[Haskell-cafe] A program which never crashes (even when a function calls error)
[It is a philosophical question, not a practical programming problem.] I'm used, in imperative programming languages with exceptions (like Python) to call any function without fear of stopping the program because I can always catch the exceptions with things like (Python): while not over: try: code which may raise an exception... except Exception e: do something clever How to do it in Haskell? How can I call functions like Prelude.head while being sure my program won't stop, even if I call head on an empty list (thus calling error)? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A program which never crashes (even when a function calls error)
On 8/1/06, Stephane Bortzmeyer [EMAIL PROTECTED] wrote: How to do it in Haskell? How can I call functions like Prelude.head while being sure my program won't stop, even if I call head on an empty list (thus calling error)? Try looking at Control.Exception. For example: module Test where import Control.Exception import Prelude hiding (catch) example = (do print (head (tail a)) return ok) `catch` (\e - do putStrLn (Caught exception: ++ show e) return error) produces: *Test z - example Caught exception: Prelude.head: empty list *Test z error This might be the beginning of what you want. /g ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A program which never crashes (even when a function calls error)
On Tue, Aug 01, 2006 at 08:52:06AM +0200, Stephane Bortzmeyer wrote: To: haskell-cafe@haskell.org From: Stephane Bortzmeyer [EMAIL PROTECTED] Date: Tue, 1 Aug 2006 08:52:06 +0200 Subject: [Haskell-cafe] A program which never crashes (even when a function calls error) [It is a philosophical question, not a practical programming problem.] I'm used, in imperative programming languages with exceptions (like Python) to call any function without fear of stopping the program because I can always catch the exceptions with things like (Python): while not over: try: code which may raise an exception... except Exception e: do something clever How to do it in Haskell? How can I call functions like Prelude.head while being sure my program won't stop, even if I call head on an empty list (thus calling error)? in haskell98, you can't. if you cannot prove a list will always be non-empty, you should use pattern matching instead of head. one disadvantage of exceptions is that the byte code tends to be slow and ugly and hard to generate, in particular in pure lazy languages. but admittedly sometimes exceptions are cool. therefore ghc comes with a quite sophisticated and mature exception handling library. http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html looks a little different from python, but should do the trick. cheers, matthias signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A program which never crashes (even when a function calls error)
On Tue, 1 Aug 2006, Stephane Bortzmeyer wrote: [It is a philosophical question, not a practical programming problem.] I'm used, in imperative programming languages with exceptions (like Python) to call any function without fear of stopping the program because I can always catch the exceptions with things like (Python): while not over: try: code which may raise an exception... except Exception e: do something clever How to do it in Haskell? How can I call functions like Prelude.head while being sure my program won't stop, even if I call head on an empty list (thus calling error)? Catching errors is quite a hack. If the head of an empty list is requested, then this is considered a programming error, not a user error. Thus the best is to stop the program. Don't mix programming errors with problems caused by users (that is, exceptions). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A program which never crashes (even when a function calls error)
Hello Stephane, Tuesday, August 1, 2006, 10:52:06 AM, you wrote: except Exception e: don't look at anything except than Tackling the awkward squad: monadic input/output, concurrency, exceptions, and foreign-language calls in Haskell http://research.microsoft.com/Users/simonpj/papers/marktoberdorf/marktoberdorf.ps.gz -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A program which never crashes (even when a function calls error)
Stephane Bortzmeyer wrote: [It is a philosophical question, not a practical programming problem.] I'm used, in imperative programming languages with exceptions (like Python) to call any function without fear of stopping the program because I can always catch the exceptions with things like (Python): while not over: try: code which may raise an exception... except Exception e: do something clever How to do it in Haskell? How can I call functions like Prelude.head while being sure my program won't stop, even if I call head on an empty list (thus calling error)? Here's another way of looking at it, that I've grown fond of. If your program is a total function, then there should be no exceptions. That is, if you properly model the world, in all its messiness, then you can write a function that maps every instance of the world to some valid output, even if that output is ``Sorry.'' It might seem a daunting task, but liberal use of the Maybe class from the ground up helps. We have suffered through quite a bit of this with our hardware detector, where unexpected situations are the norm. My colleague David Fox has spent considerable time computing reasonable answers in seemingly impossible situations, to the point where you cannot turn around without bumping into a Maybe construct. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
