Comment by foldr | Hacker News Reader

Comment by foldr | Hacker News Reader

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I interpret the term compostitonally. But CL doesn’t have zero cost abstractions in that sense either.

Take the example I mentioned. Say that you’re looping through an array of pairs of 2D vectors and calculating the dot product of each pair. In C++ you can use your 2D vector class without any additional cost. In CL you either need to remove that abstraction (and deal with flat arrays of scalars) or incur the cost of boxing.

More generally, if every object with its own methods requires a boxed representation, then that severely limits the range of zero cost abstractions that you can create. If using the abstraction requires boxing then it’s not zero cost. (If Bjarne disagrees on that point, then I disagree with him!)

Anyway, I’m sure you know all this, so I’m not really sure what point you’re trying to make here. I don’t think anyone would suggest that CL is a good language for building zero cost abstractions, whatever the precise definition of the term.

Genera, TI and Xerox thought otherwise.

Unfortunely all of them messed up on management, and fighting against free UNIX source tapes.

"What you don’t use, you don’t pay for. And further: What you do use, you couldn’t hand code any better."

My original comment talked about 'portable CL'.

I am interested though: how would you define an unboxed array of structs in Genera's dialect of Lisp?

Your quote clearly applies to my example. You can avoid the boxing by hand coding the dot product computation over flat arrays; you can't avoid the boxing if you use a 2D vector abstraction (in CL).

You would use (make-array), and (zl:defstruct (:type :array)), so that you would use an array as backing store for the fields.

I don't think so? Judging by the documentation that's just the usual option (also available in CL) to have struct fields stored in an array, list, name value pair list, etc. There's no suggestion that it would accomplish unboxing in general. I guess it is possible that if each field of the struct had the same type (say, a float), then the backing storage for the struct would be an unboxed array of floats, and an array of such structs would then come out with its backing storage as an unboxed multidimensional array of floats. But:

* I'd really want to see this actually working to believe it. The documentation doesn't make clear what would happen in this circumstance. (Time to spin up OpenGenera? Ha!)

* At best this works for structures with homogenous field types. My example of a 2D vector happens to fit that criterion, but it is also useful to have unboxed arrays of structures with heterogenous field types.

http://www.bitsavers.org/pdf/symbolics/software/genera_8/Sym... (page 450 according to the on-page numbering).

Accessors take care of reading/writing from the backing storage mechanism.

If you want to be really picky, don't forget the whole OS was written in Lisp, and even the C, Pascal and Ada compilers targeted it, and much follow the same semantics. The same folder has the manuals for C and Pascal.

And if you want to be really sure how it goes, there is the low level forms for Assembly like coding, e.g. sys:art-q, which is used to pack C structures into arrays.

(:type :array) is documented as the default. So there must be something more required to create an unboxed array of structs than just that.

If, for whatever reason, it is important to you to persuade the internet that the Lisp dialect of a long-defunct operating system was able to define unboxed arrays of structs, then I think you should at least show example code demonstrating this. The more salient point, however, is that Common Lisp can't do this.

> Say that you’re looping through an array of pairs of 2D vectors and calculating the dot product of each pair. In C++ you can use your 2D vector class without any additional cost. In CL you either need to remove that abstraction (and deal with flat arrays of scalars) or incur the cost of boxing.

Or maybe use compiler macros to remove the abstraction without the cost of boxing?

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