Error vs. Exception
There is confusion about the distinction of errors and exceptions for a long time, repeated threads in Haskell-Cafe and more and more packages that handle errors and exceptions or something between. Although both terms are related and sometimes hard to distinguish, it is important to do it carefully. This is like the confusion between parallelism and concurrency.
The first problem is that exception seem to me to be the historically younger term. Before there were only errors, independent from whether they are programming or I/O or user errors. In this article we want to use the term "exception" for expected but irregular situations at runtime and the term "error" for mistakes in the running program, that can be resolved only by fixing the program.
The history may have led to the identifiers we find today in the Haskell language and standard Haskell modules.
- Exceptions:
Prelude.catch,Control.Exception.catch,Control.Exception.try,IOError,Control.Monad.Error - Errors:
error,Control.Exception.catch,Debug.Trace.trace
Note, that the catch> function from Prelude handles exceptions exclusively,
whereas its counterpart from Control.Exception also catches certain kinds of undefined values.
Prelude> catch (error "bla") (\msg -> putStrLn $ "catched " ++ show msg)
*** Exception: bla
Prelude> Control.Exception.catch (error "bla") (\msg -> putStrLn $ "catched " ++ show (msg::Control.Exception.SomeException))
catched bla
This is unsafe, since Haskell's error is just sugar for undefined,
that shall help spotting a programming error.
A program should work as well when all errors and undefineds
are replaced by infinite loops.
However infinite loops in general cannot be catched, whereas calls to sugared functions like error can.
Even more confusion was initiated by Java programming language
to use the term "exceptions" for programming errors like the NullPointerException
and introducing the distinction between
checked and unchecked exceptions.
error handling = debugging
Examples
examples: GHC errors and exceptions
Bruker library
Modula-3 arithmetic library
When exceptions become errors
unhandled exceptions
When errors become exceptions
web servers