issues with configurations

Duncan Coutts duncan.coutts at
Fri Aug 11 11:41:02 EDT 2006

On Fri, 2006-08-11 at 15:40 +0100, Simon Marlow wrote:
> Duncan Coutts wrote:
> > On Fri, 2006-08-11 at 12:15 +0100, Simon Marlow wrote:
> >>One goal (that I keep having to remind myself of) is that a .cabal file should 
> >>be modifyable by an IDE such as Visual Haskell.  I can't see Visual Haskell 
> >>being able to successfully modify that complicated build-depends expression.
> > 
> > 
> > Is that really any different from the conditionals in the configuration
> > tests?
> Visual Haskell would ignore complicated configuration sections.  It can handle 
> configuration sections that just test a single flag, because this fits in with 
> the Visual Studio notion of "configurations".  It also needs to be able to grok 
> and modify the build-depends field.  The idea would be that if you want to do 
> anything more complicated, then you add a configuration section to the .cabal 
> file by hand.

That's fair enough. Not having '?' deps is good for that I agree. I only
introduced that as a way of making configuration sections independent,
but there are probably nicer ways of doing that.

> >>The whole thing is easier to understand IMO, and it's simple to implement too: 
> >>no ordering, conditionals can be evalutated independently.
> >  
> > Well they're only independent if you take the view that package()
> > conditionals test what's available in the environment rather than what
> > we're going to actually use.
> Yes, that's the intended semantics (sorry for not saying that explicitly).  It's 
> what makes simpler, but possibly too general.

Right. That's the semantics that I objected to and tried to explain why
in another recent email.

> >>I'm back to the two-argument package() conditional, because it's actually 
> >>useful.  package(Hunit>=1, 1.1) is not the same as package(HUnit-1.1) or even 
> >>package(HUnit>=1 && 1.1).  It says "the dependency HUnit>=1 resolves to version 
> >>1.1", which is exactly what you wanted to know.  This is how you write 
> >>conditionals that test the versions of packages you actually depend on, which is 
> >>the criticism raised by Duncan at the beginning of this thread.  I believe my 
> >>original syntax addressed this concern.
> > 
> > Ok if we have package(Hunit>=1, 1.1) then do we not then have an
> > ordering issue?
> No, it's evaluated based on the environment only.  That's why the dependency is 
> repeated from the build-depends field.

I think I get that. 

"the dependency HUnit>=1 resolves to version >1.1".

s/resolves to/would resolve to

That means that supposing I were to build-depend on HUnit>=1 then I
would get a version satisfying >1.1. So if that evaluates to true it's
not to say that I do depend on HUnit>=1 or that we are going to use
>1.1. Indeed we may actually end up depending on version 0.9 and this
test will still be true.

> > I think it's quite easy to make package tests be only internal, that is
> > what deps have we resolved for the package, and not mix that up with
> > what happens to be available in the environment. Then as a separate
> > thing have some facility to make the default decisions about which
> > configurations to use to depend on the environment, but in a clearly
> > separated way so that users or packagers can override the defaults.
> Ok - so is it a goal that you want to allow auto-optional dependencies but allow 
> the packaging environment to turn them off?  If so, then I agree you need 
> something like the scheme you suggest, I just want to be clear about why we need 
> that (especially if auto-optional deps are considered evil).

I don't mind too much if we have auto-optional dependencies but if we do
then we must have a way of controlling them manually. It's probably nice
to have. I expect most devs and users would like it. Most of the time
for end users it does the right thing.

> > So let me suggest something with flags but without '?' syntax:
> > 
> > flag: debug
> > default: False
> > 
> > configuration: flag(debug)
> > build-depends: HUnit-1.1
> > 
> > configuration: using(HUnit==1.1)
> > ghc-options: -DHUNIT11
> > 
> > flag: gui
> > default: os(windows)
> >       || (available(gtk>=0.9.10) && available(cairo>=0.9.10))
> > 
> > configuration: flag(gui) && os(windows)
> > build-depends: Win32>=1.1
> > ghc-options: -DUSE_WIN32_GUI
> > 
> > configuration: flag(gui) && !os(windows)
> > build-depends: gtk>=0.9.10, cairo>=0.9.10, glib>=0.9.10
> > ghc-options: -DUSE_GTK_GUI
> with this you can still write silly tings - an available() condition for a 
> package you don't care about, for example.

Yes, that's true. I don't think it has any particularly bad consequences
though and developers are not likely to do it (unlike auto-optional deps
which devs will use if they're available).

> To summarise there are a couple of issues.  I'll use the abbreviations AOD for 
> auto-optional dependencies and SC for silly conditionals (predicates on packages 
> that aren't a dependency).
> Ordering requirement on configurations:
>    Option O1.  No ordering, requires 2-argument package() predicate.
>                Allows AOD and SC, but we could (possibly) detect these.
>    Option O2.  Ordering matters, we have using(P) predicates,
>                doesn't allow AOD or SC.
> Default settings for flags:
>    Option D1.  All flags default to off.
>    Option D2.  Defaults can be set based on the availability
>                of packages.  Allows AOD and SC, but both can be
>                disabled with a command-line switch.
> If we can do without AOD, then we just choose between O1 and O2 - not a 
> straightforward choice, but I went the O1 route because it's easier to implement 
> and more declarative, on the other hand adding detection of AODs would mean more 
> complexity.

Yes, I think that summarises it quite well.

> If we want AOD, then probably O2/D2 (the dcoutts proposal) is the sensible 
> choice. On the other hand, it means adding yet more stuff to Cabal...

Aye, ho hum. I've not spotted any nice simple schemes that cope with the
proposed use cases.


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