Project overview

Our implementation of the Primer programming language is not like most other programming language implementations. We don't provide a typical command-line compiler, or readline-based REPL/interpreter, for example.

Programming language as a service

Instead, our implementation of the Primer programming language is intended for use as a sort of über language server, but not in the more familiar sense of the Microsoft Language Server Protocol (LSP). Unlike LSP servers, the Primer language server is the single source of truth for a given Primer program. The Primer language server provides the following services to its clients:

• It stores and retrieves programs from a program store, backed by a database engine. In our implementation of Primer, there's no concept of programs stored in flat text files in a hierarchical filesystem.

• It provides the program's abstract syntax tree (AST) to the client, along with a list of type-safe, atomic AST editing actions that can be performed at the currently-selected node, making it relatively easy to implement structure editors.

• It type checks the entire program, and can automatically fix up programs after editing actions are performed, guaranteeing that programs are always well-formed, modulo holes (which are similar in concept to GHC's typed holes, but implemented quite differently).

• It provides both a small-step reducible expression (redex) evaluator, for tracing program evaluation in learning and debugging contexts, and a "big-step" evaluator, for running programs to (bounded) termination.

In a nutshell, the Primer language server provides all of the functionality that one needs to read, write, run, debug, and store programs via a single, comprehensive API. Developers of Primer programming environments can focus on writing fluid, cohesive UIs and delivering a great user experience (UX), without getting bogged down in gluing together a number of disparate tools, each parsing and analyzing the same program over and over in inefficient, often brittle ways.

In its current implementation, the Primer language server runs as a native executable, typically in a containerized Linux environment, but in the not-too-distant future, we expect to generate a JavaScript and/or WebAssembly executable that can run most of these services directly in the browser, eliminating the most expensive and least scalable aspects of the backend service.

Project organization

This Primer project is a collection of Haskell packages, which together implement a Primer language service:

• primer implements the Primer language, including the type checker, the two evaluators, and an API that exposes these features to other Haskell programs.

• primer-api implements a Haskell API for building client applications around primer, such as editors.

• primer-miso is a web frontend for editing and running Primer programs.

(Additionally, the primer-benchmark package is used by the project maintainers to track performance regressions and improvements, but plays no part in the implementation of the Primer language service.)

The project also includes various tools for testing, linting, and formatting code; and for managing the project's database schema. These tools, and their related development workflows, are covered in detail in other sections of this guide.