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Although LISP has some of the most simple syntax I've seen, I am still confused about the fundamentals. I've done research, and I've come to the conclusion that there are two datatypes: "atoms" and lists. However, I have also come across the term "S-expression", which seems to describe both atoms and lists. So, what exactly is an S-expression? Is it a datatype? In addition, I am not sure how to distinguish function calls from data lists in LISP. For example, (1 2 3) is a list, while (f 2 3) could be some function. But how am I supposed to know whether f is a function name or some datatype? Since lists and functions use the same syntax, I am not sure how to differentiate between the two. Finally, most importantly, I need a mental model for thinking about how LISP works. For example, what are the fundamental datatypes? What are the built-in procedures used to do things with the fundamental datatypes? How can we see data and procedures as distinct? For example, in Java, instance variables at the top of classes are used to represent data, while methods are the procedures that manipulate the data. What does this look like in LISP?

(I'm new, so I'm not sure if this question is too broad or not)

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    Lisp is homoiconic, so code is data. Doesn't quite answer the question, but it is a significant departure from most languages in popular use today. Also a list declaration looks like (list 1 2 3), not (1 2 3). Commented Jul 23, 2015 at 19:11
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    Reading this book will answer all your questions: cs.cmu.edu/~dst/LispBook You can download the book as PDF from there. Commented Jul 23, 2015 at 19:19

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I second the recommendations for LispBook and Practical Common Lisp. Both great books. Once you have understood the basics, I really, really recommend Paul Graham's "On Lisp".

To guide you toward answers on your specific questions:

  1. Data types: Lisp has a rich set of data types (numeric types such as integer, rational, float - character and string - arrays and hash-tables - plenty more), but in my opinion, assuming you are already comfortable with integer and string, you should start by reading up on symbol and cons.

  2. symbol: Realise that a symbol is both an identifier and a value. Understand that each symbol can "point to" one data value and, at the same time, "point to" one function (and has three other properties that you don't need to worry about yet)

  3. cons: Discover that this magical thing called a 'cons cell' is just a struct with two pointers. One called "the Address part of the Register" and the other called "the Decrement part of the register". Don't worry about what those names mean (they're no longer relevant), just know that the car function returns the Contents of the Address part of the Register, and cdr returns the Decrement part. Now forget all of that and just remember that modern Lispers think of the cons cell as a struct with two pointers, one called the car and the other called the cdr.

  4. Lists: "There is no spoon." There is no list either. Realise that what we think of as a list is nothing more than a set of cons cells, with the car of each cons pointing to one member of the list, and the cdr of each cons pointing to the next cons (except for the last cons, whose cdr is "the null pointer" nil). This is vital to understand list manipulation, nested lists, tree structures, etc.

  5. Atoms: For now, think of an atom as a number, a character, a string or a symbol. That's enough to get you started you can dig deeper into the details later.

  6. S-expressions: An s-expr is defined as "either an atom, or a cons cell pointing to two s-expressions". Don't worry about what is or isn't an s-expr for now, but checkout wikipedia for a brief intro

  7. Lists vs function calls: Man, oh, man! I struggled with this when I got started! But it's actually quite easy with a bit of practice: "Five times two" is just an English phrase, right? But if I asked you to "evaluate" it, you would probably say "ten". "1 + 2" is a mathematical expression, but a mathematician would evaluate it to 3. "5" is just a number, but if you type it into a calculator and press "=", the calculator will evaluate it to the answer 5. In Lisp (+ 1 2) is just a list. It is a list containing the symbol +, the number 1 and the number 2. BUT - if you ask Lisp to evaluate that list, it will call the function +, passing it 1 and 2 as parameters. A large part of getting comfortable with Lisp is learning when and where s-expressions are evaluated, and where they aren't. For now - anything you type into the REPL will be evaluated, parameters to function calls will be evaluated, parameters to macro calls will probably not be evaluated, you can use quote to prevent evaluation. This will become easier with practice.

  8. Essential functions: Although I said the "list" doesn't really exist ("Its conses all the way down, Mr. Fry"), learn the list based stuff first, car, cdr, cons, list, append, reverse and the applicative stuff mapcar, mapcons, apply. These are a great way to start thinking in both lists and functional programming.

  9. Classes, member data and methods: I recommend leaving object orientation for later. First learn the basics of Lisp. Fight the urge to worry about data encapsulation, access control and polymorphism until you have become friends with the language itself. When you are ready, read "On Lisp", Chapter 25 will walk you through adding object orientation to Lisp yourself, showing how Lisp really is the programmable programming language. The book will then introduce you to CLOS, the standard object-orientation system built into Common Lisp. Get to know CLOS, but certainly browse around for other OO libraries. Lisp is the only language I know where you can actually choose how your want the language to implement OO.

I'm going to stop here. Get comfortable with the first 8 concepts above and a strong mental model will will have sorted itself out.

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Everything that is self evaluating is data. eg 2, "hello". Everything that is quoted is data. eg '(f 3 4), 'test, or the longhand version (quote test) Everything else that is fed to the REPL needs to be code. eg (f 3 4) is code. It's s-expression and indistinguishable to the quoted data above, but it isn't quoted so it has to be code.

There are some special forms like if, let, lambda, defun, ... that you just need to learn how works. How do you know that if isn't a method in a C dialect like Java or C#? You just have to know them by heart.

You also need to know some essential functions. Usually they are introduced in each and every tutorial together with the special forms. I recommend you read Land of Lisp and Practical Common Lisp for Common Lisp and Realm of Racket and How to design programs for Racket. For pure Scheme I recommend The wizard book (SICP). Don't do them all at the same time. Learning the other ones are easy when you have learned one good enougn.

Now, learning a Lisp is more difficult than learning another C dialect. It's because it is a totally different language with different ways to do stuff. Eg. you won't find for or while loops and variables don't have types but the objects they refer to have. You need to learn how to program almost as if you didn't know a C dialect. (Actually, knowing a C dialect might make the learning harder)

Good luck!

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An "S-expression" is 'an atom, or a list of S-expressions' (there are some more complications here, I'll skip those for now, basically boiling down to something called "read macros", where a difference between the textual representation and the internal representation can be done, to simplify human writing).

(1 2 3) is a list. But, (f 1 2 3) is also a list. However, in some (most?) circumstances, it can also be a function call (rather than a function).

I think you mostly have the syntax down, the rest is semantics (in a very technical sense). At this point, I would point you at some good reading material, there's plenty of good books around (I started with an earlier edition of Winston-Horn's "Lisp".

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