[Haskell-cafe] A new cabal odissey: cabal-1.8 breaking its own neck by updating its dependencies

[Tillmann Rendel]

Hi Paolo,

Paolo Giarrusso wrote:
>> cabal install p1 p2 is supposed to find a single consistent install plan for
>> p1 and p2 and the transitive dependencies of either of them. This is useful
>> if you plan to use p1 and p2 in a single project.
>
> Ahah! Then it's a feature. The need for consistency stems from a bug:
> in a tracker entry you linked to,
> http://hackage.haskell.org/trac/hackage/ticket/704, duncan argues that
> "we also want to be able to do things like linking multiple versions
> of a Haskell package into a single application".

I think this is a slightly different matter.

Consider a package pair, which defines an abstract datatype of pairs in 
version 1:

   module Pair (Pair, fst, snd, pair) where
     data Pair a b = Pair a b

     fst (Pair a b) = a
     snd (Pair a b) = b
     pair a b = Pair a b

In version 2 of pair, the internal representation of the datatype is 
changed:

   module Pair (Pair, fst, snd, pair) where
     data Pair a b = Pair b a
     fst (Pair b a) = a
     snd (Pair b a) = b
     pair a b = Pair b a

Now we have a package foo which depends on pair-1:

   module Foo (foo) where
     import Pair

     foo = pair 42 '?'

And a package bar which depends on pair-2:

   module Bar (bar) where
     import Pair

     bar = fst

Now, we write a program which uses both foo and bar:

   module Program where
     import Foo
     import Bar
     main = print $ bar $ foo

Even with the technical ability to link all of foo, bar, pair-1 and 
pair-2 together, I don't see how this program could be reasonably 
compiled. Therefore, I think that the notion of consistent install plans 
is relevant semantically, not just to work around some deficiency in the 
linking system.

   Tillmann