[Haskell-cafe] Re: Proposal: register a packageasprovidingseveralAPI versions

[Claus Reinke]

>> if that is your definition of compatible, you can never throw any
>> packages away
> 
> Is this a problem?

apparently, yes. no two versions of base with ghc, only the
newest set of core packages with each ghc release. and how
much time do you want to spend on finding, re-building, and
re-installing old packages everytime you move to a new machine?

it isn't (just) about space on a disk, it is about downloads 
and management, not to mention sanity of mind;-)

a simpler way of putting the responsibility issue:

- every package writer is responsible for not reducing
    the usability of his/her package at every update;
    as with a function type, that works both ways:
    use the simplest/newest dependencies available,
    and keep your package useable by existing clients

so we're not just talking about packages at certain
points in their lifetime, we're talking about the lifetime
of packages in the context of their usage contexts/
package databases.

perhaps we should treat package databases as 
distributed revision control system repos with 
interlinked dependencies? then, just as darcs did, 
we could focus on collections of patches that 
create consistent new repos.

instead of "this is package P-2.11, deal with it", 
it would be something like "add package P-2.11; 
replace uses of P-2.{0-10} with uses of P-2.11; 
remove any packages in that range; rebuild all
packages that used  P-2.{0-7} as internal types 
have changed; keep all packages P-1.* as this
is not a drop-in replacement for them".

claus