Fine-grained persistent structures are good at sharing and save/restore, but are full of allocations, indirections, and cache inefficiency.
Coarse-grained ones seem like they'd degrade to being the same as a flat structure, because there aren't *that* many local variables.
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So a flat hash table where you individually delete the new variables when exiting the scope seems like the least-bad option, but it leaves me with the nagging sense that something better must be Out There.
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What I *really* want to do is push local variables onto a stack and then pop off an entire scope at a time, but then lookups ???
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I like the idea from twitter.com/typeswitch/sta of associating each variable with an index/ID and then using arrays/vecs for everything (shades of ECS), and you can do scope-popping for those, but you still need to do the initial var->ID association somehow.
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[I'm also not sure that that's /necessarily/ superior to storing a var->{struct-of-all-associated-things} mapping directly; most likely that depends on the actual usage patterns in the compiler.]
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Let me know if you find a solution! I want it for Pikelet. Atm I do names->debruijn indices using the slow lookup, fast push/pop method in my elaborator. I do need persistent data structures for variable environments though :(
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For NBE?
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Hmm. IIRC `im` uses something like 64 elements per node. How much sharing do you actually get this way? (I.e., how many variables are there typically?)
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good question - haven't really stress-tested it
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was just like, 'ehhhh don't have to think about this right now' haha