Improve selection of arbitrary variant

2 files changed, 52 insertions, 2 deletions

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedTypeLevelExpressionNode.java b/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedTypeLevelExpressionNode.java
index c2e484c..67f56e6 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedTypeLevelExpressionNode.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedTypeLevelExpressionNode.java
@@ -1,6 +1,6 @@
 package org.zwobble.hobgoblin.compiler.ast.untyped;
 
-public sealed interface UntypedTypeLevelExpressionNode permits org.zwobble.hobgoblin.compiler.ast.untyped.UntypedTypeLevelReferenceNode, org.zwobble.hobgoblin.compiler.ast.untyped.UntypedConstructedTypeNode {
+public sealed interface UntypedTypeLevelExpressionNode permits org.zwobble.hobgoblin.compiler.ast.untyped.UntypedConstructedTypeNode, org.zwobble.hobgoblin.compiler.ast.untyped.UntypedTypeLevelReferenceNode {
     public interface Builder {
         public org.zwobble.hobgoblin.compiler.ast.untyped.UntypedTypeLevelExpressionNode build();
 
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
index 6e09b7d..653da8d 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
@@ -408,7 +408,7 @@ public class JavaTypesGenerator implements Generator {
             }
 
             case SumType sumType -> {
-                var firstVariant = context.variants(sumType).getFirst();
+                var firstVariant = selectArbitraryVariant(sumType, context);
                 yield arbitraryValue(firstVariant, context);
             }
 
@@ -422,6 +422,52 @@ public class JavaTypesGenerator implements Generator {
         };
     }
 
+    private Type selectArbitraryVariant(SumType sumType, Context context) {
+        var seenTypes = new HashSet<Type>();
+        seenTypes.add(sumType);
+        // TODO: better error
+        return context.variants(sumType).stream()
+            .filter(variant -> isNonRecursivelyConstructable(variant, seenTypes, context))
+            .findFirst()
+            .orElseThrow();
+    }
+
+    private boolean isNonRecursivelyConstructable(Type type, Set<Type> seenTypes, Context context) {
+        if (seenTypes.contains(type)) {
+            return false;
+        }
+
+        return switch (type) {
+            case ConstructedNativeType constructedNativeType ->
+                true;
+
+            case SimpleNativeType simpleNativeType ->
+                true;
+
+            case StructType structType -> {
+                var newSeenTypes = new HashSet<>(seenTypes);
+                newSeenTypes.add(type);
+                var result =  context.fieldsOf(structType).stream()
+                    .allMatch(field -> isNonRecursivelyConstructable(field.type(), newSeenTypes, context));
+                yield result;
+            }
+
+            case SumType sumType -> {
+                var newSeenTypes = new HashSet<>(seenTypes);
+                newSeenTypes.add(type);
+                var result = context.variants(sumType).stream()
+                    .anyMatch(variant -> isNonRecursivelyConstructable(variant, seenTypes, context));
+                yield result;
+            }
+
+            case TypeLevelValueType typeLevelValueType ->
+                false;
+
+            case TypeParam typeParam ->
+                false;
+        };
+    }
+
     private String lowerCamelCaseToUpperCamelCase(String name) {
         return name.substring(0, 1).toUpperCase(Locale.ROOT) + name.substring(1);
     }
@@ -437,6 +483,10 @@ public class JavaTypesGenerator implements Generator {
             this.typesInfo = typesInfo;
         }
 
+        public List<Field> fieldsOf(StructType type) {
+            return this.typesInfo.fieldsOf(type);
+        }
+
         public List<SumType> variantOf(StructType type) {
             return this.typesInfo.variantOf(type);
         }