[racket] Question about modules

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Thu Apr 7 13:47:42 EDT 2011

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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