feat: 'let'

lisp.c

@@ -99,6 +99,8 @@ void node_print(Node node) {
         Nob_String_View func_name = node.as_function.name;
         printf(SV_Fmt, SV_Arg(func_name));
     } break;
+    default:
+        UNREACHABLE("Unknown node type");
     }
 }
 
@@ -357,6 +359,67 @@ int eval_one(Node input, Scopes *scopes, Node *result) {
             return 0;
         }
 
+        if (nob_sv_eq(head.as_symbol,
+                      (Nob_String_View){.count = 3, .data = "let"})) {
+            if (children->count != 2 + 1) {
+                printf("'let' expects two arguments!\n");
+                return 1;
+            }
+
+            Node definitions = children->items[0 + 1];
+            if (definitions.kind != NODE_KIND_LIST) {
+                printf("First argument of 'let' must be a list!\n");
+                return 1;
+            }
+
+            Node expression = children->items[1 + 1];
+            if (expression.kind != NODE_KIND_LIST) {
+                printf("Second argument of 'let' must be a list!\n");
+                return 1;
+            }
+
+            // it's fine to allocate this on the stack because we will throw it
+            // off the scope stack after parsing 'let' anyways therefore it'll
+            // outlive the recursive call
+            Scope scope = {0};
+
+            nob_da_foreach(Node, def, definitions.as_list) {
+                if (def->kind != NODE_KIND_LIST || def->as_list->count != 2 ||
+                    def->as_list->items[0].kind != NODE_KIND_SYMBOL) {
+                    printf("'let' expects a list of key-value pairs, e.g. "
+                           "'(let ((x 42) (y 2)) (+ x y))'\n");
+                    return 1;
+                }
+
+                Node key = def->as_list->items[0];
+                Node value_node = def->as_list->items[1];
+
+                Node value = {0};
+                int ret = eval_one(value_node, scopes, &value);
+                if (ret != 0)
+                    return ret;
+
+                ScopeObject obj =
+                    (ScopeObject){.name = key.as_symbol, .node = value};
+                nob_da_append(&scope, obj);
+            }
+
+            Node expression_value = {0};
+            {
+                nob_da_append(scopes, &scope);
+
+                int ret = eval_one(expression, scopes, &expression_value);
+                if (ret != 0) {
+                    return ret;
+                }
+
+                UNUSED(nob_da_pop(scopes));
+            }
+
+            *result = expression_value;
+            return 0;
+        }
+
         Nodes args = {0};
         for (size_t i = 1; i < children->count; i++) {
             Node n = {0};