sixten VS felix

Immutable data structures don't necessarily require more memory: they can avoid deep copies. They are also automatically thread safe without expensive (slow) locking mechanisms. They also don't necessarily reduce cache locality. The reduced cache locality in the case of Haskell (I think) mainly comes from the representation of objects in its implementation (improved STG) which uses extensive boxing and jumps that hinder both spatial and temporal locality (require review/comment from GHC/Computer Architecture experts, take it with a grain of salt). Objects can be much more efficiently represented if not for the need to implement lazy (call-by-need) semantics. See sixten and futhark for examples.

[Sixten](https://github.com/ollef/sixten): functional programming with unboxed data by default.

I can't answer this well and don't know of any resources, but I have seen it before in the parser for sixten:

    https://github.com/ollef/sixten/blob/60d46eee20abd62599badea85774a9365c81af45/src/Frontend/Parse.hs#L458

Sixten is a language that allows precise control over memory layout of algebraic data types.

For something more peripherally related, check out Sixten. Its focus is on using unboxed value representations, which is in spirit close to what you are proposing, and some of its ideas might be good inspiration.

[felix] https://github.com/felix-lang/felix Statically typed scripting language with coroutines, functional and procedural programming, polymorphism, explicit kinding, compact linear types, row polymorphism, type classes, C/C++ or better performance.