inits

[Tim Toorop]

Chris Kuklewicz wrote:
> Aaron Denney wrote:
>   
>> On 2006-04-08, Nils Anders Danielsson <nad at cs.chalmers.se> wrote:
>>     
>>> On Fri, 07 Apr 2006, "Spencer Janssen" <spencerjanssen at gmail.com> wrote:
>>>
>>>       
>>>>> inits xs = [] : (zipWith take [1..] $ map (const xs) xs)
>>>>>           
>>>> As this version performs much better and will work as a drop in
>>>> replacement, I suggest that it be included in the hierarchical
>>>> libraries.
>>>>         
>>> It is not a drop in replacement. The original inits is strict, this
>>> one isn't.
>>>
>>> The specification of inits (from the Haskell 98 report):
>>>
>>>   inits                   :: [a] -> [[a]]
>>>   inits []                =  [[]]
>>>   inits (x:xs)            =  [[]] ++ map (x:) (inits xs)
>>>       
>> Is that a property many programs depend on?  I'd actually call that a
>> bug of the original.
>>
>>     
>
> It may break some things:
>
> head (Data.List.inits undefined) is an error
>
> head (New.Spiffy.inits undefined) is []
>
>   
Is the head of the inits of undefined really an error?
Since the head of inits []  is also []  ...
But if you really want that undefined to produce an error.. you could just :
        inits' xn@(_:_) = zipWith take [0..] $ map (const xn) $ undefined:xn
        inits' _        = undefined