Advance warning to package authors of breakage

[Duncan Coutts]

On Mon, 2009-11-16 at 12:28 +0100, Mikhail Glushenkov wrote:
> Hi,
> 
> On Mon, Nov 16, 2009 at 1:47 AM, Duncan Coutts
> <duncan.coutts at googlemail.com> wrote:
> >
> > These packages all use "build-depends: base >= 3 && < 5" (except for
> > ghc-core which cheekily uses "build-depends: base < 10" --- a practise
> > we will ban if it becomes any more popular).
> 
> I have one question. It looks like the recommended way to solve the
> problem is to use something like:
> 
> Flag separateSYB
>   Description: Data.Generics available in separate package.
> 
> [...]
>     if flag(separateSYB)
>        build-Depends: base >= 3 && < 4
>     else
>        build-Depends: base >= 4 && < 5, syb
> 
> When the user types 'cabal install pointful' (without providing
> -fseparateSYB), which version of base will be the executable built
> against?

It prefers the default value of the flag. If that's impossible then it
tries with the non-default flag value. When you do not specify a default
flag value, the default is True.

So as you've written it, in the absence of any constraints it'll pick
the separateSYB=True configuration, which as you've written it, will
*not* use the separate syb package. I think you intended to write it the
other way around.

Duncan