Cabal is not a solution to this problem because
1. you want your code to work via ghci and runhaskell and perhaps via
searchpath.
2. you may want to move a module from one package to another and you
don't want to have to examine the cabal file to figure out how to do
that. The source file should have enough information.
Extensions that change the interpretation of an already valid haskell
source file really need to be declared explicitly e.g.
undecidableinstances.
Extensions that change syntax are effectively declared by the use of
that syntax. If you can parse the source, then you know which
extensions it uses.
-Alex-
Duncan Coutts wrote:
On Fri, 2007-11-23 at 16:26 +0100, Wolfgang Jeltsch wrote:
Am Freitag, 23. November 2007 03:37 schrieben Sie:
On Fri, 2007-11-23 at 01:50 +0100, Wolfgang Jeltsch wrote:
Dont’t just think in terms of single modules. If I have a Cabal package,
I can declare used extensions in the Cabal file. A user can decide not
to start building at all if he/she sees that the package uses an
extension unsupported by the compiler.
Indeed. In theory Cabal checks all the extensions declared to be used by
the package are supported by the selected compiler. In practise I'm not
sure how well it does this or what kind of error message we get.
The problem is, of course, that you are not forced to specify all used
extensions in the Cabal file since you can still use language pragmas.
Sometimes it is even desirable to use LANGUAGE pragmas instead of information
in the Cabal file. For example, even if some modules use undecidable
instances, I might not want all modules of the package to be compiled
with -XUndecidableInstances since this could hide problems with my class
structure.
Our tentative plan there is to separate the extensions field into those
used in some module, and those applied by cabal to every module. So that
would allow you to specify a feature in one file but not all, while
still declaring to the outside world that the package uses the feature.
As for enforcing that, that may come almost for free when we get
dependency chasing as we'll be looking for imports anyway. It shouldn't
be much harder to look for language pragmas too.
Duncan
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