From e337992410a9400ceef9b281795a27a2eb024704 Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Wed, 9 Sep 2020 15:26:04 -0700
Subject: [PATCH] Add sources for unification type errors.

---
 code/compiler/13/ast.cpp  | 24 ++++++++++++++++--------
 code/compiler/13/type.cpp | 12 ++++++------
 code/compiler/13/type.hpp |  4 +++-
 3 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/code/compiler/13/ast.cpp b/code/compiler/13/ast.cpp
index 10db5cb..3f6b9f1 100644
--- a/code/compiler/13/ast.cpp
+++ b/code/compiler/13/ast.cpp
@@ -99,11 +99,19 @@ type_ptr ast_binop::typecheck(type_mgr& mgr, type_env_ptr& env) {
     type_ptr ftype = env->lookup(op_name(op))->instantiate(mgr);
     if(!ftype) throw type_error(std::string("unknown binary operator ") + op_name(op), loc);
 
-    type_ptr return_type = mgr.new_type();
-    type_ptr arrow_one = type_ptr(new type_arr(rtype, return_type));
-    type_ptr arrow_two = type_ptr(new type_arr(ltype, arrow_one));
+    // For better type errors, we first require binary function,
+    // and only then unify each argument. This way, we can
+    // precisely point out which argument is "wrong".
 
-    mgr.unify(arrow_two, ftype);
+    type_ptr return_type = mgr.new_type();
+    type_ptr second_type = mgr.new_type();
+    type_ptr first_type = mgr.new_type();
+    type_ptr arrow_one = type_ptr(new type_arr(second_type, return_type));
+    type_ptr arrow_two = type_ptr(new type_arr(first_type, arrow_one));
+
+    mgr.unify(ftype, arrow_two, loc);
+    mgr.unify(first_type, ltype, left->loc);
+    mgr.unify(second_type, rtype, right->loc);
     return return_type;
 }
 
@@ -140,7 +148,7 @@ type_ptr ast_app::typecheck(type_mgr& mgr, type_env_ptr& env) {
 
     type_ptr return_type = mgr.new_type();
     type_ptr arrow = type_ptr(new type_arr(rtype, return_type));
-    mgr.unify(arrow, ltype);
+    mgr.unify(arrow, ltype, left->loc);
     return return_type;
 }
 
@@ -190,7 +198,7 @@ type_ptr ast_case::typecheck(type_mgr& mgr, type_env_ptr& env) {
         type_env_ptr new_env = type_scope(env);
         branch->pat->typecheck(case_type, mgr, new_env);
         type_ptr curr_branch_type = branch->expr->typecheck(mgr, new_env);
-        mgr.unify(branch_type, curr_branch_type);
+        mgr.unify(curr_branch_type, branch_type, branch->expr->loc);
     }
 
     input_type = mgr.resolve(case_type, var);
@@ -361,7 +369,7 @@ type_ptr ast_lambda::typecheck(type_mgr& mgr, type_env_ptr& env) {
         full_type = type_ptr(new type_arr(std::move(param_type), full_type));
     }
 
-    mgr.unify(return_type, body->typecheck(mgr, var_env));
+    mgr.unify(return_type, body->typecheck(mgr, var_env), body->loc);
     return full_type;
 }
 
@@ -433,5 +441,5 @@ void pattern_constr::typecheck(type_ptr t, type_mgr& mgr, type_env_ptr& env) con
         constructor_type = arr->right;
     }
 
-    mgr.unify(t, constructor_type);
+    mgr.unify(constructor_type, t, loc);
 }
diff --git a/code/compiler/13/type.cpp b/code/compiler/13/type.cpp
index c4ff35a..856c463 100644
--- a/code/compiler/13/type.cpp
+++ b/code/compiler/13/type.cpp
@@ -109,7 +109,7 @@ type_ptr type_mgr::resolve(type_ptr t, type_var*& var) const {
     return t;
 }
 
-void type_mgr::unify(type_ptr l, type_ptr r) {
+void type_mgr::unify(type_ptr l, type_ptr r, const std::optional<yy::location>& loc) {
     type_var *lvar, *rvar;
     type_arr *larr, *rarr;
     type_base *lid, *rid;
@@ -126,26 +126,26 @@ void type_mgr::unify(type_ptr l, type_ptr r) {
         return;
     } else if((larr = dynamic_cast<type_arr*>(l.get())) &&
             (rarr = dynamic_cast<type_arr*>(r.get()))) {
-        unify(larr->left, rarr->left);
-        unify(larr->right, rarr->right);
+        unify(larr->left, rarr->left, loc);
+        unify(larr->right, rarr->right, loc);
         return;
     } else if((lid = dynamic_cast<type_base*>(l.get())) &&
             (rid = dynamic_cast<type_base*>(r.get()))) {
         if(lid->name == rid->name && lid->arity == rid->arity) return;
     } else if((lapp = dynamic_cast<type_app*>(l.get())) &&
             (rapp = dynamic_cast<type_app*>(r.get()))) {
-        unify(lapp->constructor, rapp->constructor);
+        unify(lapp->constructor, rapp->constructor, loc);
         auto left_it = lapp->arguments.begin();
         auto right_it = rapp->arguments.begin();
         while(left_it != lapp->arguments.end() &&
                 right_it != rapp->arguments.end()) {
-            unify(*left_it, *right_it);
+            unify(*left_it, *right_it, loc);
             left_it++, right_it++;
         }
         return;
     }
 
-    throw unification_error(l, r);
+    throw unification_error(l, r, loc);
 }
 
 type_ptr type_mgr::substitute(const std::map<std::string, type_ptr>& subst, const type_ptr& t) const {
diff --git a/code/compiler/13/type.hpp b/code/compiler/13/type.hpp
index 3ab4714..0edae1c 100644
--- a/code/compiler/13/type.hpp
+++ b/code/compiler/13/type.hpp
@@ -4,6 +4,8 @@
 #include <string>
 #include <vector>
 #include <set>
+#include <optional>
+#include "location.hh"
 
 struct type_mgr;
 
@@ -88,7 +90,7 @@ struct type_mgr {
     type_ptr new_type();
     type_ptr new_arrow_type();
 
-    void unify(type_ptr l, type_ptr r);
+    void unify(type_ptr l, type_ptr r, const std::optional<yy::location>& loc = std::nullopt);
     type_ptr substitute(
             const std::map<std::string, type_ptr>& subst,
             const type_ptr& t) const;