Function evaluate

02-array-seq/lispy/py3.9/lis.py:138–169

Evaluate an expression in an environment.

(exp: Expression, env: Environment)

# tag::EVAL_IF_TOP[]
def evaluate(exp: Expression, env: Environment) -> Any:
    "Evaluate an expression in an environment."
    if isinstance(exp, Symbol):      # variable reference
        return env[exp]
# end::EVAL_IF_TOP[]
    elif not isinstance(exp, list):  # constant literal
        return exp
# tag::EVAL_IF_MIDDLE[]
    elif exp[0] == 'quote':          # (quote exp)
        (_, x) = exp
        return x
    elif exp[0] == 'if':             # (if test conseq alt)
        (_, test, consequence, alternative) = exp
        if evaluate(test, env):
            return evaluate(consequence, env)
        else:
            return evaluate(alternative, env)
    elif exp[0] == 'lambda':         # (lambda (parm…) body…)
        (_, parms, *body) = exp
        return Procedure(parms, body, env)
    elif exp[0] == 'define':
        (_, name, value_exp) = exp
        env[name] = evaluate(value_exp, env)
# end::EVAL_IF_MIDDLE[]
    elif exp[0] == 'set!':
        (_, name, value_exp) = exp
        env.change(name, evaluate(value_exp, env))
    else:                          # (proc arg…)
        (func_exp, *args) = exp
        proc = evaluate(func_exp, env)
        args = [evaluate(arg, env) for arg in args]
        return proc(*args)


class Procedure: