While working on the paper „Evolution of Functional UI Paradigms“, published at the FUNARCH workshop of ICFP 2025, I had a thought about pure functions that I would like to briefly explain here. As an argument in favor of pure functions, functional programmers often cite better testability: pure functions require no complicated test setup or mocks, are deterministic, parallelizable, etc. In tendency, this is certainly correct. Pure functions are often better suited for testing than their impure counterparts. However, this connection is not a neccessary implication in either direction: There are pure functions that are hard to test, and there are impure functions that are easy to test. Thus purity itself cannot be the substance that ensures a piece of program is properly testable. But then what is this substance?

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On March 13, 2026, BOB, Active Group‘s conference about the best in software development, will take place — once again in Berlin and once again at the Scandic Berlin Potsdamer Platz.

The Call for Contributions is open. Send us your proposal for a talk or tutorial (by November 17, 2025) — the program committee is looking forward to it!

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This is an English translation of the German version of this post.

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Welcome to the New Year! We‘ll get started right with the first article of 2014 on a build system written in Haskell called shake.

Larger software projects (almost) all use a build system to automatically create a finished software product from source code. This includes, for example, compiling source files, linking object files, generating documentation, or assembling distribution archives.

This blog article provides an introduction to the shake build system, written in Haskell. This system has the advantage that dependencies between build artifacts can arise dynamically, i.e., while the build system is running. With make, probably the best-known build system, dependencies must be known before invoking the build system, which in practice often leads to limitations and problems.

We use shake at our company to compile our product Checkpad MED. Here shake plays to its full strengths, as an important component of the Checkpad infrastructure is code generation. Thanks to dynamic dependencies, it is possible to compile the program that generates the code, generate the code itself, and compile and link the generated code with a single invocation of the build system.

Neil Mitchell, the author of shake, developed a variant of the tool for use at Standard Chartered to efficiently compile really large software projects. Details on this as well as detailed information on shake‘s internal architecture can be found in this article.

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