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Add error hint when accessing the default value of a union type with no default #1369

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Add error hint when accessing the default value of a union type with no default #1369

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43e9f2e
Add error hint when accessing the default value of a union type with …
HT154 Dec 14, 2025
7c1aede
Handle unions with default element but still no default value
HT154 Dec 15, 2025
5bb88d3
Update error message
HT154 Dec 15, 2025
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29 changes: 27 additions & 2 deletions pkl-core/src/main/java/org/pkl/core/ast/member/DefaultPropertyBodyNode.java
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
* Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
Expand All @@ -19,6 +19,9 @@
import com.oracle.truffle.api.frame.VirtualFrame;
import com.oracle.truffle.api.source.SourceSection;
import org.pkl.core.ast.ExpressionNode;
import org.pkl.core.ast.type.TypeNode.ConstrainedTypeNode;
import org.pkl.core.ast.type.TypeNode.TypeAliasTypeNode;
import org.pkl.core.ast.type.TypeNode.UnionTypeNode;
import org.pkl.core.runtime.Identifier;
import org.pkl.core.runtime.VmUtils;
import org.pkl.core.util.Nullable;
Expand Down Expand Up @@ -50,8 +53,30 @@ public Object executeGeneric(VirtualFrame frame) {
}

CompilerDirectives.transferToInterpreter();

// attempt to give a hint when a union type is missing a default
String aliasName = null;
String unionTypeSource = null;
if (typeNode != null) {
var tn = typeNode.getTypeNode();
while (true) {
if (tn instanceof TypeAliasTypeNode typeAlias) {
aliasName = typeAlias.getVmTypeAlias().getSimpleName();
tn = typeAlias.getVmTypeAlias().getTypeNode();
} else if (tn instanceof ConstrainedTypeNode constrained) {
tn = constrained.getBaseTypeNode();
} else {
break;
}
}
if (tn instanceof UnionTypeNode union) {
unionTypeSource = union.getSourceSection().getCharacters().toString();
}
}

throw exceptionBuilder()
.undefinedPropertyValue(propertyName, VmUtils.getReceiver(frame))
.undefinedPropertyValue(
propertyName, VmUtils.getReceiver(frame), unionTypeSource, aliasName)
.build();
}
}
4 changes: 4 additions & 0 deletions pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java
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Original file line number Diff line number Diff line change
Expand Up @@ -2598,6 +2598,10 @@ public Object executeAndSet(VirtualFrame frame, Object value) {
return childNode.createDefaultValue(language, headerSection, qualifiedName);
}

public TypeNode getBaseTypeNode() {
return childNode;
}

public SourceSection getBaseTypeSection() {
return childNode.getSourceSection();
}
Expand Down
14 changes: 12 additions & 2 deletions pkl-core/src/main/java/org/pkl/core/runtime/VmExceptionBuilder.java
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
* Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
Expand Down Expand Up @@ -124,9 +124,19 @@ public VmExceptionBuilder undefinedValue() {
return withExternalMessage("undefinedValue");
}

public VmExceptionBuilder undefinedPropertyValue(Identifier propertyName, Object receiver) {
public VmExceptionBuilder undefinedPropertyValue(
Identifier propertyName,
Object receiver,
@Nullable String unionTypeSource,
@Nullable String typeAliasName) {
kind = VmException.Kind.UNDEFINED_VALUE;
this.receiver = receiver;
if (unionTypeSource != null && typeAliasName != null) {
return withExternalMessage(
"undefinedPropertyValueUnionAlias", propertyName, unionTypeSource, typeAliasName);
} else if (unionTypeSource != null) {
return withExternalMessage("undefinedPropertyValueUnion", propertyName, unionTypeSource);
}
return withExternalMessage("undefinedPropertyValue", propertyName);
}

Expand Down
8 changes: 8 additions & 0 deletions pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
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Original file line number Diff line number Diff line change
Expand Up @@ -436,6 +436,14 @@ Undefined value.
undefinedPropertyValue=\
Tried to read property `{0}` but its value is undefined.

undefinedPropertyValueUnion=\
Tried to read property `{0}` but its value is undefined.\n\
Union type `{1}` has no default member.

undefinedPropertyValueUnionAlias=\
Tried to read property `{0}` but its value is undefined.\n\
Union type `{1}` (from type alias `{2}`) has no default member.

cannotFindModule=\
Cannot find module `{0}`.

