presto-main/src/main/java/io/prestosql/operator/scalar/MapTransformKeyFunction.java/MapTransformKeyFunction/MapTransformKeyFunction
class MapTransformKeyFunction:    private MapTransformKeyFunction()
    {
        super(new FunctionMetadata(
                new Signature(
                        "transform_keys",
                        ImmutableList.of(typeVariable("K1"), typeVariable("K2"), typeVariable("V")),
                        ImmutableList.of(),
                        mapType(new TypeSignature("K2"), new TypeSignature("V")),
                        ImmutableList.of(
                                mapType(new TypeSignature("K1"), new TypeSignature("V")),
                                functionType(new TypeSignature("K1"), new TypeSignature("V"), new TypeSignature("K2"))),
                        false),
                false,
                ImmutableList.of(
                                        new FunctionArgumentDefinition(false),
                                        new FunctionArgumentDefinition(false)),
                false,
                false,
                "apply lambda to each entry of the map and transform the key",
                SCALAR));
    }