From d024723fb80ed28e6eea333f83481397a0cba7ea Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Fri, 14 Jul 2023 21:09:42 +0100
Subject: Type if expression properly

---
 .../java/org/shedlang/compiler/typechecker/TypeChecker.java  |  9 +++++++--
 src/main/java/org/shedlang/compiler/types/Types.java         | 12 ++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

(limited to 'src/main/java')

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index b6233c04..359c49d1 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -497,11 +497,16 @@ public class TypeChecker {
         var typedElseBody = typedElseBodyResult.value();
         context = typedElseBodyResult.context();
 
+        var branchTypes = Stream.concat(
+            typedConditionalBranches.stream()
+                .map(conditionalBranch -> conditionalBranch.body().type()),
+            Stream.of(typedElseBody.type())
+        ).toList();
+
         var typed = new TypedIfNode(
             typedConditionalBranches,
             typedElseBody,
-            // TODO: consider type of conditional branches
-            typedElseBody.type(),
+            Types.commonSupertype(branchTypes),
             untyped.source()
         );
 
diff --git a/src/main/java/org/shedlang/compiler/types/Types.java b/src/main/java/org/shedlang/compiler/types/Types.java
index f192c98e..92ae294d 100644
--- a/src/main/java/org/shedlang/compiler/types/Types.java
+++ b/src/main/java/org/shedlang/compiler/types/Types.java
@@ -1,5 +1,8 @@
 package org.shedlang.compiler.types;
 
+import java.util.HashSet;
+import java.util.List;
+
 public class Types {
     private Types() {
     }
@@ -18,4 +21,13 @@ public class Types {
     public static Type effectType(Effect effect) {
         return new TypeLevelValueType(effect);
     }
+
+    public static Type commonSupertype(List<Type> subtypes) {
+        var uniqueSubtypes = new HashSet<>(subtypes);
+        if (uniqueSubtypes.size() == 1) {
+            return subtypes.get(0);
+        } else {
+            return TOP;
+        }
+    }
 }
-- 
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