A curmudgeon tries a language server

This article does that thing bloggers are told not to do: it discusses several different things at once:

• How Lisp programmers do programming differently.
• How Haskell code reduces the need for inspecting state.
• How to integrate a project-local LSP server with Emacs.
• How to achieve live reload with foreign-store and GHCid.

The reason I put all these things together is they are tightly related! Read on to see how.

Full article (10–25 minute read): A curmudgeon tries a language server

Flashcard of the week

Oh, this is a good one.

What does Parnas suggest as a requirement for program A to be allowed to use program B?

This question is dense in specific terminology:

• When Parnas says "program", he doesn't necessarily mean complete software packages with user-facing executable files. A "program", as far as Parnas cares, is a bundle of internally related code. It can be a library, a supporting package, a third-party client, or even a module that is part of the same project.

• When Parnas says program A "uses" program B, he means a strong dependency relationship. Program A only "uses" program B if the successful execution of program A depends on a correct version of program B being present.

  For example, if program A makes a fire-and-forget style call to program B, but does not wait for the response, then program A does not "use" program B in the Parnas sense, even though it clearly attempts to invoke it. We can see A does not use B in that situation, because even if we removed B from the system, A would be none the wiser.

The question in the flashcard is about designing for modularity: We have program A and program B, and we are trying to decide whether we should allow program A to use program B. There's a question we should ask ourselves to know whether to: "Could there be any useful subset of the system we're building that contains A but not B?"

If there is, we cannot allow A to use B, because someone else might come along and want to deploy a version of the system that only contains the subset that depends on A. If we have built A with strong dependence on B, they are forced to pull B along for the ride, which is bad modularity.

Said slightly differently: if there is a useful subset that only contains A and not B, we cannot make A use B, because we want to allow for deploying a subsetted system with only A, without having to change the code. That's what modularity is about: designing for extension and contraction.

Thus, the requirement for A to use B is

If the system is useless unless it contains both A and B, we can allow A to use B.

This gets a little abstract and it's not always so easy to judge, but I think it's brilliant.

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