Documentation

The Reader monad

The request for a value of type e from the current environment This can be expressed as a GADT because the type of values returned in response to a (Reader e a) request is not any a; we expect in reply the value of type e, the value from the environment. So, the return type is restricted: 'a ~ e'

One can also define this as

data Reader e v = (e ~ v) => Reader

^ without GADTs, using explicit coercion as is done here.

newtype Reader e v = Reader (e->v)

^ In the latter case, when we make the request, we make it as Reader id. So, strictly speaking, GADTs are not really necessary.

Get the current value from a Reader. The signature is inferred (when using NoMonomorphismRestriction).

Locally rebind the value in the dynamic environment This function is like a relay; it is both an admin for Reader requests, and a requestor of them

Request the environment value using a transformation function.

The handler of Reader requests. The return type shows that all Reader requests are fully handled.