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

C. K. Young · Accepted Answer · 2011-06-14 00:58:02Z

3

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.

answered Jun 14, 2011 at 0:58

C. K. Young

224k47 gold badges394 silver badges446 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

tekknolagi Over a year ago

how does defining functions work though? how does the interpreter understand (define (square x) (* x x)) ?

C. K. Young Over a year ago

@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 Over a year ago

how does it hold the user defined function in memory and then use it when the user calls (square 5) ?

C. K. Young Over a year ago

@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 Over a year ago

@tekknolagi: The arguments passed to the Python lambda, of course. (Read up on how to use variable-arity functions in Python.)

|

Collectives™ on Stack Overflow

lisp interpreter in python

1 Answer 1

9 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

9 Comments

Your Answer

Sign up or log in

Post as a guest

Related