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Semantic Subtyping in Luau
Luau is the first programming language to put the power of semantic subtyping in the hands of millions of creators.
Off-the-shelf semantic subtyping is slightly different from what is implemented in Luau, because it requires models to be set-theoretic, which requires that inhabitants of function types “act like functions.” There are two reasons why we drop this requirement.
Set-theoretic semantic subtyping does not support this normalization, and instead normalizes functions to disjunctive normal form (unions of intersections of functions). We do not do this for ergonomic reasons: overloaded functions are idiomatic in Luau, but DNF is not, and we do not want to present users with such non-idiomatic types.
For these two reasons (which are largely about ergonomics rather than anything technical) we drop the set-theoretic requirement, and use pragmatic semantic subtyping.
Unexpectedly, this is not always true in set-theoretic models, due to uninhabited types. In set-theoretic models, if x has type never then f(x) has type never. We do not want to burden users with the idea that function application has a special corner case, especially since that corner case can only arise in dead code.
The other difference between Luau’s type system and off-the-shelf semantic subtyping is that Luau does not support all negated types.
The Better Parts. Douglas Crockford. JS Fest 2018
"the bugs that keep you up at night, the type system does not help with those".. and "actually helps to cause those" "you have to circumvent the type system" "you tend to give the type system the value of all of the bugs that it found, but we don't subtract the value of the bugs that it missed" "and we don't consider the value of bugs that it caused" "full accounting of types, they're just not work it"
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Template for CMU SCS PhD thesis (2006)
21 ordered
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Luau is the first programming language to put the power of semantic subtyping in the hands of millions of creators. Minimizing false positives One of the issues with type error reporting in tools like the Script Analysis widget in Roblox Studio is false positives. These are warnings that are artifacts of the analysis, and don’t correspond […]
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