Evaluate an expression in an environment.
(exp: Expression, env: Environment)
| 135 | ################ Evaluator |
| 136 | |
| 137 | def evaluate(exp: Expression, env: Environment) -> Any: |
| 138 | "Evaluate an expression in an environment." |
| 139 | if isinstance(exp, Symbol): # variable reference |
| 140 | return env[exp] |
| 141 | elif not isinstance(exp, list): # constant literal |
| 142 | return exp |
| 143 | elif exp[0] == 'quote': # (quote exp) |
| 144 | (_, x) = exp |
| 145 | return x |
| 146 | elif exp[0] == 'if': # (if test conseq alt) |
| 147 | (_, test, consequence, alternative) = exp |
| 148 | if evaluate(test, env): |
| 149 | return evaluate(consequence, env) |
| 150 | else: |
| 151 | return evaluate(alternative, env) |
| 152 | elif exp[0] == 'lambda': # (lambda (parm…) body…) |
| 153 | (_, parms, *body) = exp |
| 154 | if not isinstance(parms, list): |
| 155 | raise SyntaxError(lispstr(exp)) |
| 156 | return Procedure(parms, body, env) |
| 157 | elif exp[0] == 'define': |
| 158 | (_, name_exp, *rest) = exp |
| 159 | if isinstance(name_exp, Symbol): # (define name exp) |
| 160 | value_exp = rest[0] |
| 161 | env[name_exp] = evaluate(value_exp, env) |
| 162 | else: # (define (name parm…) body…) |
| 163 | name, *parms = name_exp |
| 164 | env[name] = Procedure(parms, rest, env) |
| 165 | elif exp[0] == 'set!': |
| 166 | (_, var, value_exp) = exp |
| 167 | env.change(var, evaluate(value_exp, env)) |
| 168 | else: # (proc arg…) |
| 169 | (func_exp, *args) = exp |
| 170 | proc = evaluate(func_exp, env) |
| 171 | args = [evaluate(arg, env) for arg in args] |
| 172 | return proc(*args) |
| 173 | |
| 174 | |
| 175 | class Procedure: |