  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 handling of resources. An important step towards handling resources is
the adoption of linear and affine types that enforce single-threaded resource
usage. However, the few languages supporting such types require heavy type
annotations.
  We present Affe, an extension of ML that manages linearity and affinity
properties using kinds and constrained types. In addition Affe supports the
exclusive and shared borrowing of affine resources, inspired by features of
Rust. Moreover, Affe retains the defining features of the ML family: it is an
impure, strict, functional expression language with complete principal type
inference and type abstraction. Affe does not require any linearity annotations
in expressions and supports common functional programming idioms.
