Proposal: Add strict iterate'

[Bas van Dijk]

Hi David,

As suggested[1] by Takano Akio a few years ago iterate can be made strict
by applying headStrict to its result:

headStrict :: [a] -> [a]
headStrict = foldr (\x y -> seq x (x : y)) []

We might want to add this to Data.List unless there are no use-cases for a
lazy accumulator in iterate in which case I'm +1 for a strict
implementation.

Bas

[1] http://www.haskell.org/pipermail/libraries/2012-November/018775.html
On 30 Jul 2014 02:48, "David Feuer" <david.feuer at gmail.com> wrote:

> I think most if not all applications of iterate would prefer to accumulate
> strictly—iterating a constant function is not very useful and the spine of
> the result of iterate is always the same. Unlike unfoldr, which has enough
> strictness built in to allow its caller to decide whether to accumulate
> strictly, iterate is entirely lazy. I therefore propose:
>
> iterate' :: (a -> a) -> a -> [a]
> iterate' f b = unfoldr go b
>   where
>     go x = x `seq` Just (x, f x)
>
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