lisp interpreter in python

Question

I'm curious how a part of Peter Norvig's Lisp interpreter works. One can define functions in this Lisp interpreter...how does this work? I'm a beginner, and just would like a simple explanation.

There's one part that might be related where it says

elif x[0] == 'define': # (define var exp)
 (_, var, exp) = x

http://norvig.com/lispy.html

Answer 1

In this case, x[0] is define, x[1] is the variable name, and x[2] is the expression. So, in Python, _, var, exp = x is a "destructuring assignment", which destructures the array x into its constituent elements, and assigns them to the variables on the left-hand side.

• tekknolagi: how does defining functions work though? how does the interpreter understand (define (square x) (* x x)) ?
• C. K. Young: @tekknolagi: (define (square x) (* x x)) is syntactic sugar for (define square (lambda (x) (* x x))). It's pretty obvious here that var is square and exp is (lambda (x) (* x x)).
• tekknolagi: how does it hold the user defined function in memory and then use it when the user calls (square 5) ?
• C. K. Young: @tekknolagi: lambda creates a function object. In Scheme (just like in Python, JavaScript, etc.), functions are first-class: you can pass them around like other objects. Additionally (unlike other objects), you can call them.
• C. K. Young: @tekknolagi: The arguments passed to the Python lambda, of course. (Read up on how to use variable-arity functions in Python.)