More generally, Van Roy sees a layered language design with four core layers, a structure which has been independently discovered across multiple projects:

The common language has a layered structure with four layers: a strict functional core, followed by declarative concurrency, then asynchronous message passing, and finally global named state. This layered structure naturally supports four paradigms.

Van Roy draws four conclusions from his analysis here:

1. Declarative programming is at the very core of programming languages.
2. Declarative programming will stay at the core for the foreseeable future, because distributed, secure, and fault-tolerant programming are essential topics that need support from the programming language
3. Deterministic concurrency is an important form of concurrency that should not be ignored. It is an excellent way to exploit the parallelism of multi-core processors.
4. Message-passing concurrency is the correct default for general-purpose concurrency instead of shared-state concurrency.

Adrian Colyer

Programming Paradigms for Dummies: What Every Programmer Should Know

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Declarative interfaces are also composable