Package management

Brandon Moore brandon_m_moore at
Thu Apr 28 08:01:25 CEST 2011

> From: Albert Y. C. Lai <trebla at>

> To: glasgow-haskell-users at
> Sent: Wed, April 27, 2011 9:53:38 PM
> Subject: Re: Package management
> On 11-04-26 05:05 PM, Brandon Moore wrote:
> > There are already hashes to  keep incompatible builds of a package separate.
> > Would anything break  if
> > existing packages were left alone when a new version was installed?  
> > preferring the most
> > recent if a package flag specifies  version but not hash).
> Whether we allow multiple builds of same package  same version, or allow 
>multiple versions of same package, we face the same  dilemmas:
> First dilemma: If the package comes with C code, you get a name  clash at the C 
>level. Because C function names in such packages are unlikely to  vary by 
>version. Well, the linker sees two extern C functions both called  
>get_current_timezone_seconds (real example from the time package),  great.
> Second dilemma: If the package comes with instance code like  "instance Read 
>(IO a)", you get overlapping instances.
> Third dilemma:  Data types and type classes defined by the package cannot be 
>safely exchanged  between two users because the two users depend on two 
>different builds. Perhaps  it is safe in most cases, but you can't be too  sure.

All of these problems only happen if you try to use multiple versions of the 
package in the same program.
Compilers and dependency solvers should continue to reject any attempt to use 
multiple versions of the same package.

I am only worried about the more basic situation, where each program needs just 
one version of
the library, but different programs might need it to be compiled against 
different versions of the dependencies.

What can happen now is that you build B-1.0 and build A-1.3 against B-1.0 for 
one program that
needs B-1.0, and then rebuild A-1.3 against B-2.0 to compile another program 
that needs B-2.0,
but this unregisters the first version of A-1.3, which has to be recompiled if 
you want to build P1 again.

Is there any reason not to leave both registered?



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