ExceptT package:mtl-prelude

A monad transformer that adds exceptions to other monads. ExceptT constructs a monad parameterized over two things:

• e - The exception type.
• m - The inner monad.

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Convert a ExceptT computation to MaybeT, discarding the value of any exception.

Map the unwrapped computation using the given function.

• runExceptT (mapExceptT f m) = f
  (runExceptT m)

Convert a MaybeT computation to ExceptT, with a default exception value.

The inverse of ExceptT.

Transform any exceptions thrown by the computation using the given function.