function evaluate($expr, $env = []) {
if (is_string($expr)) {
if (is_numeric($expr)) {
$val = (float) $expr;
return [$val, $env];
}
if ('true' === $expr) {
return [true, $env];
}
if ('false' === $expr) {
return [false, $env];
}
if ('null' === $expr) {
return [null, $env];
}
$var = $expr;
return [$env[$var], $env];
}
$fn = array_shift($expr);
$args = $expr;
if ('define' === $fn) {
list($var, $val) = $args;
list($val, $env) = evaluate($val, $env);
$env = array_merge($env, [$var => $val]);
return [null, $env];
}
if ('lambda' === $fn) {
list($lambda_args, $code) = $args;
$closure = ['closure', $lambda_args, $code, $env];
return [$closure, $env];
}
if ('if' === $fn) {
list($cond, $then, $else) = $args;
$cond = evaluate($cond, $env)[0];
if ($cond === true) {
$val = evaluate($then, $env)[0];
} else {
$val = evaluate($else, $env)[0];
}
return [$val, $env];
}
!
$fn = evaluate($fn, $env)[0];
$args = array_map(function ($arg) use ($env) {
return evaluate($arg, $env)[0];
}, $args);
return apply($fn, $args, $env);
}
!
function apply($fn, $args, $env) {
list($_, $lambda_args, $code, $closure_env) = $fn;
!
$env = array_merge($env, $closure_env);
$env = array_merge($env, array_combine($lambda_args, $args));
return evaluate_code($code, $env);
}
function evaluate_code($code, $env = []) {
foreach ($code as $expr) {
list($val, $env) = evaluate($expr, $env);
}
return [$val, $env];
}
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