support for (doc)

src/CoreFunc.php

@@ -6,17 +6,16 @@ use Closure;
 class CoreFunc extends Func
 {
     protected string $name;
-    protected string $doc;
     protected int $minArgs;
     protected int $maxArgs;
 
     public function __construct(string $name, string $doc, int $minArgs, int $maxArgs, Closure $closure)
     {
+        parent::__construct($closure, $doc);
+
         $this->name = $name;
-        $this->doc = $doc;
         $this->minArgs = $minArgs;
         $this->maxArgs = $maxArgs;
-        $this->closure = $closure;
     }
 
     public function call(array $args)

src/Func.php

@@ -6,10 +6,12 @@ use Closure;
 abstract class Func
 {
     protected Closure $closure;
+    protected ?string $doc;
 
-    public function __construct(Closure $closure)
+    public function __construct(Closure $closure, ?string $doc = null)
     {
         $this->closure = $closure;
+        $this->doc = $doc;
     }
 
     public function getClosure(): Closure
@@ -17,6 +19,11 @@ abstract class Func
         return $this->closure;
     }
 
+    public function getDoc(): ?string
+    {
+        return $this->doc;
+    }
+
     public function call(array $args)
     {
         return ($this->closure)(...$args);

src/Lisp.php

@@ -29,6 +29,16 @@ class Lisp
 
     public function register(Env $env): void
     {
+        $env->set('doc', new CoreFunc('doc', 'Get documentation for a function.', 1, 1,
+            function ($a) {
+                if ($a instanceof Func) {
+                    return $a->getDoc();
+                }
+
+                throw new MadLispException('first argument to doc is not function');
+            }
+        ));
+
         $env->set('eval', new CoreFunc('eval', 'Evaluate arguments.', 1, -1,
             function (...$args) use ($env) {
                 $results = $this->eval->eval($args, $env);