Copyright  (C) Edward Kmett 20132015 (c) Google Inc. 2012 

License  BSDstyle (see the file LICENSE) 
Maintainer  Edward Kmett <ekmett@gmail.com> 
Stability  experimental 
Portability  nonportable 
Safe Haskell  Trustworthy 
Language  Haskell98 
This module supports monads that can throw extensible exceptions. The
exceptions are the very same from
Control.Exception
, and the operations
offered very similar, but here they are not limited to
IO
.
This code is in the style of both transformers and mtl, and is compatible with them, though doesn't mimic the module structure or offer the complete range of features in those packages.
This is very similar to
ErrorT
and
MonadError
, but based on features of
Control.Exception
. In particular, it handles the complex case of
asynchronous exceptions by including
mask
in the typeclass. Note that the
extensible exceptions feature relies on the RankNTypes language extension.
Synopsis
 class Monad m => MonadThrow m where
 class MonadThrow m => MonadCatch m where

class
MonadCatch
m =>
MonadMask
m
where
 mask :: (( forall a. m a > m a) > m b) > m b
 uninterruptibleMask :: (( forall a. m a > m a) > m b) > m b
 generalBracket :: m a > (a > ExitCase b > m c) > (a > m b) > m (b, c)
 data ExitCase a
 mask_ :: MonadMask m => m a > m a
 uninterruptibleMask_ :: MonadMask m => m a > m a
 catchAll :: MonadCatch m => m a > ( SomeException > m a) > m a
 catchIOError :: MonadCatch m => m a > ( IOError > m a) > m a
 catchJust :: ( MonadCatch m, Exception e) => (e > Maybe b) > m a > (b > m a) > m a
 catchIf :: ( MonadCatch m, Exception e) => (e > Bool ) > m a > (e > m a) > m a
 data Handler m a = forall e. Exception e => Handler (e > m a)
 catches :: ( Foldable f, MonadCatch m) => m a > f ( Handler m a) > m a
 handle :: ( MonadCatch m, Exception e) => (e > m a) > m a > m a
 handleAll :: MonadCatch m => ( SomeException > m a) > m a > m a
 handleIOError :: MonadCatch m => ( IOError > m a) > m a > m a
 handleJust :: ( MonadCatch m, Exception e) => (e > Maybe b) > (b > m a) > m a > m a
 handleIf :: ( MonadCatch m, Exception e) => (e > Bool ) > (e > m a) > m a > m a
 try :: ( MonadCatch m, Exception e) => m a > m ( Either e a)
 tryJust :: ( MonadCatch m, Exception e) => (e > Maybe b) > m a > m ( Either b a)
 onException :: MonadCatch m => m a > m b > m a
 onError :: MonadMask m => m a > m b > m a
 bracket :: MonadMask m => m a > (a > m c) > (a > m b) > m b
 bracket_ :: MonadMask m => m a > m c > m b > m b
 finally :: MonadMask m => m a > m b > m a
 bracketOnError :: MonadMask m => m a > (a > m c) > (a > m b) > m b

