"temporary" templates

[FeepingCreature]

On Tuesday, 26 November 2019 at 18:59:26 UTC, Paul Backus wrote:
> On Tuesday, 26 November 2019 at 18:45:35 UTC, Adam D. Ruppe 
> wrote:
>> On Tuesday, 26 November 2019 at 18:35:42 UTC, Stefan Koch 
>> wrote:
>>> In _general_ it's not possible to categorize a template  as 
>>> being temporary or  not.
>>> For language semantic reasons it is a requirement  for every 
>>> re-instantiated template to forward  to  exactly the same 
>>> symbol as was generated during the first instantiation.
>>
>> What if it kept *just* the symbol, but discarded its innards 
>> when under memory pressure, then could regenerate them if 
>> evaluated again?
>
> This will cause compilation to become non-deterministic, since 
> the result of template evaluation can depend on the order in 
> which declarations are semantically analyzed:
>

...

Snapshot the lexical environment at the point of instantiation? 
Could make namespaces linked list of arrays so you can 
copy-on-write them for relatively cheap. Compilation is pure 
enough that this should be sufficient.

Imo templates are mostly split into two halves: large templates 
that are usually evaluated once with the same parameters, and 
small templates that are evaluated repeatedly with the same 
parameters. So a logic of "discard template contents beyond a 
certain cutoff size" should already do it.

And of course, mixin templates don't need to save anything 
because they're always reevaluated anyways.