Question

Haskell is givinig me a headache today. I want to handle an exception. When it gets to the top it prints like this:

*** Exception: ../p/trip/Trip.hs:(88,16)-(89,50): Non-exhaustive patterns in function split

To me it looks like it is PatternMatchFail, but this doesn’t work:

handle (\(PatternMatchFail _) -> return env) f 

I mean, it compiles, but doesn’t handle the exception. What am I doing wrong? Is this the wrong exception or what? Is there a way to catch any exception?

Answer 1

If fixing the source of the error is not an option, you should look at this: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html

I believe using ‘handle’ or ‘try’ or ‘catch’ or whatever from Control.Exception is the key here, the functions in the standard prelude only deal with IO-Exceptions, not with errors in pure code.

In Haskell98, pure code cannot deal with exceptions. Pure functions must return a value, an exception is a failure to return a value.

Example:

import qualified Control.Exception as C x ::String x = undefined y = 'return value'  main = do C.handle (\_ -> return 'caught') (C.evaluate x) >>= print           C.handle (\_ -> return 'caught') (C.evaluate y) >>= print 

The call to evaluate is to force the evaluation of x and y, haskell being lazy and all.

If you let the evaluation of x be deferred until later (lazily), the exception will also be thrown later, in a different place (in this case it is ‘print’ that uses the value), where it may not be caught.