diff --git a/code/compiler/13/definition.cpp b/code/compiler/13/definition.cpp
index c6e69ae..dc36f20 100644
--- a/code/compiler/13/definition.cpp
+++ b/code/compiler/13/definition.cpp
@@ -1,4 +1,5 @@
 #include "definition.hpp"
+#include <cassert>
 #include "error.hpp"
 #include "ast.hpp"
 #include "instruction.hpp"
@@ -62,7 +63,10 @@ void definition_data::insert_constructors() const {
     type_app* return_app = new type_app(std::move(this_type_ptr));
     type_ptr return_type(return_app);
     for(auto& var : vars) {
-        if(var_set.find(var) != var_set.end()) throw 0;
+        if(var_set.find(var) != var_set.end())
+            throw std::runtime_error(
+                    std::string("type variable ") +
+                    var + std::string(" used twice in data type definition."));
         var_set.insert(var);
         return_app->arguments.push_back(type_ptr(new type_var(var)));
     }
@@ -121,8 +125,7 @@ void definition_group::typecheck(type_mgr& mgr, type_env_ptr& env) {
         dependency_graph.add_function(def_defn.second->name);
 
         for(auto& dependency : def_defn.second->nearby_variables) {
-            if(defs_defn.find(dependency) == defs_defn.end())
-                throw 0;
+            assert(defs_defn.find(dependency) != defs_defn.end());
             dependency_graph.add_edge(def_defn.second->name, dependency);
         }
     }
diff --git a/code/compiler/13/env.cpp b/code/compiler/13/env.cpp
index 818cf5d..74b1650 100644
--- a/code/compiler/13/env.cpp
+++ b/code/compiler/13/env.cpp
@@ -1,9 +1,10 @@
 #include "env.hpp"
+#include <cassert>
 
 int env_var::get_offset(const std::string& name) const {
     if(name == this->name) return 0;
-    if(parent) return parent->get_offset(name) + 1;
-    throw 0;
+    assert(parent);
+    return parent->get_offset(name) + 1;
 }
 
 bool env_var::has_variable(const std::string& name) const {
@@ -13,8 +14,8 @@ bool env_var::has_variable(const std::string& name) const {
 }
 
 int env_offset::get_offset(const std::string& name) const {
-    if(parent) return parent->get_offset(name) + offset;
-    throw 0;
+    assert(parent);
+    return parent->get_offset(name) + offset;
 }
 
 bool env_offset::has_variable(const std::string& name) const {
diff --git a/code/compiler/13/main.cpp b/code/compiler/13/main.cpp
index 6f844ca..ec21e2b 100644
--- a/code/compiler/13/main.cpp
+++ b/code/compiler/13/main.cpp
@@ -103,12 +103,12 @@ void output_llvm(llvm_context& ctx, const std::string& filename) {
         std::error_code ec;
         llvm::raw_fd_ostream file(filename, ec, llvm::sys::fs::F_None);
         if (ec) {
-            throw 0;
+            throw std::runtime_error("failed to open object file for writing");
         } else {
             llvm::CodeGenFileType type = llvm::CGFT_ObjectFile;
             llvm::legacy::PassManager pm;
             if (targetMachine->addPassesToEmitFile(pm, file, NULL, type)) {
-                throw 0;
+                throw std::runtime_error("failed to add passes to pass manager");
             } else {
                 pm.run(ctx.module);
                 file.close();
@@ -132,10 +132,13 @@ void gen_llvm(global_scope& scope) {
 
 int main(int argc, char** argv) {
     if(argc != 2) {
-        std::cout << "please enter a file to compile." << std::endl;
+        std::cerr << "please enter a file to compile." << std::endl;
     }
     parse_driver driver(argv[1]);
-    driver.run_parse();
+    if(!driver.run_parse()) {
+        std::cerr << "failed to open file " << argv[1] << std::endl;
+        exit(1);
+    }
 
     type_mgr mgr;
     type_env_ptr env(new type_env);
@@ -154,8 +157,10 @@ int main(int argc, char** argv) {
         scope.compile();
         gen_llvm(scope);
     } catch(unification_error& err) {
-        err.pretty_print(std::cout, driver, mgr);
+        err.pretty_print(std::cerr, driver, mgr);
     } catch(type_error& err) {
-        err.pretty_print(std::cout, driver);
+        err.pretty_print(std::cerr, driver);
+    } catch(std::runtime_error& err) {
+        std::cerr << err.what() << std::endl;
     }
 }
diff --git a/code/compiler/13/parse_driver.hpp b/code/compiler/13/parse_driver.hpp
index 9fd1f97..a12b41b 100644
--- a/code/compiler/13/parse_driver.hpp
+++ b/code/compiler/13/parse_driver.hpp
@@ -26,9 +26,9 @@ struct parse_driver {
     parse_driver(const std::string& file)
         : file_name(file), file_offset(0) {}
 
-    void run_parse() {
+    bool run_parse() {
         file_stream.open(file_name);
-        if(!file_stream.good()) throw 0;
+        if(!file_stream.good()) return false;
         line_offsets.push_back(0);
         yyscan_t scanner;
         scanner_init(this, &scanner);
@@ -36,6 +36,7 @@ struct parse_driver {
         parser();
         scanner_destroy(&scanner);
         read_file = string_stream.str();
+        return true;
     }
 
     int get() {
diff --git a/code/compiler/13/parsed_type.cpp b/code/compiler/13/parsed_type.cpp
index c69e50a..cd73792 100644
--- a/code/compiler/13/parsed_type.cpp
+++ b/code/compiler/13/parsed_type.cpp
@@ -1,15 +1,26 @@
 #include "parsed_type.hpp"
+#include <sstream>
 #include "type.hpp"
 #include "type_env.hpp"
+#include "error.hpp"
 
 type_ptr parsed_type_app::to_type(
         const std::set<std::string>& vars,
         const type_env& e) const {
     auto parent_type = e.lookup_type(name);
-    if(parent_type == nullptr) throw 0;
+    if(parent_type == nullptr)
+        throw type_error(std::string("no such type or type constructor ") + name);
     type_base* base_type;
-    if(!(base_type = dynamic_cast<type_base*>(parent_type.get()))) throw 0;
-    if(base_type->arity != arguments.size()) throw 0;
+    if(!(base_type = dynamic_cast<type_base*>(parent_type.get())))
+        throw type_error(std::string("invalid type ") + name);
+    if(base_type->arity != arguments.size()) {
+        std::ostringstream error_stream;
+        error_stream << "invalid application of type ";
+        error_stream << name;
+        error_stream << "(" << base_type->arity << " arguments expected, ";
+        error_stream << "but " << arguments.size() << " were provided)";
+        throw type_error(error_stream.str());
+    }
 
     type_app* new_app = new type_app(std::move(parent_type));
     type_ptr to_return(new_app);
@@ -22,7 +33,8 @@ type_ptr parsed_type_app::to_type(
 type_ptr parsed_type_var::to_type(
         const std::set<std::string>& vars,
         const type_env& e) const {
-    if(vars.find(var) == vars.end()) throw 0;
+    if(vars.find(var) == vars.end())
+        throw type_error(std::string("the type variable ") + var + std::string(" was not explicitly declared."));
     return type_ptr(new type_var(var));
 }
 
diff --git a/code/compiler/13/type_env.cpp b/code/compiler/13/type_env.cpp
index f42e46a..4c5d3ad 100644
--- a/code/compiler/13/type_env.cpp
+++ b/code/compiler/13/type_env.cpp
@@ -1,5 +1,6 @@
 #include "type_env.hpp"
 #include "type.hpp"
+#include "error.hpp"
 
 void type_env::find_free(const type_mgr& mgr, std::set<std::string>& into) const {
     if(parent != nullptr) parent->find_free(mgr, into);
@@ -61,14 +62,15 @@ void type_env::bind(const std::string& name, type_scheme_ptr t, visibility v) {
 }
 
 void type_env::bind_type(const std::string& type_name, type_ptr t) {
-    if(lookup_type(type_name) != nullptr) throw 0;
+    if(lookup_type(type_name) != nullptr)
+        throw type_error("redefinition of type");
     type_names[type_name] = t;
 }
 
 void type_env::generalize(const std::string& name, const group& grp, type_mgr& mgr) {
     auto names_it = names.find(name);
-    if(names_it == names.end()) throw 0;
-    if(names_it->second.type->forall.size() > 0) throw 0;
+    assert(names_it != names.end());
+    assert(names_it->second.type->forall.size() == 0);
 
     std::set<std::string> free_in_type;
     std::set<std::string> free_in_env;