Kindly Bent to Free Us
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Systems programming often requires the manipulation of resources like file handles, network connections, or dynamically allocated memory. Programmers need to follow certain protocols to handle these resources correctly. Violating these protocols causes bugs ranging from type mismatches over data races to use-after-free errors and memory leaks. These bugs often lead to security vulnerabilities. While statically typed programming languages guarantee type soundness and memory safety by design, most of them do not address issues arising from improper resource handling. Linear and affine types guarantee single-threaded resource usage, but they are rarely deployed as they are too restrictive for real-world applications. We present Affe, an extension of ML with constrained types that manages linearity and affinity properties through kinds. In addition Affe supports the exclusive and unrestricted borrowing of affine resources, inspired by features of Rust. Moreover, Affe retains the defining features of the ML family: an impure, strict functional expression language with complete principal type inference and type abstraction through modules. Our language does not require any linearity annotations in expressions and supports common functional programming idioms.
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