The "killer feature" of alists is that they are persistent data structures; when we extend a shorter alist to make a longer one, the shorter one is unaffected. Moreover, if the program has only a reference to the tail portion of a longer alist, the head portion can be garbage-collected.
With this, we can correctly implement lexical closures: we simply take the current environment (head of the alist) ssociate it with the lambda body and arguments, and we are done.
yes! also when you only have two or three things in them, they are faster than hashing, especially on machines without a data cache. clojure uses hamts for this, thus getting this fp-persistence† as well as efficient lookup in large dictionaries, and chris wellons posted a concise, fast c hamt implementation last year at https://nullprogram.com/blog/2023/09/30/
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† 'persistent' unfortunately also means 'stored in nonvolatile memory', so i feel the need to disambiguate
With this, we can correctly implement lexical closures: we simply take the current environment (head of the alist) ssociate it with the lambda body and arguments, and we are done.