Multiple dependencies on the same package (was discussion on #haskell)

[Einar Karttunen]

Hello

There was a discussion about the semantics of the package
dependencies on #haskell. Continuing here for clarity.

build-depends: foo > 1 && < 3
vs
build-depends: foo > 1, foo < 3

and the same with configuration stuff:

packages(foo >1 && < 3)
vs
packages(foo > 1) && packages(foo < 3)


Current Cabal and the configurations patch interpret these as:
* "packages(foo >1 && < 3)"
*  there exists a package foo such that the version is >1 and <3

* "packages(foo > 1) && packages(foo < 3)"
* there exists a package foo such that the version is >1 *and*
* there exists a package foo such that the version is <3

This means that the semantics are very simple, each package
and depend can be handled in isolation. This has not caused
problems but should be documented more clearly.

The alternative solution is to make package names have
per-configuration and per-build-depend-line specific scope
and add an environment to the semantics. This would eliminate
a class of errors, but on the other hand make reasoning
about the semantics harder.

e.g. is this a tautology:

configuration: (package(foo > 10) || flag) || (package(foo < 10) || !flag)

Under the current semantics it is a tautology, since regardless
of how the foo match goes we have:
(x || a) || (y || !a) <=> (x || y || (a || !a)) <=> True

With the new semantics this would depend on the evaluation strategy.
Clearly there is no version of foo which satisfies the dependencies.

- Einar Karttunen