Re: [MirageOS-devel] How to implement protocols?

[Thomas Gazagnaire <thomas@xxxxxxxxxxxxxx>]

>> The whole point is to have multiple instances of the functor in the same 
>> program. We want the ability to link multiple TCP/IP stacks (instantiated 
>> with different arguments) to create virtual networks for benchmarks or tests.
> 
> I would just like to remind that functors are not the only way of structuring 
> programs. Function application is a very versatile glue aswell.

So the alternative you propose is to thread function parameters everywhere. I'm 
not sure this is much better.

To rephrase the nice slide decks from Pierce and Harper:

Also, on slide 25:
"path hacks! Much nicer to express linking in a real programming language. 
"Agreed. But functors are not the only possible ânice linking language.â Why 
not invent a real language with primitives for manipulating linking contexts in 
the style of search paths?" 

Instead to invent a language to manipulate linking context, we use an eDSL and 
that's Functoria. Functoria manipulates functors, but also opam package and 
ocamlfind libraries, so you can implement path hacks in there if you want.

slide 26:
"Functors can arise when a library module needs to refer to a client module 
(because the library implementor doesnât know what name will be chosen for the 
client module)."
"Somewhat, but perhaps one could also address this sort of application with 
âpath hacksâ or a more sophisticated linking language"

The problem we have in Mirage is not a problem of client name, it is a problem 
of client implementation. We already have defined all the possible client 
signatures that an application can use, that's mirage-types. Should we provide 
a base implementation for every signature defined in mirage-types and ask the 
users to build against them, then use path hacks to switch from the base 
implementation to the real implementation at link time? (as a side note, what 
happens with cmx and cross-module inlining in that case?) That might be 
possible, if only we had  good build system :p (the myocamlbuild.ml doing that 
kind of thing is here[1])

slide 27:
"Functors are needed when we want to implement the same signature multiple 
times and use more than one of the implementations in the same run of a 
program." 
actually this appears very often in Mirage, especially when you want to test 
the various implementations of you signature in the same test program or when 
you want to superpose various layers of implementations (virtual dispatcher, 
etc...). 

I agree we are over functorizing in some cases (Console and IO are good 
candidates) but for the network and storage stack, we have concrete exemple 
where most if the functors are actually needed. I'm happy to experiment with 
simpler system if it actually exists. Experimenting with path hacks in 
functoria (or with the next build system) could be a first step in that 
direction (even if I'm a bit worried to loose some cool possibilities).

Best,
Thomas
[1]: https://github.com/mirage/mirage/blob/one-build/scripts/myocamlbuild.ml


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