Its got to be a snippet. Suprisingly hard to track down with DDG or Google, though. Here's something pretty similar [0]. It's from a course powerpoint, and my g

Its got to be a snippet. Suprisingly hard to track down with DDG or Google, though. Here's something pretty similar [0]. It's from a course powerpoint, and my guess would be a student or web searcher modified it. def eval(x, env=global_env): "Evaluate an expression in an environment." if isa(x, Symbol): return env.lookup(x) elif not isa(x, list): return x elif x[0] == 'quote': return x[1] elif x[0] == 'if': return eval((x[2] if eval(x[1], env) else x[3]), env) elif x[0] == 'set!': env.set(x[1], eval(x[2], env)) elif x[0] == 'define': env.define(x[1], eval(x[2], env)) elif x[0] == 'lambda’: return lambda *args: eval(x[2], Env(x[1], args, env)) elif x[0] == 'begin': return [eval(exp, env) for exp in x[1:]] [-1] else: exps = [eval(exp, env) for exp in x] proc = exps.pop(0) return proc(*exps) [edit] And here's a second, hosted on Github. [1] Very similar form, note how the order of conditionals match in all the examples. This one says it was written as prep for SICP. Is the proto-source from the older Scheme version of the class? [edit2] Ahh.. proto-source is Peter Norvig's lis.py. [2] [3] Above example explicitly references it blog. [4] def eval(x, env=global_env): "Evaluate an expression in an environment." if isinstance(x, Symbol): # variable reference return env.find(x)[x] elif not isinstance(x, List): # constant literal return x elif x[0] == 'quote': # (quote exp) (_, exp) = x return exp elif x[0] == 'if': # (if test conseq alt) (_, test, conseq, alt) = x exp = (conseq if eval(test, env) else alt) return eval(exp, env) elif x[0] == 'define': # (define var exp) (_, var, exp) = x env[var] = eval(exp, env) elif x[0] == 'set!': # (set! var exp) (_, var, exp) = x env.find(var)[var] = eval(exp, env) elif x[0] == 'lambda': # (lambda (var...) body) (_, parms, body) = x return Procedure(parms, body, env) else: # (proc arg...) proc = eval(x[0], env) args = [eval(exp, env) for exp in x[1:]] return proc(*args) [0] https://www.csee.umbc.edu/courses/331/fall11/notes/schemeInPython.ppt.pdf https://www.csee.umbc.edu/courses/331/fall11/notes/schemeInP... [1] https://github.com/eigenhombre/smallscheme/blob/master/smallscheme/scheme.py https://github.com/eigenhombre/smallscheme/blob/master/small... [2] https://norvig.com/lispy.html https://norvig.com/lispy.html [3] https://norvig.com/lis.py https://norvig.com/lis.py [4] http://johnj.com/scheme-in-python.html http://johnj.com/scheme-in-python.html