lens-process

This package is still in Beta!

This package is intended to be on the lighter side, with few dependencies aside from lens. For full disclosure, this is the minimal dependency graph of lens-process:

Motivation

lens-process provides optics for the process package. These optics provide convenient lenses, traversals, and prisms, as well as classy variants for significant classifiable portions of the library for convenience. In addition, we provide some combinators for working with CreateProcess types. The intention of this package is to create a well-typed optical layer for process, reflecting the shape of certain types of commands at the type level. For instance, consider the following:

myStdInProcess
  :: forall a
  . CommandProcess
  -> (Handle -> IO StdStream)
  -> IO (Either Text a)
myStdInProcess cp f g = do
  (mhin, _, _, _) <- createProcess cp
  case mhin of
    Nothing -> error "oh no!"
    Just t -> f t

This is very standard process code. However, if anyone else encounters this code, it is immediately apparent that information is lacking from the type signature. What Handle are we using? What type of command are we running? It is not reflected. Consider instead this replacement:

myStdInProcess
  :: forall a b c
  . (HasStdIn a, IsUseHandle b)
  => a -> (b -> IO c) -> IO c
myStdInProcess cp f = do
  handler <- createProcess cp
  case handler ^? _Stdin . _Just . re _UsesHandle of
    Nothing -> error "oh no!"
    Just t -> f t

What have we gained here? Well, for one, I know I'm working with something that only has a _Stdin, and that i will be piping whatever its _Stdin entry is into a UseHandle. Then, my function handles it accordingly. Much information has been gained! Classy optics gives us the necessary information to proceed as planned with more confidence.