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Unknown::Values - Use 'unknown' values instead of undef ones


version 0.102


    use Unknown::Values;

    my $value = unknown;
    my @array = ( 1, 2, 3, $value, 4, 5 );
    my @less    = grep { $_ < 4 } @array;   # (1,2,3)
    my @greater = grep { $_ > 3 } @array;   # (4,5)

    my @underpaid;
    foreach my $employee (@employees) {
        # this will never return true if salary is "unknown"
        if ( $employee->salary < $threshold ) {
            push @underpaid => $employee;


    use Unknown::Values ':FATAL';
    my $value = unknown;

    if ( 3 < $value ) { ... } # fatal error

    if ( is_unknown $value ) { # not a fatal error


    # see documentation Unknown::Values::Instance::Object
    use Unknown::Values ':OBJECT';    # NULL Object pattern

    my $employee = unknown;

    if ( $employee->salary < $threshold ) {
        # we will never get to here


This code is alpha. Some behavior may change. The module name may change.

This module provides you with two new keywords, unknown and is_unknown.

unknown is conceptually similar to the SQL NULL value. From the point of view of logic, this often an improvement over undef values. Consider the following code, used to give underpaid employees a pay raise:

    foreach my $employee (@employees) {
        if ( $employee->annual_salary < $threshold ) {

Who got a salary increase? Ordinarily that would be:

  • Every employee with a salary less than $threshold.

  • Every employee with an undefined salary.

Why are we giving salary increases to employees whose salary is undefined? Consider the types of employees who might have undefined annual salaries:

  • Unpaid interns

  • Volunteers

  • Hourly employees

    We don't know in advance how many hours a week they will work.

  • CEO

    Maybe it's a private company so his salary is confidential.

  • New employee

    Their salary has not yet been entered in the database.

If, however, the $employee->salary method returns unknown, the comparison will always return false, thus ensuring that anyone with an unknown salary will not have their salary increased.

As another example, consider the following statements:

    my @numbers = ( 1,2,3,4,unknown,5,6,unknown,7 );
    my @less    = grep { $_ < 5 } @numbers; # 1,2,3,4
    my @greater = grep { $_ > 4 } @numbers; # 5,6,7

In other words, unknown comparisons return false because we can't know how they compare to other values. Now replace the above with undef:

    my @numbers = ( 1,2,3,4,undef,5,6,undef,7 );
    my @less    = grep { $_ < 5 } @numbers; # 1,2,3,4,undef,undef
    my @greater = grep { $_ > 4 } @numbers; # undef,5,6,undef,7

In other words, you're probably getting garbage.



    my $value = unknown;

A safer replacement for undef. Conceptually, unknown behaves very similarly to SQL's NULL.

Note that comparisons will return false, but stringification is always a fatal This ensures that you cannot accidentally use unknown values as hash keys or array indices:

    my $unknown = Person->fetch($id);
    print $unknown;             # fatal
    $cache{$unknown}   = $id;   # fatal
    $ordered[$unknown] = $id;   # fatal


    if ( is_unknown $value ) { ... }

Test if a value is unknown. Do not use $value->isa(...) because the class is blessed into is not guaranteed.



Use unknown instead of undef when you don't want the value to default to false.


Test whether a given value is unknown.

    my $value1 = unknown;
    my $value2 = undef;
    my $value3 = 0;
    my $value4 = 1;

    if ( is_unknown $value1 ) {
        ... this is the only one for which this function returns true

Defaults to $_:

    foreach (@things) {
        if ( is_unknown ) {
            # do something

If you have specified use Unknown::Values ':FATAL', this is the only safe use for unknown values. Any other use is fatal.

NULL Objects

If you're a fan of the NULL object pattern, you can do this:

    use Unknown::Values ':OBJECT';

    my $unknown = unknown;
    if ( $unknown->foo->bar->baz > $limit ) {
        # we will never get here

See Unknown::Values::Instance::Object for more information.


unknown values sort to the end of the list, unless you reverse the sort.

    my @sorted = sort { $a <=> $b } ( 4, 1, unknown, 5, unknown, unknown, 7 );
    eq_or_diff \@sorted, [ 1, 4, 5, 7, unknown, unknown, unknown ],
      'Unknown values should sort at the end of the list';
    my @sorted = sort { $b <=> $a } ( 4, 1, unknown, 5, unknown, unknown, 7 );
    eq_or_diff \@sorted, [ unknown, unknown, unknown, 7, 5, 4, 1 ],
      '... but the sort to the front in reverse';

This is a bit arbitrary, but some decision had to be made and I thought that you'd rather deal with known values first:

    my @things = sort @other_things;
    foreach (@things) {
        last if is_unknown;
        # work with known values

Note that if you specify use Unknown::Values 'fatal', sorting an unknown value is fatal.


