[racket] Question about modules

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Thu Apr 7 15:01:00 EDT 2011



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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