Lib A can link to an instantiation of Lib B version 1 and Lib Z can link to an instantiation of Lib B version 2
On Apr 7, 2011, at 3:01 PM, Robby Findler wrote: > How? > > Robby > > On Thu, Apr 7, 2011 at 12:47 PM, Matthias Felleisen > <matth...@ccs.neu.edu> wrote: >> >> Units would solve this problem. >> >> >> On Apr 7, 2011, at 7:32 AM, Robby Findler wrote: >> >>> On Wed, Apr 6, 2011 at 2:10 PM, Mark Engelberg <mark.engelb...@gmail.com> >>> wrote: >>>> The following two lines trigger an error: >>>> >>>> (require (planet dherman/memoize:3:1)) >>>> (require (planet soegaard/math:1:4/math)) >>>> >>>> The error says something along the lines that the soegaard package >>>> requires an older version of the memoize library, and the two can't >>>> coexist. >>>> >>>> Shouldn't modules protect me from this? Shouldn't it be possible for >>>> the math library to use its own required version of the memoize >>>> library, without it screwing up my ability to require the latest >>>> version of the memoize library? >>> >>> In the fully general case, this is not possible: you need to >>> instantiate two copies of the same library and if that library has >>> some external state (say a database connection) or even internal state >>> that somehow goes from one version of the library to the other version >>> via the other packages (imagine that there was a generative >>> define-struct in there somewhere and values somehow make their way >>> from one version's maker to another version's selector), then strange >>> things start happening and the library breaks, through no real fault >>> of its own. >>> >>> In the world of Racket modules, you never have two instantiations of a >>> particular module at once -- they always share the same state (unless >>> you monkey around at lower level using the introspection facilities) >>> so when planet is asked to load two different versions into a single >>> program, it decides to be conservative and signal an error, rather >>> than violate this. >>> >>> That said, there are two things to note: >>> >>> - Planet has a kind of escape hatch where a library can declare that >>> it is fine with being instantiated multiple times. See section 6.2.3 >>> of the planet docs. >>> >>> - Planet was probably too conservative on this front and probably in >>> Planet 2.0 the story will be different (and better). >>> >>> Robby >>> >>> _________________________________________________ >>> For list-related administrative tasks: >>> http://lists.racket-lang.org/listinfo/users >> >> _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
