I quite like the argument that take is a total function and
as such all its return values are from teh specificed range. I can also
see the logic in
take n [] = [] where n > 0
taking n from nothing, or the empty set, returns nothing!
The same should apply to head and tail. head or tail of [] should
be [].
What does the list think?
Cheers, Paul
At 16:43 12/09/2007, you wrote:
Hi
On 12 Sep 2007, at 11:44, ChrisK wrote:
Conor McBride wrote:
I'd like operations to
complain
about bogus input, rather than producing bogus output.
Then you want a runtime assertion checking error helpful Data.List
replacement.
Could you use Control.Exception.assert and make a wrapper for
Data.List?
http://www.haskell.org/ghc/docs/latest/html/users_guide/sec-
assertions.html
Hmmm. It might be quite annoying to make it a wrapper if it's just
a
question
of appealing to error rather than returning dummy values in failure
cases.
Defining the domain of a function can be quite like defining the
function
itself. Also, sometimes there can be problems arriving at a Bool.
For
example,
zipping colists is a productive coprogram, and it can raise errors
in
off-diagonal cases, but you can't compute in advance whether you're
on the
diagonal.
A more serious point is that in some cases we might want take to
underapproximate, or zip to truncate (or tail [] = [] ?). I don't
think there's
always a clear "library" choice here. I tend to be pragmatic
about
it. I was
just pointing out that it does sometimes make sense to use less
defined
functions when one has a more specific domain in mind. I'm usually
happy to
write the fussy variants myself, if I'm that agitated.
Funny old world
Conor
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