TIL: `join` over `(->)`

I discovered a neat pattern: using join with the function arrow ((-)) as a Monad. When you join over functions, you get: This lets you pass the same argument twice: once to determine what to do, and another to actually do it. In , we have validators with the signature type Validator = input - Validation. Most of our validation helpers are written in a point-free, un-applied style, which works…