GoKawiilRambles around computer science
Diverting trains of thought, wasting precious time
Mon, 10 Nov 2025
Reversing abstractions: an existential crisis
Computer science in general, and language implementation in particular, are founded on the idea of realising abstractions in a way I'll call “forwards” and also “existential”. This post is about how these aren't the only way to think about programming infrastructure and abstractions.
The theory goes that you pick some desired abstraction—it could be a programming language, maybe an abstract data type, or something else. Then you figure out the realisation. In the case of a programming language, this could be memory layouts, calling conventions and a compiler that generates object code modelling these. For abstract data types, it's some specific concrete data structure and the mapping from abstract to concrete operations.
That's the “forwards” part. It's also “existential” when you keep the details of the realisation to yourself. Client code (e.g. source code in our new language) knows there is a realisation but the implementation choices themselves are hidden. In turn this allows the implementer to change their minds, while having indemnified themselves against any fallout when this breaks things. (Cynical, me?)
I find myself drawing an ASCII-art diagram like this.
existential . abstraction v / | \ v //|||\\ v . . . . . . . possible realisations -- pick one! hide it!
So far, so textbook. However, this hiding isn't always done. Examples of “opting out” of hiding are ABIs and debuggers.
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