diff --git a/bin/pre.slime b/bin/pre.slime
index 3b8f203..c815b44 100644
--- a/bin/pre.slime
+++ b/bin/pre.slime
@@ -124,6 +124,17 @@ by the key 'from' and ends with the number defined in 'to'."
   (when (< from to)
       (pair from (range :from (+ 1 from) :to to))))
 
+(defun range-while (:keys from :defaults-to 0 to)
+  "Returns a sequence of numbers starting with the number defined
+by the key 'from' and ends with the number defined in 'to'."
+  (define result (list from))
+  (mutate from (incr from))
+  (while (< from to)
+    (prog
+     (append result (copy from))
+     (mutate from (incr from))))
+  result)
+
 (defun map (fun seq)
   "Takes a function and a sequence as arguments and returns a new
 sequence which contains the results of using the first sequences
diff --git a/src/ast.c b/src/ast.c
index 8fd7c14..90085c1 100644
--- a/src/ast.c
+++ b/src/ast.c
@@ -156,6 +156,7 @@ typedef enum {
     Built_In_Read,
     Built_In_Rest,
     Built_In_Subtraction,
+    Built_In_Try,
     Built_In_Type,
     Built_In_While,
 } Built_In_Name;
@@ -198,6 +199,7 @@ char* Built_In_Name_to_string(Built_In_Name name) {
     case Built_In_Read:           return "read";
     case Built_In_Rest:           return "rest";
     case Built_In_Subtraction:    return "-";
+    case Built_In_Try:            return "try";
     case Built_In_Type:           return "type";
     case Built_In_While:          return "while";
     }
@@ -315,6 +317,7 @@ Ast_Node* create_ast_node_built_in_function(char* name) {
     else if (string_equal(name, "quote"))        type = Built_In_Quote;
     else if (string_equal(name, "read"))         type = Built_In_Read;
     else if (string_equal(name, "rest"))         type = Built_In_Rest;
+    else if (string_equal(name, "try"))          type = Built_In_Try;
     else if (string_equal(name, "type"))         type = Built_In_Type;
     else if (string_equal(name, "while"))        type = Built_In_While;
     else return nullptr;
diff --git a/src/eval.c b/src/eval.c
index 53802f4..bb3d80a 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -640,11 +640,21 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
                 if (arguments_length < 2) {
                     report_error(Error_Type_Wrong_Number_Of_Arguments);
                 }
-                Ast_Node* condition = arguments->value.pair->first;
+                Ast_Node* condition_part = arguments->value.pair->first;
+                Ast_Node* condition;
                 Ast_Node* then_part = arguments->value.pair->rest;
                 Ast_Node* result = create_ast_node_nil();
-                while (eval_expr(condition, env)->type != Ast_Node_Type_Nil) {
-                    result = eval_expr(then_part->value.pair->first, env);
+
+                while (true) {
+                    try {
+                        condition = eval_expr(condition_part, env);
+                    }
+                    if (condition->type == Ast_Node_Type_Nil) {
+                        break;
+                    }
+                    try {
+                        result = eval_expr(then_part->value.pair->first, env);
+                    }
                 }
                 return result;
 
@@ -684,6 +694,27 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
                 }
                 return arguments->value.pair->first;
             }
+            case Built_In_Try: {
+                try {
+                    arguments_length = list_length(arguments);
+                }
+                if (arguments_length != 2) {
+                    report_error(Error_Type_Wrong_Number_Of_Arguments);
+                }
+
+                Ast_Node* try_part   = arguments->value.pair->first;
+                Ast_Node* catch_part = arguments->value.pair->rest->value.pair->first;
+                Ast_Node* result;
+
+                result = eval_expr(try_part, env);
+                if (error) {
+                    delete_error();
+                    try {
+                        result = eval_expr(catch_part, env);
+                    }
+                }
+                return result;
+            }
             case Built_In_Macro_Define:
             case Built_In_Define: {
                 try {