Re: Proposal: making inits and tails less strict

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 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