class
(
Typeable
e,
Show
e) =>
Exception
e
where
 toException :: e > SomeException
 fromException :: SomeException > Maybe e
 displayException :: e > String
 data SomeException = Exception e => SomeException e
Typeclass
The mtl style typeclass
class Monad m => MonadThrow m where Source #
A class for monads in which exceptions may be thrown.
Instances should obey the following law:
throwM e >> x = throwM e
In other words, throwing an exception shortcircuits the rest of the monadic computation.
throwM :: Exception e => e > m a Source #
Throw an exception. Note that this throws when this action is run in
the monad
m
, not when it is applied. It is a generalization of
Control.Exception
's
throwIO
.
Should satisfy the law:
throwM e >> f = throwM e
Instances
class MonadThrow m => MonadCatch m where Source #
A class for monads which allow exceptions to be caught, in particular
exceptions which were thrown by
throwM
.
Instances should obey the following law:
catch (throwM e) f = f e
Note that the ability to catch an exception does
not
guarantee that we can
deal with all possible exit points from a computation. Some monads, such as
continuationbased stacks, allow for more than just a success/failure
strategy, and therefore
catch
cannot
be used by those monads to properly
implement a function such as
finally
. For more information, see
MonadMask
.
catch :: Exception e => m a > (e > m a) > m a Source #
Provide a handler for exceptions thrown during execution of the first
action. Note that type of the type of the argument to the handler will
constrain which exceptions are caught. See
Control.Exception
's
catch
.
Instances
MonadCatch IO Source #  
MonadCatch STM Source #  
e ~ SomeException => MonadCatch ( Either e) Source # 
Since: 0.8.3 
MonadCatch m => MonadCatch ( ListT m) Source #  
MonadCatch m => MonadCatch ( MaybeT m) Source # 
Catches exceptions from the base monad. 
Monad m => MonadCatch ( CatchT m) Source #  
MonadCatch m => MonadCatch ( IdentityT m) Source #  
( Error e, MonadCatch m) => MonadCatch ( ErrorT e m) Source # 
Catches exceptions from the base monad. 
MonadCatch m => MonadCatch ( ExceptT e m) Source # 
Catches exceptions from the base monad. 
MonadCatch m => MonadCatch ( ReaderT r m) Source #  
MonadCatch m => MonadCatch ( StateT s m) Source #  
MonadCatch m => MonadCatch ( StateT s m) Source #  
( MonadCatch m, Monoid w) => MonadCatch ( WriterT w m) Source #  
( MonadCatch m, Monoid w) => MonadCatch ( WriterT w m) Source #  
( MonadCatch m, Monoid w) => MonadCatch ( RWST r w s m) Source #  
( MonadCatch m, Monoid w) => MonadCatch ( RWST r w s m) Source #  
class MonadCatch m => MonadMask m where Source #
A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuationbased monads are invalid instances of this class.
Instances should ensure that, in the following code:
fg = f `finally` g
The action
g
is called regardless of what occurs within
f
, including
async exceptions. Some monads allow
f
to abort the computation via other
effects than throwing an exception. For simplicity, we will consider aborting
and throwing an exception to be two forms of "throwing an error".
If
f
and
g
both throw an error, the error thrown by
fg
depends on which
errors we're talking about. In a monad transformer stack, the deeper layers
override the effects of the inner layers; for example,
ExceptT e1 (Except
e2) a
represents a value of type
Either e2 (Either e1 a)
, so throwing both
an
e1
and an
e2
will result in
Left e2
. If
f
and
g
both throw an
error from the same layer, instances should ensure that the error from
g
wins.
Effects other than throwing an error are also overriden by the deeper layers.
For example,
StateT s Maybe a
represents a value of type
s > Maybe (a,
s)
, so if an error thrown from
f
causes this function to return
Nothing
,
any changes to the state which
f
also performed will be erased. As a
result,
g
will see the state as it was before
f
. Once
g
completes,
f
's error will be rethrown, so
g
' state changes will be erased as well.
This is the normal interaction between effects in a monad transformer stack.
By contrast,
liftedbase
's
version of
finally
always discards all of
g
's nonIO effects, and
g
never sees any of
f
's nonIO effects, regardless of the layer ordering and
regardless of whether
f
throws an error. This is not the result of
interacting effects, but a consequence of
MonadBaseControl
's approach.
mask :: (( forall a. m a > m a) > m b) > m b Source #
Runs an action with asynchronous exceptions disabled. The action is
provided a method for restoring the async. environment to what it was
at the
mask
call. See
Control.Exception
's
mask
.
uninterruptibleMask :: (( forall a. m a > m a) > m b) > m b Source #
Like
mask
, but the masked computation is not interruptible (see
