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> Well, you can say the same about git right? The difference is that in git the repo structure is metadata. The data (the contents of particular commits) is c

by lisper 4y ago

> Well, you can say the same about git right? The difference is that in git the repo structure is metadata. The data (the contents of particular commits) is completely independent of the repo structure and so you can change the latter without changing the former. Furthermore, when you try to build a program, you only ever deal with a single commit. You never build a program from multiple commits at once. Program linking has nothing to do with the repo structure. But in unison, the "repo structure" is no longer metadata, it's part of the program. It is now part and parcel of your linker. So when you change the "repo structure" you are changing the behavior of the program. Git works precisely because it separates data (program) from metadata (repo structure). When you glom them together the way unisom does, you get some gnarly problems to which I don't see any immediately evident solutions.

eternalban 4y ago
There is a solution to this problem of content addressable structures. I was (paper, head) designing the same general approach 10 years ago, borrowing a sort of ~homoiconicity from Lisp (code and data are just tuples of structured hashes) as well. The problem, explicitly and concisely is this: Referential elements of structures introduce cascading re-hashing. There is however a solution to this by adding a layer of (invariant) semantic indirection for referential (as opposed to inlined embedded ‘value’ tuples) types. With this approach, at this point, we obtain 2 benefits: ‘object’ re-evaluation (rehash) is only necessary when object structure changes (not the referents); and in an object graph, cascades will only impact the immediate referents’ ‘value’ mappings. caveat that I felt extremely queasy seeing this post on hn — a session of self criticism ensued regarding the topic of “laziness” :D — so haven’t read beyond the upsetting intro to Unison.

by lisper 4y ago

eternalban 4y ago

There is a solution to this problem of content addressable structures. I was (paper, head) designing the same general approach 10 years ago, borrowing a sort of ~homoiconicity from Lisp (code and data are just tuples of structured hashes) as well. The problem, explicitly and concisely is this: Referential elements of structures introduce cascading re-hashing. There is however a solution to this by adding a layer of (invariant) semantic indirection for referential (as opposed to inlined embedded ‘value’ tuples) types. With this approach, at this point, we obtain 2 benefits: ‘object’ re-evaluation (rehash) is only necessary when object structure changes (not the referents); and in an object graph, cascades will only impact the immediate referents’ ‘value’ mappings. caveat that I felt extremely queasy seeing this post on hn — a session of self criticism ensued regarding the topic of “laziness” :D — so haven’t read beyond the upsetting intro to Unison.