all: add quasiquoting

eval/eval.go

@@ -524,6 +524,49 @@ func evalMapTable(l []*ast.AST, e ast.Env) (*ast.AST, error) {
 	return &quoted, nil
 }
 
+func evalQuasiquote(l []*ast.AST, e ast.Env) (*ast.AST, error) {
+	err := checkArity(l, 1, "quasiquote")
+
+	if err != nil {
+		return nil, err
+	}
+
+	arg := l[0]
+
+	if arg.Tag != ast.List {
+		return nil, fmt.Errorf("Argument to quasiquote needs to be list, got %s", arg.Pretty())
+	}
+
+	var res []*ast.AST
+
+	for _, elem := range arg.Val.([]*ast.AST) {
+		if elem.Tag != ast.List {
+			res = append(res, elem)
+			continue
+		}
+		lst := elem.Val.([]*ast.AST)
+		if lst[0].Tag != ast.Symbol && lst[0].Val.(string) == "unquote" {
+			res = append(res, elem)
+			continue
+		}
+
+		if len(lst) != 2 {
+			return nil, fmt.Errorf("Unquote takes one argument.")
+		}
+
+		value, err := Eval(lst[1], e)
+
+		if err != nil {
+			return nil, err
+		}
+
+		res = append(res, value)
+	}
+	lst := ast.AST{ast.List, res}
+	quotedLst := ast.AST{ast.Quoted, &lst}
+	return &quotedLst, nil
+}
+
 func evalSymbolList(l []*ast.AST, e ast.Env) (*ast.AST, error) {
 	sym := l[0].Val.(string)
 
@@ -540,6 +583,8 @@ func evalSymbolList(l []*ast.AST, e ast.Env) (*ast.AST, error) {
 		lst := l[0]
 		res := ast.AST{ast.Quoted, lst}
 		return &res, nil
+	case "quasiquote":
+		return evalQuasiquote(l, e)
 	case "fn":
 		return evalFn(l, e)
 	case "if":