Better shim libraries

[Ericson2314]

I'd love to ditch names like transformers-compat and instead allow libraries to declare private dependencies on new versions of themselves.

The harder part of this is private dependencies of course but #4104 also helps in that by making version bounds that exclude internal components and error today. That way tomorrow we are free to assign brand new meaning to them (i.e. actually depend on a newer version of me, please) tomorrow without breaking any existing packages.

[phadej]

Sorry, I don't understand this issue, can you give an example how you want things to be?

[Ericson2314]

Sorry for not responding earlier. Also I realized I misunderstood transformers-compat in that it works in the opposite fashion.

The goal is say I have a library foo, with releases versions 0.1.9.0 and 0.2.0.0. But I don't want to leave my 0.1.x.y users high and dry, and I only broke a few interfaces in 0.2.0.0. The purpose of this feature would be to allow me to write a 0.1.10.0 that depends on 0.2.*.* and mostly reexports compatible definitions, but wraps/replaces incompatible ones when necessary.

[Ericson2314]

@phadej Ah, I just discovered

cabal/cabal-install/Distribution/Solver/Types/PackagePath.hs

Line 49 in b32cfbc

-- This makes it possible to have base shims.

I think what I want is a generalization of this, to allow any package depending on a new generation of itself.

[phadej]

I.e. you want to have

name: mylib
version: 0.1.10

library:
  build-depends: mylib >= 0.2 && <0.3

IMHO it's not worth it. There are quite a lot of difficulties (with shadowing modules, types, you cannot shim classes/instances anyway). I'd just create a compat library (in either direction).

[phadej]

Also, transformers-compat works around a problem maybe not so obvious: in some setups transformers isn't upgradeable (i.e. if you depend on ghc lib).

And indeed, transformers-compat adds functionality to old transformers, which is introduced in the new one.

And e.g. Data.Functor.Classes is an example of a module you cannot shim, even if you want very very much. (huge difference between trransformers-0.4 and transformers-0.5 / base-4.9).