What is the difference between Lisp-1 and Lisp-2?

Question

I have tried to understand the difference between Lisp-1 and Lisp-2 and how this relates to Clojure but I still do not understand properly. Can anyone enlighten me?

Fleeta

Answer 1

According to wikipedia:

Whether a separate namespace for functions is an advantage is a source of contention in the Lisp community. It is usually referred to as the Lisp-1 vs. Lisp-2 debate. Lisp-1 refers to Scheme's model and Lisp-2 refers to Common Lisp's model.

It's basically about whether variables and functions can have the same name without clashing. Clojure is a Lisp-1 meaning that it does not allow the same name to be used for a function and a variable simultaneously.

Bromleigh

Isn't a Lisp-2 more confusing having functions and variables with the same names then? – Fleeta

Part of the reason why is that programmers using Lisp-2 dialects do not go out of their way to have functions and variables having the same names. list is often used as a function parameter and nobody things, OMG that is so confusing since (list ...) is a standard function. Many functions that have list as a variable don't use the list function, or don't use it near that variable. Even when that does happen it's not too bad: (list foo list). This isn't any more confusing than a sentence like "fight the good fight" where the same word appears as a noun and verb. – Origan

@Zubair Java has even more name spaces. You can define a class, a method and a variable with the same name. – Himself

As a mnemonic, I think of Lisp 1s as having 1 namespace, and Lisp 2s having 2 namespaces (one for functions and one for variables). – Foreplay

@NickMcCurdy As a mnemonic, I think of a bicycle as having two wheels, and tricycle having three. – Origan

@Fleeta Lisp-2 preceded lisp-1 as far as I know. – Mathematical

Answer 2

You might like to read this paper by Richard Gabriel. It is a summary of the issues that the Lisp community were discussing in Lisp1 vs Lisp2. It's a bit dense and slow moving in the first few sections, but is much easier to read by the time you get past section 5.

Basically, Lisp1 has a single environment that maps symbols to values, and those values can be either "regular" or functions. Lisp2 has (at least) two namespaces (symbols have a slot for their a function value and one for a regular value). So, in Lisp2, you can have a function named foo and a value named foo, whereas in Lisp1, the name foo can refer only to a single value (function or otherwise).

There are several tradeoffs and differences of taste between the two, but read the paper for the details. Christian Queinnec's book, "Lisp in Small Pieces" also has discussion of the differences woven through the text.

Colonel

One of the more enigmatic sections of the Gabriel paper, is section 11 on Multiprocessing. In that section, he implies that Lisp1 is more conducive to a functional style of programming, hence more conducive to multiprocessing. Obviously, this is of interest wrt Clojure. But I'm not really sure why Lisp1 is more conducive to FP. Anyone have insight into this? – Colonel

probably because the major point of FP is to treat functions as first class values, hence it's much more convenient and conceptually cleaner to treat them in the same way as all other values – Consideration

@PeterMcLain When users of Lisp-1 dialects say tht Lisp-1 is more conductive to functional programming, what they mean is that you don't have to stuff the code full of funcall and function operators. These disappear in a Lisp-1. – Origan

An example what happens when a Schemer tries to write Lisp: emacs.stackexchange.com/q/28979/2787 – Himself

you used the phrase "at least" about Lisp-2. The simple differentiation between the 2 cases is: whether the symbol is at the head position of an s-expression, or not (if so, then lookup in the function namespace, else in the variable namespace). Can you give some examples of other, than those 2 cases? – Scrofula

@DanielDinnyes same name may refer to a value or to a function or (and that is the "at least" part) to other stuff, like restart designators, goto labels, etc. if I'm not mistaken. But to what you've said, there's also quoted arguments to mapcar and the like, some arcane rules of resolution there. Again IIANM. – Fernanda

I just like that the paper uses the term "perspicuously" with the wonderfully ironic meaning "easier to understand". – Hardeman

@Hardeman Where is the irony? In that "perspicuously" is a rarely used word of many syllables denoting clarity? – Origan

@PeterMcLain I cannot recommend Quinnec's Lisp In Small Pieces enough. It's definitely required reading for anyone who wants a deep understanding of Lisp (the family of languages). It's also really important to follow through the references, too. – Aldo

Answer 3

In a Lisp-1 a symbol can only have 1 value. In a Lisp-2 a symbol can have two values which are 1) the value when the symbol is interpreted as a function and 2) when the symbol is interpreted as a value.

Down in the implementation in a Lisp-1 the structure that contains the value associated with a symbol has just one slot for a pointer to the data and type. In a Lisp-2 there are two pointers, one the function value, the other the variable value.

In Common Lisp, a Lisp-2, you can call (symbol-function ‘a) (symbol-value ‘a) to get the function and value separately. If ‘a is defined as the add function and (setq a 3)then (a a a 1) would return 7.

In a lisp 1 you couldn’t do that because a could only be one thing, and the context does not change the meaning.

In Common Lisp #’ is an abbreviation for symbol-function. (setf (symbol-function ‘a) #’+ ) would set a to be the function sum.

If you wanted, you could create a function a that would set a to the value last returned by a call to a, perhaps if a was expensive to compute.

In a Lisp-2 if you assigned a function to a symbol then to call that function you use the funcall operator.

Roping

when copying and pasting into lisp, take care with the quotes in the examples. Looks likd the single quotes don't copy correctly on my Mac. Just edit the quotes to single quotes when trying the examples. – Roping