Personal tools

Error vs. Exception

From HaskellWiki

Revision as of 23:33, 5 December 2009 by Lemming (Talk | contribs)

Jump to: navigation, search

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
error
s and
undefined
s

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

Contents

1 Examples

examples: GHC errors and exceptions

Bruker library

Modula-3 arithmetic library

2 When exceptions become errors

unhandled exceptions

3 When errors become exceptions

web servers

4 See also