Strict static types reduce a giant class of errors (including most of the footguns I'm always triggering in my dayjob with Python). Purity reduces another giant

Strict static types reduce a giant class of errors (including most of the footguns I'm always triggering in my dayjob with Python). Purity reduces another giant class: It makes it easy (indeed kind of forces you) to restrict your IO to a thin, top-level error, hence keeping the vast majority of the code purely functional and thus easy to test. Exactly how Haddock produces such wonderful automatic documentation, I don't know, but good God it does. The strict static types clearly help -- you can see exactly what every function inputs and outputs, what every data type needs, etc. And then you can jump to the definitions of any of those things, and if need be (almost never), the source code that defines them.