteed to throw "foo".

The rule of thumb is to use evaluate to force or handle exceptions in lazy values. If, on the other hand, you are forcing a lazy value for efficiency reasons only and do not care about exceptions, you may use return $! x.

catch #

Arguments

:: Exception e 
=> IO a

The computation to run

-> (e -> IO a)

Handler to invoke if an exception is raised

-> IO a 

This is the simplest of the exception-catching functions. It takes a single argument, runs it, and if an exception is raised the "handler" is executed, with the value of the exception passed as an argument. Otherwise, the result is returned as normal. For example:

  catch (readFile f)
        (\e -> do let err = show (e :: IOException)
                  hPutStr stderr ("Warning: Couldn't open " ++ f ++ ": " ++ err)
                  return "")

Note that we have to give a type signature to e, or the program will not typecheck as the type is ambiguous. While it is possible to catch exceptions of any type, see the section "Catching all exceptions" (in Control.Exception) for an explanation of the problems with doing so.

For catching exceptions in pure (non-IO) expressions, see the function evaluate.

Note that due to Haskell's unspecified evaluation order, an expression may throw one of several possible exceptions: consider the expression (error "urk") + (1 `div` 0). Does the expression throw ErrorCall "urk", or DivideByZero?

The answer is "it might throw either"; the choice is non-deterministic. If you are catching any type of exception then you might catch either. If you are calling catch with type IO Int -> (ArithException -> IO Int) -> IO Int then the handler may get run with DivideByZero as an argument, or an ErrorCall "urk" exception may be propagated further up. If you call it again, you might get the opposite behaviour. This is ok, because catch is an IO computation.

data ErrorCall #

This is thrown when the user calls error. The first String is the argument given to error, second String is the location.

Bundled Patterns

pattern ErrorCall :: String -> ErrorCall 

Instances

Instances details
Exception ErrorCall

Since: base-4.0.0.0

Instance details

Defined in GHC.Exception

Show ErrorCall

Since: base-4.0.0.0

Instance details

Defined in GHC.Exception

Eq ErrorCall

Since: base-4.7.0.0

Instance details

Defined in GHC.Exception

Ord ErrorCall

Since: base-4.7.0.0

Instance details

Defined in GHC.Exception

throw :: forall (r :: RuntimeRep) (a :: TYPE r) e. Exception e => e -> a #

Throw an exception. Exceptions may be thrown from purely functional code, but may only be caught within the IO monad.

class (Typeable e, Show e) => Exception e where #

Any type that you wish to throw or catch as an exception must be an instance of the Exception class. The simplest case is a new exception type directly below the root:

data MyException = ThisException | ThatException
    deriving Show

instance Exception MyException

The default method definitions in the Exception class do what we need in this case. You can now throw and catch ThisException and ThatException as exceptions:

*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException

In more complicated examples, you may wish to define a whole hierarchy of exceptions:

---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler

data SomeCompilerException = forall e . Exception e => SomeCompilerException e

instance Show SomeCompilerException where
    show (SomeCompilerException e) = show e

instance Exception SomeCompilerException

compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException

compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
    SomeCompilerException a <- fromException x
    cast a

---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler

data SomeFrontendException = forall e . Exception e => SomeFrontendException e

instance Show SomeFrontendException where
    show (SomeFrontendException e) = show e

instance Exception SomeFrontendException where
    toException = compilerExceptionToException
    fromException = compilerExceptionFromException

frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException

frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
    SomeFrontendException a <- fromException x
    cast a

---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception

data MismatchedParentheses = MismatchedParentheses
    deriving Show

instance Exception MismatchedParentheses where
    toException   = frontendExceptionToException
    fromException = frontendExceptionFromException

We can now catch a MismatchedParentheses exception as MismatchedParentheses, SomeFrontendException or SomeCompilerException, but not other types, e.g. IOException:

*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses

Minimal complete definition

Nothing

Methods

toException :: e -> SomeException #

fromException :: SomeException -> Maybe e #

displayException :: e -> String #

Render this exception value in a human-friendly manner.

Default implementation: show.

Since: base-4.8.0.0

Instances

Instances details
Exception TextException 
Instance details

Defined in Z.Data.Text.Base

Exception VectorException 
Instance details

Defined in Z.Data.Vector.Base

Exception Base64DecodeException 
Instance details

Defined in Z.Data.Vector.Base64

Exception HexDecodeException 
Instance details

Defined in Z.Data.Vector.Hex

Exception AlreadyExists Source # 
Instance details

Defined in Z.IO.Exception

Exception HardwareFault Source # 
Instance details

Defined in Z.IO.Exception

Exception IllegalOperation Source # 
Instance details

Defined in Z.IO.Exception

Exception InappropriateType Source # 
Instance details

Defined in Z.IO.Exception

Exception Interrupted Source # 
Instance details

Defined in Z.IO.Exception

Exception InvalidArgument Source # 
Instance details

Defined in Z.IO.Exception

Exception NoSuchThing Source # 
Instance details

Defined in Z.IO.Exception

Exception OtherError Source # 
Instance details

Defined in Z.IO.Exception

Exception PermissionDenied Source # 
Instance details

Defined in Z.IO.Exception

Exception ProtocolError Source # 
Instance details

Defined in Z.IO.Exception

Exception ResourceBusy Source # 
Instance details

Defined in Z.IO.Exception

Exception ResourceExhausted Source # 
Instance details

Defined in Z.IO.Exception

Exception ResourceVanished Source # 
Instance details

Defined in Z.IO.Exception

Exception SomeIOException Source # 
Instance details

Defined in Z.IO.Exception

Exception SystemError Source # 
Instance details

Defined in Z.IO.Exception

Exception TimeExpired Source # 
Instance details

Defined in Z.IO.Exception

Exception UnexpectedEOF Source # 
Instance details

Defined in Z.IO.Exception

Exception UnsatisfiedConstraints Source # 
Instance details

Defined in Z.IO.Exception

Exception UnsupportedOperation Source # 
Instance details

Defined in Z.IO.Exception

Exception NestedAtomically

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception NoMethodError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception NonTermination

