presto-main/src/main/java/io/prestosql/type/FunctionType.java/FunctionType/getDisplayName
class FunctionType:    @Override
    public String getDisplayName()
    {
        ImmutableList<String> names = getTypeParameters().stream()
                .map(Type::getDisplayName)
                .collect(toImmutableList());
        return "function(" + Joiner.on(",").join(names) + ")";
    }