Ambiguous type variable error msg

Question

I don\'t think it is a bug, but I am a bit puzzled as to why that doesn\'t work. A bonus question is why does it mention variable e? There is no variable e.

    P         


        

Answer 1

The type of Control.Exception.handle is:

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

The problem you are seeing is that the lambda expression (\_ -> return "err") is not of type e -> IO a where e is an instance of Exception. Clear as mud? Good. Now I'll provide a solution which should actually be useful :)

It just so happens in your case that e should be Control.Exception.ErrorCall since undefined uses error which throws ErrorCall (an instance of Exception).

To handle uses of undefined you can define something like handleError:

handleError :: (ErrorCall -> IO a) -> IO a -> IO a
handleError = handle

It's essentially an alias Control.Exception.handle with e fixed as ErrorCall which is what error throws.

It looks like this when run in GHCi 7.4.1:

ghci> handleError (\_ -> return "err") undefined
"err"

To handle all exceptions a handleAll function can be written as follows:

handleAll :: (SomeException -> IO a) -> IO a -> IO a
handleAll = handle

Catching all exceptions has consequences described well in this excerpt of the Control.Exception documentation:

Catching all exceptions

It is possible to catch all exceptions, by using the type SomeException:

catch f (\e -> ... (e :: SomeException) ...)

HOWEVER, this is normally not what you want to do!

For example, suppose you want to read a file, but if it doesn't exist then continue as if it contained "". You might be tempted to just catch all exceptions and return "" in the handler. However, this has all sorts of undesirable consequences. For example, if the user presses control-C at just the right moment then the UserInterrupt exception will be caught, and the program will continue running under the belief that the file contains "". Similarly, if another thread tries to kill the thread reading the file then the ThreadKilled exception will be ignored.

Instead, you should only catch exactly the exceptions that you really want. In this case, this would likely be more specific than even "any IO exception"; a permissions error would likely also want to be handled differently. Instead, you would probably want something like:

 e <- tryJust (guard . isDoesNotExistError) (readFile f)
 let str = either (const "") id e

There are occassions when you really do need to catch any sort of exception. However, in most cases this is just so you can do some cleaning up; you aren't actually interested in the exception itself. For example, if you open a file then you want to close it again, whether processing the file executes normally or throws an exception. However, in these cases you can use functions like bracket, finally and onException, which never actually pass you the exception, but just call the cleanup functions at the appropriate points.

But sometimes you really do need to catch any exception, and actually see what the exception is. One example is at the very top-level of a program, you may wish to catch any exception, print it to a logfile or the screen, and then exit gracefully. For these cases, you can use catch (or one of the other exception-catching functions) with the SomeException type.

Source: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html#g:4