runReader

runReader is a function commonly found in functional programming libraries, particularly those inspired by Haskell and its type classes. It is primarily used with the Reader monad or ReaderT transformer. The purpose of runReader is to execute a computation that depends on an environment and to extract the final result.

The Reader monad is a way to represent computations that need access to a shared environment. This environment is passed implicitly to all parts of the computation without needing to be explicitly passed as an argument at each step. The Reader monad allows for "reading" from this environment.

The runReader function takes two arguments: the Reader computation itself and the environment that the computation will use. When runReader is applied, it effectively unwraps the Reader computation, provides it with the specified environment, and returns the computed value. This is the point where the dependency on the environment is resolved.

For example, if a Reader computation needs to access a configuration object, runReader would be used with that configuration object to get the final result of the computation. It's the mechanism that allows a computation designed to be flexible with its environment to be instantiated with a concrete environment and produce a definite output. In essence, runReader "runs" the reader-based computation to completion by supplying the necessary context.