The Hardest Program I've Ever Written – How a code formatter works (2015)

Philip Wadler has a seminal paper [0] on implementing pretty printers by modeling them using an algebra. There's implementations of the algorithm in most programming languages nowadays, and some state of the art advances that help with performance [1] It's a very elegant algorithm, and is very pleasant to work. Elixir's formatter uses a variant of it [2], and it appears in the standard library [3]. I've personally made use of it to write a code formatter for GraphQL queries…

I read this before working on prettier and it was a bit scary to be honest :) I wasn’t convinced that the exhaustive approach with weigths was a good idea. It makes it computationally super intensive and hard to predict what the printer would do. Instead prettier runs on a simple idea: if something doesn’t fit in one line, break the outermost parent. This simple rule (implemented via the Wadler IR) is efficient to implement and makes the output generally…

The Dart formatter sounds really advanced, and reflects potential complexity of the language. I think I'd still prefer to see a formatter attempt to preserve any formatting which is already 'good enough' to pass as an output threshold. Code isn't just a recipe for a computer to do something, it's a language for explaining to other programmers what that thing is and what's important to the structure of accomplishing it. The choice of where to place a break can matter…

Great blog post and super interesting. My feeling though is the problem is they have a line limit. Maybe they should rethink their style. I'm serious. Before I worked at Google, in 30 years of programming I never worked at a company that had a line limit. Adding a line limit at Google did not make me more productive. At first I thought "hey, I guess 80 chars makes side by side comparison easier" but then I thought back, hmm…