
Control.Monad.Exception.Base 


Description 
A Monad Transformer for explicitly typed checked exceptions.
The exceptions thrown by a computation are inferred by the typechecker
and appear in the type signature of the computation as Throws constraints.
Exceptions are defined using the extensible exceptions framework of Marlow (documented in Control.Exception):
 An Extensible DynamicallyTyped Hierarchy of Exceptions, by Simon Marlow, in Haskell '06.
Example
data DivideByZero = DivideByZero deriving (Show, Typeable)
data SumOverflow = SumOverflow deriving (Show, Typeable)
instance Exception DivideByZero
instance Exception SumOverflow
data Expr = Add Expr Expr  Div Expr Expr  Val Double
eval (Val x) = return x
eval (Add a1 a2) = do
v1 < eval a1
v2 < eval a2
let sum = v1 + v2
if sum < v1  sum < v2 then throw SumOverflow else return sum
eval (Div a1 a2) = do
v1 < eval a1
v2 < eval a2
if v2 == 0 then throw DivideByZero else return (v1 / v2)
GHCi infers the following types
eval :: (Throws DivideByZero l, Throws SumOverflow l) => Expr > EM l Double
eval `catch` \ (e::DivideByZero) > return (1) :: Throws SumOverflow l => Expr > EM l Double
runEM(eval `catch` \ (e::SomeException) > return (1))
:: Expr > Double


Synopsis 



Documentation 




A Monad Transformer for explicitly typed checked exceptions.
 Constructors   Instances  



Run a computation explicitly handling exceptions







Instances  



Instances  





Run a safe computation



Run a safe computation checking even unchecked (UncaughtException) exceptions



The throw primitive



Rethrow an exception keeping the call trace



The catch primitive



Like catch but makes the call trace available



Sequence two computations discarding the result of the second one.
If the first computation rises an exception, the second computation is run
and then the exception is rethrown.



Like finally, but performs the second computation only when the first one
rises an exception



:: Monad m   => EMT l m a  acquire resource
 > a > EMT l m b  release resource
 > a > EMT l m c  computation
 > EMT l m c  



Capture an exception e, wrap it, and rethrow.
Keeps the original monadic call trace.





UncaughtException models unchecked exceptions
In order to declare an unchecked exception E,
all that is needed is to make e an instance of UncaughtException
instance UncaughtException E
Note that declaring an exception E as unchecked does not automatically
turn its children unchecked too. This is a shortcoming of the current encoding.
  Instances  



A monad of explicitly typed, checked exceptions



Run a computation explicitly handling exceptions





Run a safe computation



Run a computation checking even unchecked (UncaughtExceptions) exceptions



Constructors   Instances  



FailException is thrown by Monad fail
 Constructors   Instances  


data MonadZeroException  Source 

MonadZeroException is thrown by MonadPlus mzero
 Constructors   Instances  



This function may be used as a value for mplus in MonadPlus




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