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
