borrow checking #15282
borrow checking #15282
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I can't comment on the implementation but we'll likely want to have an easy to approach API and story.
openArrayis long to type and very obscure apart from Oberon, Modula and Pascal devs, I would love to see it replaced either by view or span (given that slice is taken). C# and C++ also use span and call views the overarching type category that doesn't own memory just like current http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0122r1.pdf . (Rust and Swift uses Slice)toOpenArrayshould be using[]except when the compiler detects that the assignment escapes (which it now can do with borrow checking, write-tracking https://nim-lang.org/blog/2020/09/01/write-tracking.html. This has been a source of slow allocation and GC pressures in Nimbus (avoid unnecessary seq allocations status-im/nimbus-eth2#1573) and is an easy way to get in trouble with multithreading especially OpenMP when we forget to setupForeignThreadGc- We probably need a StringView type
I should probably write a RFC on that.
| echo a.len | ||
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| proc main(s: seq[int]) = | ||
| var x: openArray[int] = s # 'x' is a view into 's' |
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I think a var x = s.view() or var x = s.borrow() would be nicer but I guess we can discuss API once the low-level primitive work.
| echo x[i] | ||
| take(x) | ||
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| take(x.toOpenArray(0, 1)) # slicing remains possible |
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Similarly [] on sequences should now return openArray by default unless this is assigned to a returned value that escapes and maybe we need a StringView type?
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Well I won't rename or introduce new APIs, I'm following nim-lang/RFCs#178 If you want new names or APIs please write an RFC and see how the community reacts. ;-) |
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this is so cool and pretty much the only thing I felt was missing from Nim! While this is going to be experimental at first, I think it deprecates multiple things which were more or less workarounds using pointers:
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Agreed. Also the special casing for |
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Merging this as the dangers of merge conflicts become serious. But it's hardly usable in this state and will see follow-up PRs. |
* refactoring: move procs to typeallowed.nim * frontend preparations for first class openArray support * prepare the code generator for first class openArray * code generation for first class openArray; WIP * code generation for open arrays, progress * added isViewType proc * preparations for borrow checking * added borrow checking to the front end
Todo: