apple/pkl · error

typeMismatch

Error message

typeMismatch

What it means

Pkl's internal `ApplyVmFunction1Node.executeBoolean` calls a single-argument Pkl function and requires the result to be a Boolean. If the function returns anything else, it throws a `typeMismatch` error against `base::Boolean`. This guards internal call sites (such as predicate/result evaluation in `result`/`eval` paths) that assume a Boolean return value.

Solutions

1. Inspect the function passed at the error location and ensure its body produces a Boolean (`true`/`false` or a comparison).
2. If the function can yield null or other values, wrap the final expression so every branch returns a Boolean.
3. If you intended a transformation rather than a predicate, use the appropriate API (map-style call) instead of one expecting a Boolean result.
4. Add an explicit type assertion on the function's return expression to surface the mismatch closer to its source.

Example fix

// before (pkl)
nums.filter((n) -> n * 2)          // returns Int -> typeMismatch (expected Boolean)

// after
nums.filter((n) -> n % 2 == 0)     // returns Boolean

// pkl
assert(functionResult is Boolean, "predicate must return a Boolean")

function isBooleanResult(f: (Any) -> Any, x: Any): Boolean = f(x) is Boolean

When it happens

Trigger: A single-argument function is invoked through a code path that expects a Boolean result (executeBoolean, reached from `result`/`eval` evaluation) but the function body evaluates to a non-Boolean value, e.g. a lambda used as a predicate returns Int, String, or null.

Common situations: Passing a lambda that returns a number or string where a Boolean predicate is required; a function whose last expression is conditional and one branch yields null; using a mapping function instead of a filter/predicate function; typos like returning the compared values instead of the comparison.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• typeMismatch
• typeMismatch
• typeMismatch
• cannotExportValue
• cannotFlattenCollectionWithNonCollectionElement

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/48fa1a0e4fcca890. Report an issue: GitHub.

Appendix: source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction1Node.java:45

import org.pkl.core.runtime.VmCollection;
import org.pkl.core.runtime.VmFunction;

@NodeChild("functionNode")
@NodeChild("argumentNode")
public abstract class ApplyVmFunction1Node extends ExpressionNode {
  public abstract Object execute(VmFunction function, Object arg1);

  public static ApplyVmFunction1Node create() {
    //noinspection ConstantConditions
    return ApplyVmFunction1NodeGen.create(null, null);
  }

  public final boolean executeBoolean(VmFunction function, Object arg1) {
    var result = execute(function, arg1);
    if (result instanceof Boolean b) return b;

    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder().typeMismatch(result, BaseModule.getBooleanClass()).build();
  }

  public final String executeString(VmFunction function, Object arg1) {
    var result = execute(function, arg1);
    if (result instanceof String string) return string;

    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder().typeMismatch(result, BaseModule.getStringClass()).build();
  }

  public final Long executeInt(VmFunction function, Object arg1) {
    var result = execute(function, arg1);
    if (result instanceof Long l) return l;

    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder().typeMismatch(result, BaseModule.getIntClass()).build();
  }

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