In Haskell, we use the Control.Exception module to handle exceptions. The general syntax for using try/catch/finally in Haskell is as follows:
import Control.Exception
main = do
result <- try $ do
-- code that may throw an exception
return "Success" -- return value if no exception is thrown
case result of
Left exception -> do
putStrLn $ "Caught exception: " ++ show (exception :: SomeException)
Right success -> do
putStrLn $ "Success: " ++ success
finally Block
Here,
try wraps the code block that might throw an exception. If an exception is thrown, then the result will be a Left value containing the exception, otherwise, a Right value containing the result of the code block is returned. The case statement is used to pattern match on the result to handle the exception if one occurred, or to handle the success value.
The finally block is optional, and is used to execute some code regardless of whether or not an exception was thrown. Here's an example with a message:
import Control.Exception
main = do
result <- try $ do
num1 <- readLn
num2 <- readLn
if num2 == 0
then throwIO DivideByZero
else return $ num1 div num2
case result of
Left DivideByZero -> do
putStrLn "Caught DivideByZero exception. Cannot divide by zero!"
Left exception -> do
putStrLn $ "Caught exception: " ++ show (exception :: SomeException)
Right quotient -> do
putStrLn $ "The result of dividing " ++ show num1 ++ " by " ++ show num2 ++ " is: " ++ show quotient
finally putStrLn "Exiting program."
In this example, we're trying to divide two numbers provided by user input, but we first check if the second number is zero, and throws a
DivideByZero exception if it is. If no exception is thrown, we print out the quotient. If an exception is thrown, we print out an appropriate message. Finally, we print out a message indicating that the program is exiting, regardless of whether or not an exception was thrown.