From 7f0b4097352f429c31526c31b281c78b7ba4d8c9 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sun, 2 Aug 2026 11:54:31 +0100
Subject: Generate proper errors when arbitrary value cannot be found

---
 .../compiler/analysis/ArbitraryValueAnalysis.java      | 15 ++++++++++++---
 .../analysis/errors/SumTypeHasNoVariantsError.java     | 18 ++++++++++++++++++
 .../compiler/analysis/errors/TypeIsInfiniteError.java  | 18 ++++++++++++++++++
 3 files changed, 48 insertions(+), 3 deletions(-)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/SumTypeHasNoVariantsError.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/TypeIsInfiniteError.java

(limited to 'src/main')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/analysis/ArbitraryValueAnalysis.java b/src/main/java/org/zwobble/hobgoblin/compiler/analysis/ArbitraryValueAnalysis.java
index c1d7cc3..ecfdd76 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/analysis/ArbitraryValueAnalysis.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/analysis/ArbitraryValueAnalysis.java
@@ -1,5 +1,8 @@
 package org.zwobble.hobgoblin.compiler.analysis;
 
+import org.zwobble.hobgoblin.compiler.analysis.errors.SumTypeHasNoVariantsError;
+import org.zwobble.hobgoblin.compiler.analysis.errors.TypeIsInfiniteError;
+import org.zwobble.hobgoblin.compiler.sources.NullSource;
 import org.zwobble.hobgoblin.compiler.typechecker.TypesInfo;
 import org.zwobble.hobgoblin.compiler.types.*;
 
@@ -9,13 +12,19 @@ import java.util.Set;
 
 public class ArbitraryValueAnalysis {
     public static Type selectArbitraryVariant(SumType sumType, TypesInfo typesInfo) {
+        var sumVariants = typesInfo.sumVariants(sumType);
+        if (sumVariants.isEmpty()) {
+            // TODO: proper source
+            throw new SumTypeHasNoVariantsError(sumType, NullSource.INSTANCE);
+        }
+
         var seenTypes = new HashSet<Type>();
         seenTypes.add(sumType);
-        // TODO: better error
-        return typesInfo.sumVariants(sumType).stream()
+        return sumVariants.stream()
             .filter(variant -> isNonRecursivelyConstructable(variant.valueType(), seenTypes, typesInfo))
             .findFirst()
-            .orElseThrow()
+            // TODO: proper source
+            .orElseThrow(() -> new TypeIsInfiniteError(sumType, NullSource.INSTANCE))
             .containerType();
     }
 
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/SumTypeHasNoVariantsError.java b/src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/SumTypeHasNoVariantsError.java
new file mode 100644
index 0000000..7624767
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/SumTypeHasNoVariantsError.java
@@ -0,0 +1,18 @@
+package org.zwobble.hobgoblin.compiler.analysis.errors;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+import org.zwobble.hobgoblin.compiler.types.SumType;
+
+public class SumTypeHasNoVariantsError extends SourceError {
+    private final SumType type;
+
+    public SumTypeHasNoVariantsError(SumType type, Source source) {
+        super(type + " has no variants", source);
+        this.type = type;
+    }
+
+    public SumType type() {
+        return type;
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/TypeIsInfiniteError.java b/src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/TypeIsInfiniteError.java
new file mode 100644
index 0000000..87e96cc
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/analysis/errors/TypeIsInfiniteError.java
@@ -0,0 +1,18 @@
+package org.zwobble.hobgoblin.compiler.analysis.errors;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+import org.zwobble.hobgoblin.compiler.types.Type;
+
+public class TypeIsInfiniteError extends SourceError {
+    private final Type type;
+
+    public TypeIsInfiniteError(Type type, Source source) {
+        super(type.describe() + " is infinite", source);
+        this.type = type;
+    }
+
+    public Type type() {
+        return type;
+    }
+}
-- 
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