[Haskell-cafe] Re: Is "take" behaving correctly?

[PR Stanley]

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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