The Secret Life of Types in Swift
I’m going to attempt to start by giving an overview of how types work in Swift, from the parser down to the lower layers of code generation in the frontend. Swift is a strong, statically-typed language with an advanced type system more reminisicent of functional languages such as OCaml and Haskell than something like C, so this seems like as good a place to start as any.
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Types in Swift form a mini-language in of themselves, with a grammar consisting of nominal types as leaves, and structural types such as function types as interior nodes. Types are formed from TypeLocs and TypeReprs early on in semantic analysis. Further down in the compiler, sugar is removed and types are canonicalized, simplifying structural walks and equality comparisons. Substitution is a fundamental operation frequently used in the implementation of generics, and it is important to think about the role of types and declarations when performing substitutions for member access. Various higher-order operations simplify tedious boilerplate when manipulating types throughout the compiler.
Now, let’s peel back a layer and dive into the type system of SIL, the Swift Intermediate Language. SIL adds a layer of detail missing from formal types, drawing a distinction between values and addresses, and making function types more explicit by introducing explicit annotations for argument and return value conventions.
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At this point, we still cannot compile our code, but at least we can detect a type mismatch at the level of SILFunctionTypes, instead of just mis-compiling incorrect code. A situation where the formal types of the expressions match, but the lowered types do not is called an “abstraction difference”. Abstraction differences are handled by SILGen wrapping the substituted function value inside a re-abstraction thunk.
The re-abstraction thunk forwards arguments, calls the function, and forwards the result, taking care to handle any abstraction differences in the arguments and results. If the substituted argument is trivial but the original argument is passed indirectly, the thunk will load the value from its address and pass it to the substituted function. Similarly, if the substituted result is trivial but the original result is returned indirectly, the thunk takes the substituted result value, and stores it into the indirect return address given to the thunk.