Control.Exception
's
uninterruptibleMask
. WARNING:
Only use if you need to mask exceptions around an interruptible operation
AND you can guarantee the interruptible operation will only block for a
short period of time. Otherwise you render the program/thread unresponsive
and/or unkillable.
:: m a 
acquire some resource 
> (a > ExitCase b > m c) 
release the resource, observing the outcome of the inner action 
> (a > m b) 
inner action to perform with the resource 
> m (b, c) 
A generalized version of
bracket
which uses
ExitCase
to distinguish
the different exit cases, and returns the values of both the
use
and
release
actions. In practice, this extra information is rarely needed,
so it is often more convenient to use one of the simpler functions which
are defined in terms of this one, such as
bracket
,
finally
,
onError
,
and
bracketOnError
.
This function exists because in order to thread their effects through the
execution of
bracket
, monad transformers need values to be threaded from
use
to
release
and from
release
to the output value.
NOTE
This method was added in version 0.9.0 of this
library. Previously, implementation of functions like
bracket
and
finally
in this module were based on the
mask
and
uninterruptibleMask
functions only, disallowing some classes of
tranformers from having
MonadMask
instances (notably
multiexitpoint transformers like
ExceptT
). If you are a
library author, you'll now need to provide an implementation for
this method. The
StateT
implementation demonstrates most of the
subtleties:
generalBracket acquire release use = StateT $ s0 > do ((b, _s2), (c, s3)) < generalBracket (runStateT acquire s0) ((resource, s1) exitCase > case exitCase of ExitCaseSuccess (b, s2) > runStateT (release resource (ExitCaseSuccess b)) s2  In the two other cases, the base monad overridesuse
's state  changes and the state reverts tos1
. ExitCaseException e > runStateT (release resource (ExitCaseException e)) s1 ExitCaseAbort > runStateT (release resource ExitCaseAbort) s1 ) ((resource, s1) > runStateT (use resource) s1) return ((b, c), s3)
The
StateT s m
implementation of
generalBracket
delegates to the
m
implementation of
generalBracket
. The
acquire
,
use
, and
release
arguments given to
StateT
's implementation produce actions of type
StateT s m a
,
StateT s m b
, and
StateT s m c
. In order to run those
actions in the base monad, we need to call
runStateT
, from which we
obtain actions of type
m (a, s)
,
m (b, s)
, and
m (c, s)
. Since each
action produces the next state, it is important to feed the state produced
by the previous action to the next action.
In the
ExitCaseSuccess
case, the state starts at
s0
, flows through
acquire
to become
s1
, flows through
use
to become
s2
, and finally
flows through
release
to become
s3
. In the other two cases,
release
does not receive the value
s2
, so its action cannot see the state changes
performed by
use
. This is fine, because in those two cases, an error was
thrown in the base monad, so as per the usual interaction between effects
in a monad transformer stack, those state changes get reverted. So we start
from
s1
instead.
Finally, the
m
implementation of
generalBracket
returns the pairs
(b, s)
and
(c, s)
. For monad transformers other than
StateT
, this
will be some other type representing the effects and values performed and
returned by the
use
and
release
actions. The effect part of the
use
result, in this case
_s2
, usually needs to be discarded, since those
effects have already been incorporated in the
release
action.
The only effect which is intentionally not incorporated in the
release
action is the effect of throwing an error. In that case, the error must be
rethrown. One subtlety which is easy to miss is that in the case in which
use
and
release
both throw an error, the error from
release
should
take priority. Here is an implementation for
ExceptT
which demonstrates
how to do this.
generalBracket acquire release use = ExceptT $ do (eb, ec) < generalBracket (runExceptT acquire) (eresource exitCase > case eresource of Left e > return (Left e)  nothing to release, acquire didn't succeed Right resource > case exitCase of ExitCaseSuccess (Right b) > runExceptT (release resource (ExitCaseSuccess b)) ExitCaseException e > runExceptT (release resource (ExitCaseException e)) _ > runExceptT (release resource ExitCaseAbort)) (either (return . Left) (runExceptT . use)) return $ do  The order in which we perform those twoEither
effects determines  which error will win if they are bothLeft
s. We want the error from release
to win. c < ec b < eb return (b, c)
Since: 0.9.0
Instances
MonadMask IO Source #  
e ~ SomeException => MonadMask ( Either e) Source # 
Since: 0.8.3 
Defined in Control.Monad.Catch 

