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
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