On 5 November 2017 at 21:32, Michał Marczyk <[email protected]>
wrote:
> Now, if xs' head has not been realized, (with-meta xs {:foo 1}) can
> conceivably operate in one of two ways:
>
> 1. it can copy over xs' thunk (the nullary function embedded in the lazy
> seq object xs that is called – and expected to return a sequence or nil –
> when xs needs to be realized) into a new lazy seq object ys;
>
> 2. it can realize the head of xs and call .withMeta on the result.
>
> The first approach, however, breaks the promise that realizing the head of
> xs will have the effect of realizing the head of ys, as neither xs nor ys
> would subsequently have any way of knowing whether the other's head has
> been realized when they finally need to realize their own heads, and so the
> second one to be realized would be forced to recompute the sequence.
>
> Thus the implementation takes the second approach.
>
Why can't it just memoize the thunk when creating a new instance?
--
James Reeves
booleanknot.com
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