MonadMask m => MonadMask ( MaybeT m) Source # 
Since: 0.10.0 
Defined in Control.Monad.Catch 

Monad m => MonadMask ( CatchT m) Source # 
Note: This instance is only valid if the underlying monad has a single exit point!
For example,

Defined in Control.Monad.Catch.Pure 

MonadMask m => MonadMask ( IdentityT m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. IdentityT m a > IdentityT m a) > IdentityT m b) > IdentityT m b Source # uninterruptibleMask :: (( forall a. IdentityT m a > IdentityT m a) > IdentityT m b) > IdentityT m b Source # generalBracket :: IdentityT m a > (a > ExitCase b > IdentityT m c) > (a > IdentityT m b) > IdentityT m (b, c) Source # 

( Error e, MonadMask m) => MonadMask ( ErrorT e m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. ErrorT e m a > ErrorT e m a) > ErrorT e m b) > ErrorT e m b Source # uninterruptibleMask :: (( forall a. ErrorT e m a > ErrorT e m a) > ErrorT e m b) > ErrorT e m b Source # generalBracket :: ErrorT e m a > (a > ExitCase b > ErrorT e m c) > (a > ErrorT e m b) > ErrorT e m (b, c) Source # 

MonadMask m => MonadMask ( ExceptT e m) Source # 
Since: 0.9.0 
Defined in Control.Monad.Catch mask :: (( forall a. ExceptT e m a > ExceptT e m a) > ExceptT e m b) > ExceptT e m b Source # uninterruptibleMask :: (( forall a. ExceptT e m a > ExceptT e m a) > ExceptT e m b) > ExceptT e m b Source # generalBracket :: ExceptT e m a > (a > ExitCase b > ExceptT e m c) > (a > ExceptT e m b) > ExceptT e m (b, c) Source # 

MonadMask m => MonadMask ( ReaderT r m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. ReaderT r m a > ReaderT r m a) > ReaderT r m b) > ReaderT r m b Source # uninterruptibleMask :: (( forall a. ReaderT r m a > ReaderT r m a) > ReaderT r m b) > ReaderT r m b Source # generalBracket :: ReaderT r m a > (a > ExitCase b > ReaderT r m c) > (a > ReaderT r m b) > ReaderT r m (b, c) Source # 

MonadMask m => MonadMask ( StateT s m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. StateT s m a > StateT s m a) > StateT s m b) > StateT s m b Source # uninterruptibleMask :: (( forall a. StateT s m a > StateT s m a) > StateT s m b) > StateT s m b Source # generalBracket :: StateT s m a > (a > ExitCase b > StateT s m c) > (a > StateT s m b) > StateT s m (b, c) Source # 

MonadMask m => MonadMask ( StateT s m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. StateT s m a > StateT s m a) > StateT s m b) > StateT s m b Source # uninterruptibleMask :: (( forall a. StateT s m a > StateT s m a) > StateT s m b) > StateT s m b Source # generalBracket :: StateT s m a > (a > ExitCase b > StateT s m c) > (a > StateT s m b) > StateT s m (b, c) Source # 

( MonadMask m, Monoid w) => MonadMask ( WriterT w m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. WriterT w m a > WriterT w m a) > WriterT w m b) > WriterT w m b Source # uninterruptibleMask :: (( forall a. WriterT w m a > WriterT w m a) > WriterT w m b) > WriterT w m b Source # generalBracket :: WriterT w m a > (a > ExitCase b > WriterT w m c) > (a > WriterT w m b) > WriterT w m (b, c) Source # 

( MonadMask m, Monoid w) => MonadMask ( WriterT w m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. WriterT w m a > WriterT w m a) > WriterT w m b) > WriterT w m b Source # uninterruptibleMask :: (( forall a. WriterT w m a > WriterT w m a) > WriterT w m b) > WriterT w m b Source # generalBracket :: WriterT w m a > (a > ExitCase b > WriterT w m c) > (a > WriterT w m b) > WriterT w m (b, c) Source # 

( MonadMask m, Monoid w) => MonadMask ( RWST r w s m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. RWST r w s m a > RWST r w s m a) > RWST r w s m b) > RWST r w s m b Source # uninterruptibleMask :: (( forall a. RWST r w s m a > RWST r w s m a) > RWST r w s m b) > RWST r w s m b Source # generalBracket :: RWST r w s m a > (a > ExitCase b > RWST r w s m c) > (a > RWST r w s m b) > RWST r w s m (b, c) Source # 

