[joshmarlow]

I've done a lot of work in Python and a fair amount of ocaml and rust.

I find that in Python - probably any dynamic language - a lot of the tests I write are just to make sure the pieces fit together. It's too easy to write well tested units that don't work together because of a misplaced argument or something at the interface. That type of testing is largely not needed with static typing.

Also, when I create a data structure in Python my mental model of what it means and how it relates to other parts of the program can be kind of fuzzy. Which means the pieces may not fit like I think they do because I missed a detail.

But a strong static type system forces me to clarify my thinking much earlier in the coding process than in a dynamic language. It encourages me to encode more of my mental model into the code and that allows the compiler to double check me.

It's kind of like spell check but for design.