Map, filter and pure functions

[realhet]

On Friday, 29 November 2019 at 15:49:24 UTC, Paul Backus wrote:
> It's actually a much simpler reason: filter calls .front twice 
> for each element in its input (once to check if the value 
> satisfies the predicate, and then again to return the value if 
> it does), and the range returned by map doesn't save the value 
> of .front between calls, so it has to re-compute it each time 
> by calling the transform function.

This makes it clear.

In my case a cache which can access all the previous elements 
would be a too big thing, so I will do the filtering in a later 
stage manually. But most importantly, now I know what's going on, 
Thank You!