Flix is a functional, imperative, and logic programming language developed at Aarhus University, with funding from the Independent Research Fund Denmark, and by a community of open source contributors. The Flix language supports algebraic data types, pattern matching, parametric polymorphism, currying, higher-order functions, extensible records, channel and process-based concurrency, and tail call elimination. Two notable features of Flix are its type and effect system and its support for first-class Datalog constraints. The Flix type and effect system supports Hindley-Milner-style type inference. The system separates pure and impure code: if an expression is typed as pure then it cannot produce an effect at run-time. Higher-order functions can enforce that they are given pure (or impure) function arguments. The type and effect system supports effect polymorphism which means that the effect of a higher-order function may depend on the effect(s) of its argument(s). Flix supports Datalog programs as first-class values. A Datalog program value, i.e. a collection of Datalog facts and rules, can be passed to and returned from functions, stored in data structures, and composed with other Datalog program values. The minimal model of a Datalog program value can be computed and is itself a Datalog program value. In this way, Flix can be viewed as a meta programming language for Datalog. Flix supports stratified negation and the Flix compiler ensures stratification at compile-time. Flix also supports an enriched form of Datalog constraints where predicates are given lattice semantics. Overview Flix is a programming language in the ML-family of languages. Its type and effect system is based on Hindley-Milner with several extensions, including row polymorphism and Boolean unification. The syntax of Flix is inspired by Scala and uses short keywords and curly braces. Flix supports uniform function call syntax which allows a function call f(x, y, z) to be written as x.f(y, z). The concurrency model of Flix is inspired by Go and based on channels and processes. A process is a light-weight thread that does not share (mutable) memory with another process. Processes communicate over channels which are bounded or unbounded queues of immutable messages. While many programming languages support a mixture of functional and imperative programming, the Flix type and effect system tracks the purity of every expression making it possible to write parts of a Flix program in a purely functional style with purity enforced by the effect system. Flix programs compile to JVM bytecode and are executable