The stack trace in a lazy language is usually unhelpful, since
it describes the consumer of (hd []) not its producer.
That's why the cost-centre stack idea is likely to help.
Simon
| -----Original Message-----
| From: Fergus Henderson [mailto:[EMAIL PROTECTED]]
| Sent: 01 April 2000 16:53
| To: Simon Peyton-Jones
| Cc: '[EMAIL PROTECTED]'; [EMAIL PROTECTED]
| Subject: Re: improving error messages
|
|
| On 30-Mar-2000, Simon Peyton-Jones <[EMAIL PROTECTED]> wrote:
| > | What do you think of improving the error messages like
| > | Prelude.take: negative argument
| > | (for say, take (-1)
| [(0,'b'),(1,'a')] )
| >
| > That would be splendid. But I don't see how to do it.
| >
| > take :: Int -> [a] -> [a]
| >
| > Since take is polymorphic in a, I can't print a member of the
| > list. If you want to define your own take
| >
| > mytake :: Show a => Int -> [a] -> [a]
| >
| > that's fine, but it's a different function.
|
| Well, there's two separate issues here.
|
| One issue is what exception is thrown. One this issue, you
| are absolutely
| right: there is no way for the exception thrown to expose any
| information
| about the value or type of the argument, without breaking
| some of the nice
| properties that Haskell possesses.
|
| The other issue is what error message the implementation
| prints out if such an
| exception is not caught (as is _always_ the case for
| implementations which
| don't support ghc's `Exception' module or something equivalent).
| And on this issue, Haskell semantics don't impose any constraints.
| The message that the implementation prints out can be as informative
| as the implementor is capable of.
|
| There is no conceptual difficulty with a Haskell implemention
| printing out a
| full stack trace, complete with the values of all of the
| arguments. Of
| course, modifying an existing implementation to do so may
| well be by no means
| trivial, but it's just a matter of programming...
| If you really don't see how it could be done, I'd be happy to sketch
| the outline of a possible implementation.
|
| --
| Fergus Henderson <[EMAIL PROTECTED]> | "I have always known
| that the pursuit
| WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a
| lethal habit"
| PGP: finger [EMAIL PROTECTED] | -- the last words
| of T. S. Garp.
|