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception PatternMatchFail

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception RecConError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception RecSelError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception RecUpdError

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception TypeError

Since: base-4.9.0.0

Instance details

Defined in Control.Exception.Base

Exception Void

Since: base-4.8.0.0

Instance details

Defined in Data.Void

Exception ErrorCall

Since: base-4.0.0.0

Instance details

Defined in GHC.Exception

Exception ArithException

Since: base-4.0.0.0

Instance details

Defined in GHC.Exception.Type

Exception SomeException

Since: base-3.0

Instance details

Defined in GHC.Exception.Type

Exception AllocationLimitExceeded

Since: base-4.8.0.0

Instance details

Defined in GHC.IO.Exception

Exception ArrayException

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception AssertionFailed

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception AsyncException

Since: base-4.7.0.0

Instance details

Defined in GHC.IO.Exception

Exception BlockedIndefinitelyOnMVar

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception BlockedIndefinitelyOnSTM

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception CompactionFailed

Since: base-4.10.0.0

Instance details

Defined in GHC.IO.Exception

Exception Deadlock

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception ExitCode

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception FixIOException

Since: base-4.11.0.0

Instance details

Defined in GHC.IO.Exception

Exception IOException

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception SomeAsyncException

Since: base-4.7.0.0

Instance details

Defined in GHC.IO.Exception

data SomeException #

The SomeException type is the root of the exception type hierarchy. When an exception of type e is thrown, behind the scenes it is encapsulated in a SomeException.

Constructors

Exception e => SomeException e 

Instances

Instances details
Print SomeException 
Instance details

Defined in Z.Data.Text.Print

Exception SomeException

Since: base-3.0

Instance details

Defined in GHC.Exception.Type

Show SomeException

Since: base-3.0

Instance details

Defined in GHC.Exception.Type

type HasCallStack = ?callStack :: CallStack #

Request a CallStack.

NOTE: The implicit parameter ?callStack :: CallStack is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Since: base-4.9.0.0

data CallStack #

CallStacks are a lightweight method of obtaining a partial call-stack at any point in the program.

A function can request its call-site with the HasCallStack constraint. For example, we can define

putStrLnWithCallStack :: HasCallStack => String -> IO ()

as a variant of putStrLn that will get its call-site and print it, along with the string given as argument. We can access the call-stack inside putStrLnWithCallStack with callStack.

>>> :{
putStrLnWithCallStack :: HasCallStack => String -> IO ()
putStrLnWithCallStack msg = do
  putStrLn msg
  putStrLn (prettyCallStack callStack)
:}

Thus, if we call putStrLnWithCallStack we will get a formatted call-stack alongside our string.

>>> putStrLnWithCallStack "hello"
hello
CallStack (from HasCallStack):
  putStrLnWithCallStack, called at <interactive>:... in interactive:Ghci...

GHC solves HasCallStack constraints in three steps:

  1. If there is a CallStack in scope -- i.e. the enclosing function has a HasCallStack constraint -- GHC will append the new call-site to the existing CallStack.
  2. If there is no CallStack in scope -- e.g. in the GHCi session above -- and the enclosing definition does not have an explicit type signature, GHC will infer a HasCallStack constraint for the enclosing definition (subject to the monomorphism restriction).
  3. If there is no CallStack in scope and the enclosing definition has an explicit type signature, GHC will solve the HasCallStack constraint for the singleton CallStack containing just the current call-site.

CallStacks do not interact with the RTS and do not require compilation with -prof. On the other hand, as they are built up explicitly via the HasCallStack constraints, they will generally not contain as much information as the simulated call-stacks maintained by the RTS.

A CallStack is a [(String, SrcLoc)]. The String is the name of function that was called, the SrcLoc is the call-site. The list is ordered with the most recently called function at the head.

NOTE: The intrepid user may notice that HasCallStack is just an alias for an implicit parameter ?callStack :: CallStack. This is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Since: base-4.8.1.0

Instances

Instances details
Print CallStack 
Instance details

Defined in Z.Data.Text.Print

Methods

toUTF8BuilderP :: Int -> CallStack -> Builder () #

IsList CallStack

Be aware that 'fromList . toList = id' only for unfrozen CallStacks, since toList removes frozenness information.

Since: base-4.9.0.0

Instance details

Defined in GHC.Exts

Associated Types

type Item CallStack #

Show CallStack

Since: base-4.9.0.0

Instance details

Defined in GHC.Show

type Item CallStack 
Instance details

Defined in GHC.Exts

callStack :: HasCallStack => CallStack #

Return the current CallStack.

Does *not* include the call-site of callStack.

Since: base-4.9.0.0