( MonadMask m, Monoid w) => MonadMask ( RWST r w s m) Source #  
Defined in Control.Monad.Catch mask :: (( forall a. RWST r w s m a > RWST r w s m a) > RWST r w s m b) > RWST r w s m b Source # uninterruptibleMask :: (( forall a. RWST r w s m a > RWST r w s m a) > RWST r w s m b) > RWST r w s m b Source # generalBracket :: RWST r w s m a > (a > ExitCase b > RWST r w s m c) > (a > RWST r w s m b) > RWST r w s m (b, c) Source # 
A
MonadMask
computation may either succeed with a value, abort with an
exception, or abort for some other reason. For example, in
ExceptT e IO
you can use
throwM
to abort with an exception (
ExitCaseException
) or
throwE
to abort with a value of type
e
(
ExitCaseAbort
).
Utilities
These functions follow those from
Control.Exception
, except that they are
based on methods from the
MonadCatch
typeclass. See
Control.Exception
for API usage.
mask_ :: MonadMask m => m a > m a Source #
Like
mask
, but does not pass a
restore
action to the argument.
uninterruptibleMask_ :: MonadMask m => m a > m a Source #
Like
uninterruptibleMask
, but does not pass a
restore
action to the
argument.
catchAll :: MonadCatch m => m a > ( SomeException > m a) > m a Source #
Catches all exceptions, and somewhat defeats the purpose of the extensible exception system. Use sparingly.
NOTE This catches all exceptions , but if the monad supports other ways of aborting the computation, those other kinds of errors will not be caught.
catchIOError :: MonadCatch m => m a > ( IOError > m a) > m a Source #
Catch all
IOError
(eqv.
IOException
) exceptions. Still somewhat too
general, but better than using
catchAll
. See
catchIf
for an easy way
of catching specific
IOError
s based on the predicates in
System.IO.Error
.
catchJust :: ( MonadCatch m, Exception e) => (e > Maybe b) > m a > (b > m a) > m a Source #
A more generalized way of determining which exceptions to catch at run time.
catchIf :: ( MonadCatch m, Exception e) => (e > Bool ) > m a > (e > m a) > m a Source #
Catch exceptions only if they pass some predicate. Often useful with the
predicates for testing
IOError
values in
System.IO.Error
.
Generalized version of
Handler
catches :: ( Foldable f, MonadCatch m) => m a > f ( Handler m a) > m a Source #
Catches different sorts of exceptions. See
Control.Exception
's
catches
handle :: ( MonadCatch m, Exception e) => (e > m a) > m a > m a Source #
Flipped
catch
. See
Control.Exception
's
handle
.
handleAll :: MonadCatch m => ( SomeException > m a) > m a > m a Source #
Flipped
catchAll
handleIOError :: MonadCatch m => ( IOError > m a) > m a > m a Source #
Flipped
catchIOError
handleJust :: ( MonadCatch m, Exception e) => (e > Maybe b) > (b > m a) > m a > m a Source #
Flipped
catchJust
. See
Control.Exception
's
handleJust
.
handleIf :: ( MonadCatch m, Exception e) => (e > Bool ) > (e > m a) > m a > m a Source #
Flipped
catchIf
try :: ( MonadCatch m, Exception e) => m a > m ( Either e a) Source #
Similar to
catch
, but returns an
Either
result. See
Control.Exception
's
try
.
tryJust :: ( MonadCatch m, Exception e) => (e > Maybe b) > m a > m ( Either b a) Source #
A variant of
try
that takes an exception predicate to select
which exceptions are caught. See
Control.Exception
's
tryJust
onException :: MonadCatch m => m a > m b > m a Source #
Run an action only if an exception is thrown in the main action. The exception is not caught, simply rethrown.
NOTE
The action is only run if an
exception
is thrown. If the monad
supports other ways of aborting the computation, the action won't run if
those other kinds of errors are thrown. See
onError
.
onError :: MonadMask m => m a > m b > m a Source #
Run an action only if an error is thrown in the main action. Unlike
onException
, this works with every kind of error, not just exceptions. For
example, if
f
is an
ExceptT
computation which aborts with a
Left
, the
computation
onError f g
will execute
g
, while
onException f g
will not.
This distinction is only meaningful for monads which have multiple exit
points, such as
Except
and
MaybeT
. For monads that only have a single
exit point, there is no difference between
onException
and
onError
,
except that
onError
has a more constrained type.
Since: 0.10.0
bracket :: MonadMask m => m a > (a > m c) > (a > m b) > m b Source #
Generalized abstracted pattern of safe resource acquisition and release
in the face of errors. The first action "acquires" some value, which
is "released" by the second action at the end. The third action "uses"
the value and its result is the result of the
bracket
.
If an error is thrown during the use, the release still happens before the error is rethrown.
Note that this is essentially a typespecialized version of
generalBracket
. This function has a more common signature (matching the
signature from
Control.Exception
), and is often more convenient to use. By
contrast,
generalBracket
is more expressive, allowing us to implement
other functions like
bracketOnError
.
bracket_ :: MonadMask m => m a > m c > m b > m b Source #
Version of
bracket
without any value being passed to the second and
third actions.
finally :: MonadMask m => m a > m b > m a Source #
Perform an action with a finalizer action that is run, even if an error occurs.
bracketOnError :: MonadMask m => m a > (a > m c) > (a > m b) > m b Source #
Like
bracket
, but only performs the final action if an error is
thrown by the inbetween computation.
Reexports from Control.Exception
class ( Typeable e, Show e) => Exception e where Source #
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
Nothing
toException :: e > SomeException Source #
fromException :: SomeException > Maybe e Source #
displayException :: e > String Source #
Render this exception value in a humanfriendly manner.
Default implementation:
.
show
Since: base4.8.0.0
Instances
data SomeException Source #
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
.
Exception e => SomeException e 
Instances
Show SomeException 
Since: base3.0 
Defined in GHC.Exception.Type 

Exception SomeException 
Since: base3.0 
Defined in GHC.Exception.Type 