[dllthomas]

> Reader in Haskell helps prevent having to do that similar to environment variables or dynamic scoping

Reader in Haskell addresses some of the same concerns, but it differs in important ways. Reader is statically determined, Reader is clearly scoped to particular parts of your program (you can see whether a given function's behavior might depend on a value from a Reader in a way that's not visible with dynamic scope or environment variables), and Reader forces you to have set everything you might access. You could work around that last by making it a `Reader (Map String Dynamic)` or something, but that's not common in Haskell.

I make no claim, here, that these are obviously the right decisions - but they seem to provide enough differentiation that someone could rationally prefer one or the other.