Question

I’ve encounter a following statement by Richard Stallman:

‘When you start a Lisp system, it enters a read-eval-print loop. Most other languages have nothing comparable to read, nothing comparable to eval, and nothing comparable to print. What gaping deficiencies! ‘

Now, I did very little programming in Lisp, but I’ve wrote considerable amount of code in Python and recently a little in Erlang. My impression was that these languages also offer read-eval-print loop, but Stallman disagrees (at least about Python):

‘I skimmed documentation of Python after people told me it was fundamentally similar to Lisp. My conclusion is that that is not so. When you start Lisp, it does ‘read’, ‘eval’, and ‘print’, all of which are missing in Python.’

Is there really a fundamental technical difference between Lisp’s and Python’s read-eval-print loops? Can you give examples of things that Lisp REPL makes easy and that are difficult to do in Python?

Answer 1

In support of Stallman’s position, Python does not do the same thing as typical Lisp systems in the following areas:

• The read function in Lisp reads an S-expression, which represents an arbitrary data structure that can either be treated as data, or evaluated as code. The closest thing in Python reads a single string, which you would have to parse yourself if you want it to mean anything.

• The eval function in Lisp can execute any Lisp code. The eval function in Python evaluates only expressions, and needs the exec statement to run statements. But both these work with Python source code represented as text, and you have to jump through a bunch of hoops to “eval” a Python AST.

• The print function in Lisp writes out an S-expression in exactly the same form that read accepts. print in Python prints out something defined by the data you’re trying to print, which is certainly not always reversible.

Stallman’s statement is a bit disingenuous, because clearly Python does have functions named exactly eval and print, but they do something different (and inferior) to what he expects.

In my opinion, Python does have some aspects similar to Lisp, and I can understand why people might have recommended that Stallman look into Python. However, as Paul Graham argues in What Made Lisp Different, any programming language that includes all the capabilities of Lisp, must also be Lisp.