[Haskell-cafe] package dependencies should distinguish between API and implementation?

[Albert Y. C. Lai]

On 13-02-25 06:50 AM, Johannes Waldmann wrote:
> or in general, A (and B.1) are baked into ghc,
> but there is some B.2/B.3 out there which U wants to use.
>
> Or is this what already happens? (ghc would notice
> that B.1.foo is different from B.2.foo.
> cabal-install would warn, but proceed?
> Then the effect of the proposal would just be
> to switch off these warnings in some cases?)

GHC uses both versions of B as much as possible. If none of the 
following 3 problems occur, they co-exist peacefully.

B defines a type T and/or a type class C. GHC considers B-1.T and B-2.T 
different and not interchangeable; similarly C. This is fine if one 
version is merely implementation-depended on. This becomes a visible 
type error if both versions are API-depended on and you try to mix them. 
You will see type errors like "cannot match B-1.T with T" or "T is not 
an instance of B-1.C". (Because "T" refers to B-2's.)

B-1 and B-2 both have "instance Show (IO a) where ...". Then you get 
some kind of overlapping instances error.

B-1 and B-2 both contain C code, and both contain C extern functions of 
the same name (which is best practice) "fps_count" (real example from 
bytestring). Therefore, GHC cannot load both. (Neither can common 
linkers.) GHC panics in this case.

Clearly, implementation-depend still suffers from 2 of these problems.

As for cabal, it never considers multiple versions. Which I like. Just 
chop up the Gordian Knot.