Enable changing application state type in sub programs

[istathar]

Introduce changeProgram to go from one top level type, Program τ, to another, Program υ.

This enables you to switch from a simple type when starting your application (Program None, say) to the actual user data type you intend to run at (Program Settings or whatever) once initialization is complete, thereby avoiding the hassle of needing an mempty value for the settings type just so you can get past configure.

Credit to @guaraqe who had the idea that this would be useful.

[guaraqe]

I was going to push something almost identical, with type setupState :: Program α τ -> Program τ β -> Program α β, to be used as execute $ setupState setup action, the code was almost exactly the same but you bet it to me. Anyways, I approve!

[istathar]

Yours would be a little more "functional", certainly.

What I ran into today was the idea is having a different Settings type for each of the command modes. The binary I'm working on is already partially set up that way, which is what gave me the idea.

So for my use case, "figure out which type, then enter (with 3 different ones)" seems like a good pattern.

Will that be ok for you?

(Feel free to add a function with your design, too. We can have both!)

[guaraqe]

Of course, this one is just as good

[dmvianna]

🙏🏼 Thank you for this! I will need to switch between libraries with different states, this saves me!

[istathar]

@dmvianna no problem :) This is released now. Example at https://hackage.haskell.org/package/core-program/docs/Core-Program-Execute.html#v:changeProgram.