[Haskell-cafe] Naturality condition for inits

[Daniel Fischer]

Am Samstag, 7. März 2009 23:18 schrieb R J:
> Here's another Bird problem that's stymied me:
>
> The function "inits" computes the list of initial segments of a list; its
> type is inits :: [a] -> [[a]].  What is the appropriate naturality
> condition for "inits"?
>
> The only discussion in the text concerning naturality conditions concerns
> map, where the naturality conditions are stated in what seem to be
> quasi-commutativity laws over the composition operator, as follows:
>
>    f . head            =  head . map f, where f is strict (i.e., f _|_ =
> _|_) map f . tail        =  tail . map f
>    map f (xs ++ ys)    =  map f xs ++ map f ys
>    map f . reverse     =  reverse . map f
>    map f . concat      =  concat . map (map f)
>
> I believe that none of the following naturality conditions, extrapolated
> from those above, hold:
>
>    a. head . inits     =  inits [head]
>    b. tail . inits     =  inits . tail
>    c. reverse . inits  =  inits . reverse
>    d. concat . inits   =  inits . concat

How does inits interplay with (map f)?
Though inits . tail  =/= tail . inits, there is an interesting relation 
between inits  (tail xs) and inits xs, which?
When you also consider the function tails, there is an interesting relation 
involving inits and reverse, too.

>
> In case the definition of "inits" is relevant, my definition is:
>
>
>
> inits                  :: [a] -> [[a]]
>
> inits xs               =  [take n xs | n <- seglengths]
>
>                           where
>
>                               seglengths = [0..length xs]

Better define it recursively.

inits [] = [[]]
inits (x:xs) = []:map (x:) (inits xs)

>
> Thanks.