How does Scheme abstract data?

Question

In statically typed language, people are able to use algebraic data type to abstract data and also generate constructors, or use class, trait and mixin to deal with data abstraction.

In dynamically typed language, like Python and Ruby, they all provide a class system to users.

But what about scheme, the simplest functional language, the closest one to λ-calculi, how does it abstract data?

Do scheme programmers usually just put data in a list or a lambda abstraction, and write some accessor function to make it look like a tree or something else? like EOPL says: specifying data via interfaces.

And then how does this abstraction technique relate to abstract data type (ADT) and objects? with regard to On understanding data abstraction, revisited.

Answer 1

What SICP (and I guess, EOPL) is advocating is just using functions to access data; then you can always switch one set of functions for another, implementing the same named set of functions to work with another concrete implementation. And that (i.e. the sets of such functions) is what forms the "interfaces", and that's what you put in different source files, and by just loading the appropriate one you can switch the concrete implementation while all the other code is none the wiser. That's what makes it "abstract" datatype.

As for the algebraic data types, the old bare-bones Scheme way is to create closures (that hold and hide the data) which respond to "messages" and thus become "objects" (something about "Scheme mailboxes"). This gives us products, i.e. records, and functions we get for free from Scheme itself. For sum types, just as in C/C++, we can use tagged unions in a disciplined manner (or, again, hide the specifics behind a set of "interface" functions).

EOPL has something called "variant-case" which handles such sum types in a manner similar to pattern matching. Searching brings up e.g. this link saying

I'm using DrScheme w/ the EOPL textbook, which uses define-record and variant-​case. I've got the macro definitions from the PLT site, but am now dealing with ...

so seems relevant, as one example.