Proposal: making inits and tails less strict
ekmett at gmail.com
Fri Mar 18 02:00:16 CET 2011
On Thu, Mar 17, 2011 at 7:58 PM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:
> On 18 March 2011 06:25, Bas van Dijk <v.dijk.bas at 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
> > http://email@example.com/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.
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