Multiple dependencies on the same package (was discussion on
duncan.coutts at worc.ox.ac.uk
Thu Aug 10 12:52:18 EDT 2006
On Thu, 2006-08-10 at 19:31 +0300, Einar Karttunen wrote:
> There was a discussion about the semantics of the package
> dependencies on #haskell. Continuing here for clarity.
> build-depends: foo > 1 && < 3
> build-depends: foo > 1, foo < 3
> and the same with configuration stuff:
> packages(foo >1 && < 3)
> 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.
Isn't the semantics quite simple:
for each package mentioned in build depends, a candidate package must
satisfy all constraints.
for the current implemented semantics it's:
for each dependency there must exist a candidate package that satisfies
So "build-depends: foo > 1, foo < 3" means linking with foo twice (and
possibly different versions)! This seems like nonsense. If that's what
it means then it should be an error condition with a helpful message to
use "build-depends: foo > 1 && < 3" instead.
Similarly if we go with "packages(foo >1 && < 3)" meaning something
different from "packages(foo > 1) && packages(foo < 3)" then is the
latter not simply an error?
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