catch package:ghc-lib-parser

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.

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.

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)]

As with "jForIn" but creating a "for each in" statement.