Re: protocols and namespaces confusion (with an example from core.matrix)

[Gregg Reynolds]

On Wed, Oct 19, 2016 at 5:00 PM, Paul Gowder  wrote:

> Thanks Gregg and Alex!  I didn't realize that protocols (or the black
> magic interfacing of core.matrix) were that fancy.  Definitely going into
> my "code to really dig into" list.
>
>
It's remotely possible that you might find the following helpful:
lab.clj.polymorphism .
It was in part inspired by the technique used in core.matrix.  See the
section "Implementation techniques" in the readme.  At the time, it made
sense to me. ;}

-gregg

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Re: protocols and namespaces confusion (with an example from core.matrix)

[Paul Gowder]

Thanks Gregg and Alex!  I didn't realize that protocols (or the black magic 
interfacing of core.matrix) were that fancy.  Definitely going into my "code to 
really dig into" list.

Cheers,

-Paul

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Re: protocols and namespaces confusion (with an example from core.matrix)

[Alex Miller]

On Wednesday, October 19, 2016 at 7:23:21 AM UTC-5, Paul Gowder wrote:
>
> Hi folks, 
>
> I have a kinda basic question (I think?) that I should really know the 
> answer to after writing Clojure for like 8 months, but don't.  It's about 
> requiring namespaces with protocol definitions in them but not the actual 
> implementations of those protocols, and how it works.  
>
> tl/dr: if a protocol is defined in namespace a and implemented (extended 
> to the type at issue) in namespace b, can you call functions from it in 
> namespace c where namespace c only requires namespace a, not namespace b?
>

Yes. You are invoking the protocol function in a and as a caller, that's 
all you need to know about. HOWEVER, the implementation in b must be loaded 
before you can do so. Your namespace doesn't necessarily have to do it, but 
someone does. Each protocol is essentially a little piece of hidden state 
in the runtime. Each time you extend the protocol to an implementation, 
that state is modified. If you don't load the code that does the extension, 
the state will not be updated.

All of this also applies to multimethods.
 

> Backstory: I was poking around core.matrix recently in pursuit of 
> restarting work on a months ago issue I promised to work on, and it looks 
> like at least one namespace calls a function it shouldn't be able to call.
>
> So: clojure.core.matrix.impl.pprint requires only one namespace within 
> core.matrix, clojure.core.matrix.protocols. (I'm going to use ... hereafter 
> to stand for clojure.core.matrix.) ...protocols defines a protocol 
> PSliceSeq2, which gives the name, docstring, etc. for a function called 
> get-slice-seq. PSliceSeq2 is actually implemented for the generic object 
> case in ...impl.defaults and for ISeq in ...impl.sequence. 
>
> But ...impl.pprint calls ...protocols/get-slice-seq. And it seems to work. 
>  Even though I can't find either ...impl.defaults or ...impl.sequence 
> anywhere in the dependency tree for ...impl.pprint.
>
>
I haven't looked at the code, but something must be loading the extension 
before you invoke it.
 

> This is a puzzle to me.  I mean, the ...impl.pprint namespace will 
> typically be required from some other namespace, and presumably that 
> higher-level namespace will require everything that one needs. But still, I 
> hadn't thought that one would be able to call get-slice-seq in a function 
> in ...impl.pprint without requiring a namespace that actually defines the 
> function, as opposed to just defining the protocol.
>
> So can someone explain how this magic works?  Is the compiler just way 
> smarter than I thought it was, and capable of figuring this stuff out on 
> its own?
>
> Thanks!
>
> -Paul
>

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Re: protocols and namespaces confusion (with an example from core.matrix)

[Gregg Reynolds]

On Oct 19, 2016 7:23 AM, "Paul Gowder"  wrote:
>
> Hi folks,
>
> I have a kinda basic question (I think?) that I should really know the
answer to after writing Clojure for like 8 months, but don't.  It's about
requiring namespaces with protocol definitions in them but not the actual
implementations of those protocols, and how it works.
>
> tl/dr: if a protocol is defined in namespace a and implemented (extended
to the type at issue) in namespace b, can you call functions from it in
namespace c where namespace c only requires namespace a, not namespace b?
>
> Backstory: I was poking around core.matrix recently in pursuit of
restarting work on a months ago issue I promised to work on, and it looks
like at least one namespace calls a function it shouldn't be able to call.
>
> So: clojure.core.matrix.impl.pprint requires only one namespace within
core.matrix, clojure.core.matrix.protocols. (I'm going to use ... hereafter
to stand for clojure.core.matrix.) ...protocols defines a protocol
PSliceSeq2, which gives the name, docstring, etc. for a function called
get-slice-seq. PSliceSeq2 is actually implemented for the generic object
case in ...impl.defaults and for ISeq in ...impl.sequence.
>
> But ...impl.pprint calls ...protocols/get-slice-seq. And it seems to
work.  Even though I can't find either ...impl.defaults or ...impl.sequence
anywhere in the dependency tree for ...impl.pprint.
>
> This is a puzzle to me.  I mean, the ...impl.pprint namespace will
typically be required from some other namespace, and presumably that
higher-level namespace will require everything that one needs. But still, I
hadn't thought that one would be able to call get-slice-seq in a function
in ...impl.pprint without requiring a namespace that actually defines the
function, as opposed to just defining the protocol.
>
> So can someone explain how this magic works?  Is the compiler just way
smarter than I thought it was, and capable of figuring this stuff out on
its own?
>
Hi Paul,

can't give you specifics but (going from memory) I might be able to point
you in the right direction.  take a look at the initialization code of
core.matrix.  you have to feed it an implementation, and it dynamically
arranges bindings.  sorry I can't give you more detail but I suspect you'll
find your answer somewhere in that vicinity.

Gregg

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protocols and namespaces confusion (with an example from core.matrix)

[Paul Gowder]

Hi folks, 

I have a kinda basic question (I think?) that I should really know the 
answer to after writing Clojure for like 8 months, but don't.  It's about 
requiring namespaces with protocol definitions in them but not the actual 
implementations of those protocols, and how it works.  

tl/dr: if a protocol is defined in namespace a and implemented (extended to 
the type at issue) in namespace b, can you call functions from it in 
namespace c where namespace c only requires namespace a, not namespace b?

Backstory: I was poking around core.matrix recently in pursuit of 
restarting work on a months ago issue I promised to work on, and it looks 
like at least one namespace calls a function it shouldn't be able to call.

So: clojure.core.matrix.impl.pprint requires only one namespace within 
core.matrix, clojure.core.matrix.protocols. (I'm going to use ... hereafter 
to stand for clojure.core.matrix.) ...protocols defines a protocol 
PSliceSeq2, which gives the name, docstring, etc. for a function called 
get-slice-seq. PSliceSeq2 is actually implemented for the generic object 
case in ...impl.defaults and for ISeq in ...impl.sequence. 

But ...impl.pprint calls ...protocols/get-slice-seq. And it seems to work. 
 Even though I can't find either ...impl.defaults or ...impl.sequence 
anywhere in the dependency tree for ...impl.pprint.

This is a puzzle to me.  I mean, the ...impl.pprint namespace will 
typically be required from some other namespace, and presumably that 
higher-level namespace will require everything that one needs. But still, I 
hadn't thought that one would be able to call get-slice-seq in a function 
in ...impl.pprint without requiring a namespace that actually defines the 
function, as opposed to just defining the protocol.

So can someone explain how this magic works?  Is the compiler just way 
smarter than I thought it was, and capable of figuring this stuff out on 
its own?

Thanks!

-Paul

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