Re: Advance warning to package authors of breakage

16 Nov 2009

      On Mon, 2009-11-16 at 12:28 +0100, Mikhail Glushenkov wrote:
...
Hi,
On Mon, Nov 16, 2009 at 1:47 AM, Duncan Coutts
 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