CDKTN: `ForExpression` missing `iteratorContext` and `OperatorExpression` drops falsy right operands

[X-Guardian]

Expected Behavior

1. TerraformIterator.fromComplexList(list, "key").forExpressionForList(expr) should iterate the raw list directly (as it does when used via TerraformDynamicExpression), producing [ for key, val in <list>: <expr> ].

2. Op.gt(someExpr, 0) (and other operators with falsy right-hand operands like 0, false, "") should render correctly, e.g. ${(length(...) > 0)}.

Actual Behavior

1. forExpressionForList on a fromComplexList iterator always produces the map conversion { for key, val in <list>: val.<keyAttr> => val } instead of using the raw list. This causes a Duplicate object key error when multiple items share the same key attribute value.

2. Op.gt(someExpr, 0) renders as ${(length(...) > undefined)} because 0 is falsy, causing a Terraform Unknown variable "undefined" error.

Steps to Reproduce

Bug 1: ForExpression missing iteratorContext

1. Create a data source that returns a list of objects where the key attribute is not unique:

   const data = new DataSource(this, "data", {});
   const it = TerraformIterator.fromComplexList(data.items, "name");

2. Use forExpressionForList to filter the list:

   new Resource(this, "res", {
     value: Fn.element(it.forExpressionForList(`val.id if val.name == "foo"`), 0),
   });

3. Run cdktn synth and observe the generated JSON uses the map conversion:

   element([ for key, val in { for key, val in data.source.data.items: val.name => val }: val.id if key == "foo"], 0)
   

4. Run terraform plan and observe Duplicate object key error.

Bug 2: OperatorExpression drops falsy operands

1. Use any operator with 0, false, or "" as the right operand:

   Op.gt(Fn.lengthOf(someList), 0)

2. Run cdktn synth and observe the output contains > undefined instead of > 0.

Versions

language: typescript
cdktn-cli: 0.22.0
node: v22.16.0
cdktn: 0.22.0
constructs: 10.3.0
jsii: 5.8.9
terraform: 1.5.5
arch: arm64
os: darwin 26.3.1

Migration Context

I used cdktf and saw the same issue there

Upstream cdktf Issue

No response

Providers

N/A

Gist

No response

Possible Solutions

Both fixes are in packages/cdktn/lib/tfExpression.ts:

Bug 1: ForExpression.resolve() does not set iteratorContext

DynamicListTerraformIterator._getForEachExpression() returns a Lazy that checks context.iteratorContext — returning the raw list for "FOR_EXPRESSION" and the map conversion for the default case. TerraformDynamicExpression.resolve() correctly sets context.iteratorContext = "FOR_EXPRESSION" before resolving, but ForExpression.resolve() does not.

Fix: Add context.iteratorContext = "FOR_EXPRESSION" in ForExpression.resolve():

  public resolve(context: IResolveContext): string {
    const suppressBraces = context.suppressBraces;
    context.suppressBraces = true;
+   context.iteratorContext = "FOR_EXPRESSION";
    const key = this.resolveArg(context, FOR_EXPRESSION_KEY);
    const value = this.resolveArg(context, FOR_EXPRESSION_VALUE);
    const input = this.resolveArg(context, this.input);

Bug 2: OperatorExpression.resolve() uses truthiness check instead of undefined check

Line 325 uses this.right ? ... : undefined which treats all falsy values (0, false, "") as missing. The - operator branch (line 337) has the same bug.

Fix:

- const right = this.right ? this.resolveArg(context, this.right) : undefined;
+ const right = this.right !== undefined ? this.resolveArg(context, this.right) : undefined;

  case "-": {
-   if (right) {
+   if (right !== undefined) {
      // subtraction
      expr = `(${left} - ${right})`;
    } else {

Workarounds

• Bug 1: Use raw HCL expression strings instead of forExpressionForList:

  containerRepoId: `\${[for c in ${data.fqn}.items : c.id if c.name == "foo"][0]}`,

• Bug 2: Avoid using Op with falsy literal values. Use raw HCL strings for conditions instead.

Anything Else?

No response

References

• CDKTF Iterators documentation
• Terraform for Expressions

Help Wanted

• I'm interested in contributing a fix myself

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