RE: scheme memory address

[Andrew Bernard]

Hi Gianmaria,

 

It’s time to stop and take stock here. It’s clear you have some background in programming, but it is even more clear that you have not studied Lisp and Scheme, except in a passing manner. The Lisp family of languages is fundamentally different from languages like C. It takes a considerable mental effort to grasp the fullness of Lisp and Scheme, but the effort is repaid as there will come a day when you suddenly realise how it all hangs together, and you will never program the same way again. There’s a long history of programmers reporting such an epiphany after studying Lisp for a while, and your outlook is changed forever. It is often said that learning Lisp makes you a better programmer even if you never program in Lisp.

 

At this point, rather than engaging in long winded corrections of your approach, I would really like you to ‘reset’ (after all, this topic started about resetting lists…) and read SICP, Structured Interpretation of Computer Programs by Abelson and Sussman of MIT. It is the definitive introductory text, and I have a hard copy at my desk always. You can study the online version here, the second edition.

 

https://mitpress.mit.edu/sites/default/files/sicp/index.html

 

The book beautifully typeset here:

 

https://web.mit.edu/alexmv/6.037/sicp.pdf

 

At the language level in Lisp and Scheme, there are no pointers. It’s critical to realise this, and important. Obviously the language uses pointers and memory locations internally, because that’s what computers are, but it’s vital that you don’t try to second guess how Scheme works internally, because you will certainly be wrong. The point of Scheme and Lisp is that they increase the level of abstraction we are dealing with, and this is their power. The low level machine issues are abstracted away from you, in order to give you vastly greater power. C is very low in the level of abstraction hierarchy, and is closer to assembler in spirit, with all the access to memory you want, and all the consequent disasters. Scheme relieves you of this.

 

I’d just like to direct you to the answer here in this post on Stack Overflow:

 

https://stackoverflow.com/questions/38277997/what-does-location-means-in-r6rs-of-scheme

 

“The crucial point here is that in languages like Lisp pointers are (and should remain) “invisible” to the user (the programmer).”

 

Your questions may actually be better addressed on Stack Overflow, as they are mostly about Scheme rather than lilypond specific – the point being, there is a lot of help there on languages.

 

Next, and I think this is even more important than chat about pointers. You would benefit from studying Functional Programming. Scheme supports imperative programming but it is also a good functional programming language. Studying this style of computer language will again change the way you think forever. You will study immutable data and how that completely changes the way you program. Haskell is the most pure functional language and every object in Haskell is immutable. Think about that for a minute. You cant change variables but you can actually write programs? Indeed. [I’d recommend studying Haskell too as it will inform you Scheme programming.]

 

Next, I detected a hint in your thread that you would like Scheme to be object oriented. It’s not. Don’t go there. OO is not the panacea it was once thought to be. So it is also wise not to attempt to mould Scheme into an OO model. [Yes, there are dialects that provide OO, but in the context of lilypond, we have Scheme.]

 

Another great learning resource with truly superb documentation is Racket, a Scheme variant. Now at release 7.2 and very mature. It’s a really good environment for learning Scheme, even though it is a greatly extended language.

 

There’s a lot to be said here Gianmaria. I think the outcome here is you will either abandon completely or go on to be a really master Scheme programmer. Hopefully the latter!

 

In my next mail, I will outline an approach to your original question in a way that uses Scheme in an idiomatic way.

 

Andrew