inits

[Aaron Denney]

On 2006-04-08, Tim Toorop <timtoorop at quicknet.nl> wrote:
> Josef Svenningsson wrote:
>> On 4/8/06, Tim Toorop <timtoorop at quicknet.nl> wrote:
>>   
>>> On the other hand, I think the new spiffy inits is quite clear once you
>>> notice why its  map (const xn) $ undefined:xn
>>>
>>>     
>>
>> I think it would be even clearer if it were defined like this:
>>
>>   
>>> inits xs = zipWith take [0..] $ map (const xs) xs
>>>     
>>
>> That way you don't have this ugly special case for the first element
>> in the list. But there is an even better advantage: it is strict
>> again! So it's termination properties are exactly like the Prelude
>> one. This version *can* be used as a drop-in replacement of the
>> Prelude version.
>>
>> Cheers,
>>
>> /Josef
>>   
> Yes it is very clear ... except for the fact that it doesn't work correctly.
> It misses the last element.
> now we could write
>       inits2' xn = (zipWith take [0..] $ map (const xn) xn ) ++ [xn]
> Though I dont know if this really is clearer, and it seems to make the 
> garbage collector work slightly more
> And which I think does exactly the same as
>       inits' xn@(_:_) = zipWith take [0..] $ map (const xn) $ undefined:xn

Yes, but is there any real need to use the ugly pattern syntax?

Surely 

inits xs = zipWith take [0..] $ map (const xs) (undefined:xs)

(I still think letting head $ inits undefined be [] rather than
undefined is better than the current def.)

-- 
Aaron Denney
-><-