Expand Down
3 changes: 3 additions & 0 deletions pkl-core/src/test/files/LanguageSnippetTests/input/errors/noDefault3.pkl
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@@ -0,0 +1,3 @@
local typealias B = Listing<String> | String

b: B
4 changes: 4 additions & 0 deletions pkl-core/src/test/files/LanguageSnippetTests/input/errors/noDefault4.pkl
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@@ -0,0 +1,4 @@
local typealias B = Listing<String> | String
local typealias C = B(true)

c: C
1 change: 1 addition & 0 deletions pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault2.err
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
–– Pkl Error ––
Tried to read property `foo` but its value is undefined.
Union type `Foo|"baz"` has no default member.

x | foo: Foo|"baz"
^^^
Expand Down
17 changes: 17 additions & 0 deletions pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault3.err
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Original file line number Diff line number Diff line change
@@ -0,0 +1,17 @@
–– Pkl Error ––
Tried to read property `b` but its value is undefined.
Union type `Listing<String> | String` (from type alias `B`) has no default member.
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@bioball bioball Dec 15, 2025 •
edited
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This message doesn't explain what "Default member" is. If we want this as the error message, probably should have a suggestion of how to mark a union default member.

But, this error message feels a little strange to me for a couple reasons. This throws too:

foo: Int | Boolean | String

But having a default member doesn't help, because none of these types have default values.

Also, this error message feels oddly specific; the issue here is that the property doesn't have a default value.
There's no default value because:

  1. The class property has no declared default value
  2. The property's declared type has no default value
  3. The object instance did not assign to this property

So there's three possible resolutions:

  1. Add an explicit default in the class property (only if this is modifiable code)
  2. Change the type to have an explicit default (only if one of the member types has a default value, and this is modifiable code)
  3. Define the property in your instance

I think this current error message would probably be misleading too often.

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@HT154 HT154 Dec 15, 2025

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I updated the condition here to check that the union has no default member and updated the message a bit. How does it look now?

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@bioball bioball Jan 6, 2026

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It still feels odd to me:

  1. Adding a default marker doesn't do anything for union types like Int | Boolean | String
  2. The code might not be modifiable, so we shouldn't have verbage that suggests updating the type as a fix.

To address #1368, it feels like the error message should be something like this?

Cannot instantiate type `Listing<String> | String` because it has no default value.

But we'd really only want to show this error message when we're trying to amend it with an object body.


x | b: B
^
at noDefault3#b (file:///$snippetsDir/input/errors/noDefault3.pkl)

The above error occurred when rendering path `b` of module `file:///$snippetsDir/input/errors/noDefault3.pkl`.

xxx | renderer.renderDocument(value)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
at pkl.base#Module.output.text (pkl:base)

xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
^^^^
at pkl.base#Module.output.bytes (pkl:base)
17 changes: 17 additions & 0 deletions pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault4.err
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Original file line number Diff line number Diff line change
@@ -0,0 +1,17 @@
–– Pkl Error ––
Tried to read property `c` but its value is undefined.
Union type `Listing<String> | String` (from type alias `B`) has no default member.

x | c: C
^
at noDefault4#c (file:///$snippetsDir/input/errors/noDefault4.pkl)

The above error occurred when rendering path `c` of module `file:///$snippetsDir/input/errors/noDefault4.pkl`.

xxx | renderer.renderDocument(value)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
at pkl.base#Module.output.text (pkl:base)

xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
^^^^
at pkl.base#Module.output.bytes (pkl:base)
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