The signature of mapNotNull is wrong.

[HideakiSago]

Problem

kt.dart/lib/src/collection/kt_iterable.dart

Line 1574 in 80402d4

extension RequireNoNullsKtIterableExtension<T> on KtIterable<T?> {

kt.dart/lib/src/collection/kt_iterable.dart

Lines 1646 to 1667 in 80402d4

	/// Returns a list containing the results of applying the given [transform] function
	/// to each element in the original collection.
	KtList<R> mapNotNull<R>(R? Function(T?) transform) {
	final mapped = mapNotNullTo(mutableListOf<R>(), transform);
	// TODO ping dort-lang/sdk team to check type bug
	// When in single line: type "DartMutableList<String>' is not a subtype of type 'Null"
	return mapped;
	}
	/// Applies the given [transform] function to each element in the original collection
	/// and appends only the non-null results to the given [destination].
	C mapNotNullTo<R, C extends KtMutableCollection<R>>(
	C destination, R? Function(T?) transform) {
	for (final item in iter) {
	final result = transform(item);
	if (result != null) {
	destination.add(result);
	}
	}
	return destination;
	}
	}

This causes a build error with the following code:

      listOf("not null").mapNotNull((it) => it.startsWith("not") ? it : null); // error!!
      listOf("not null", null).mapNotNull((it) => it?.startsWith("not") == true ? it : null);

The following code in Kotlin did not cause a build error .

        listOf("not null").mapNotNull { if (it.startsWith("not")) it else null }
        listOf("not null", null).mapNotNull { if (it?.startsWith("not") == true) it else null }

It looks like the signature of RequireNoNullsKtIterableExtension.mapNotNull is wrong.

Suggestion

Quoting the implementation of mapNotNull from kotlin.collections:

/**
 * Returns a list containing only the non-null results of applying the given [transform] function
 * to each element in the original collection.
 * 
 * @sample samples.collections.Collections.Transformations.mapNotNull
 */
public inline fun <T, R : Any> Iterable<T>.mapNotNull(transform: (T) -> R?): List<R> {
    return mapNotNullTo(ArrayList<R>(), transform)
}

/**
 * Applies the given [transform] function to each element in the original collection
 * and appends only the non-null results to the given [destination].
 */
public inline fun <T, R : Any, C : MutableCollection<in R>> Iterable<T>.mapNotNullTo(destination: C, transform: (T) -> R?): C {
    forEach { element -> transform(element)?.let { destination.add(it) } }
    return destination
}

The following code seems fine.

extension _<T> on KtIterable<T> {
  /// Returns a list containing the results of applying the given [transform] function
  /// to each element in the original collection.
  KtList<R> mapNotNull2<R>(R? Function(T) transform) {
    final mapped = mapNotNullTo(mutableListOf<R>(), transform);
    // TODO ping dort-lang/sdk team to check type bug
    // When in single line: type "DartMutableList<String>' is not a subtype of type 'Null"
    return mapped;
  }

  /// Applies the given [transform] function to each element in the original collection
  /// and appends only the non-null results to the given [destination].
  C mapNotNullTo<R, C extends KtMutableCollection<R>>(C destination, R? Function(T) transform) {
    for (final item in iter) {
      final result = transform(item);
      if (result != null) {
        destination.add(result);
      }
    }
    return destination;
  }
}

Note that the code above is an extension of KtIterable<T>.

I thought mapNotNull should be defined in
extension KtIterableExtensions<T> on KtIterable<T> or
extension ChainableKtIterableExtensions<T> on KtIterable<T>.

Version

This problem happened when I upgraded from 0.9.1 to 1.0.0.

   kt_dart: ^0.9.1 -> ^1.0.0

[passsy]

Thanks for bringing this up. The signature of mapNotNull and others works now fine for non-nullable types.

Please update to kt_dart: ^1.1.0