On Thu, Mar 17, 2011 at 7:58 PM, Ivan Lazar Miljenovic <ivan.miljenovic@gmail.com> wrote:
On 18 March 2011 06:25, Bas van Dijk <v.dijk.bas@gmail.com> wrote:
> I would like to propose making inits and tails less strict:
>
>  inits                   :: [a] -> [[a]]
> -inits []                =  [[]]
> -inits (x:xs)            =  [[]] ++ map (x:) (inits xs)
> +inits xs                =  [] : case xs of
> +                                  []   -> []
> +                                  x:xs -> map (x:) (inits xs)
>
>  tails                   :: [a] -> [[a]]
> -tails []                =  [[]]
> -tails xxs@(_:xs)        =  xxs : tails xs
> +tails xxs               =  xxs : case xxs of
> +                                   []   -> []
> +                                   _:xs -> tails xs
>
> Having a lazier inits allows the elegant:
> nats = map length (inits nats)
> which loops for the current definition. This definition was due to John Tromp:
> http://www.mail-archive.com/haskell@haskell.org/msg21044.html

I'm not against this proposal, but am just curious: is there any
reason/need for this lazier definition apart from an elegant (but
possibly not actually needed/useful) trick?
 
In general with the standard library definitions have been made as lazy as they reasonably can. 

Here, clearly, the initial [] doesn't depend on the argument being resolved to a cons cell or nil.

For me this is interesting mostly because of the nearly-comonadic 

extend :: ([a] -> b) -> [a] -> [b]
extend f = map f . tails

which I would like to be as productive as possible.

-Edward