[racket] Question about modules

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Thu Apr 7 15:02:06 EDT 2011

Lib A can link to an instantiation of Lib B version 1
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
> <matthias at 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.engelberg at 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
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