An unknown value is equal to nothing becuase we don't know what it's value is (duh). This means that if an employee's salary is unknown, the following will not work:

    if ( $employee->salary == unknown ) { # eq fails, too

Use the is_unknown function instead.

    if ( is_unknown $employee->salary ) {

We also assume that inequality fails:

    if ( 6 != unknown ) {
        ... always false
    if ( 'Ovid' ne unknown ) {
        ... always false

Note: That's actually problematic because an unknown value should be equal to itself but not equal to other unknown values. From the standpoint of pure logic, it's wrong, but it's so awfully convenient that we've allowed it. We might revisit this.

Note that if you specify use Unknown::Values 'fatal', testing for equality is fatal.


Attempting to use unknown values in ways that don't make sense is a fatal error (unless you specified use Unknown::Values 'fatal', in which case, using unknown values in any way other than with is_unknown is fatal).

    my $value1;
    $value1 += 1; # results in 1

    my $value2 = unknown;
    $value2 += 1; # fatal

This is a side-effect of not allowing stuff like this if one of these values is unknown.

    my $total = $price * $tax_rate;

If you want += to work, properly initialize the variable to a value:

    my $value = 0;
    $value += 1;


Probably plenty.


Conditional assignment does not work, but THIS IS NOT A BUG!

    my $value = unknown;
    $value ||= 1;   # this is a no-op, as is //=
    $value++;       # fatal!

This is not a bug because we cannot positively state whether $value is true or defined, thus meaning that ||= and //= must both return unknown values. To fix this, either assign a value when you declare the variable:

    my $value = 1;

Or test to see if it's unknown:

    $value = 1 if is_unknown $value;


We follow Kleene's traditional 3VL (three-value logic). See t/logic.t for verification.

Note that if you specify use Unknown::Values 'fatal', all boolean checks with unknown values are fatal. Use is_unknown to test for unknown values.

Logical Negation

    !unknown is unknown

Logical And

    true    && unknown is unknown
    false   && unknown is false
    unknown && unknown is unknown

Logical Or

    true    || unknown is true
    false   || unknown is unknown
    unknown || unknown is unknown


Currently undef has three different coercions: false, zero, or the empty string. Sometimes those are correct, but not always. Further, by design, it doesn't always emit warnings:

    $ perl -Mstrict -Mwarnings -E 'my $foo; say ++$foo'
    $ perl -Mstrict -Mwarnings -E 'my $foo; say $foo + 1'
    Use of uninitialized value $foo in addition (+) at -e line 1.

And because it has no precise definition, undef might mean any of a number of things:

  • The value's not applicable

  • It's not known

  • It's not available

  • It's restricted

  • Something else?

In other words, the behavior of undef is overloaded, its meaning is ambiguous and you are not guaranteed to have warnings if you use it incorrectly.

Now think about SQL's NULL value. It's problematic, but no alternative has taken hold for simple reason: its meaning is clear and its behavior is unambiguous. It states quite clearly that 'if I don't have a value, I will treat that value as "unknown" via a set of well-defined rules'.

An unknown value behaves very much like the SQL NULL. It's behavior is consistent and predictable. It's meaning is unambiguous. If used incorrectly, it's a fatal error.


See also:

This module is an attempt to squeeze three-value logic into Perl, even though it's a bit of an awkward fit. Further, there are several reasons why something could fail to have a value, including "not yet known" (what this module is about), "not applicable" (something the programmer handles explicitly), "privileged" (you can't have the credit card number), an "empty set" (this is not zero), and so on. Empty sets are always equal to one another (there is, technically, only one empty set), but which of the others should be comparable?

<undef == undef> throws a warning, but allows the program to continue. Is throws the warning because it can't know if this comparison is appropriate or not. For the case of unknown values, we explicitly know the comparison is not appropriate and thus we don't allow it.


Should there be a fatal variant which dies even if you try to compare unknown to something else? (Currently, we confess() if we try other, improper operations such as math.


Should the compare() function return an unknown which returns false in booleans? That might be useful when chaining boolean tests.

More importantly, should every unknown return a sequentially different unknown and thus allow me to say that an unknown is equal to itself but not equal to other unknowns? this means that we could do this:

    my $value1 = unknown;
    my $value2 = $value1;

    if ( $value1 == $value2 ) {
        ... always true because it's an instance of a *single* unknown

But that gets confusing because we then have this:

    if ( $value1 == unknown ) {
        ... always false because unknown generates a new unknown

So an unknown sometimes equals unknowns and sometimes doesn't. It only matches an unknown if it's itself. On the surface this actually seems to be correct, except that we then have this:

    if ( ( 6 != $value1 ) == ( 7 != $value1 ) ) {
        ... always false

That has to be false because 6 != $value1 must return a unknown and 7 != $value1 should return a different unknown and their cascaded unknown value should fail. However, the following must be true:

    if ( ( 6 != $value1 ) == ( 6 != $value1 ) ) {
        ... always true!

Because 6 != $value1 should always return the same unknown. Here's why. We assume, for the sake of argument, that the unknown $value1 has a value, but we don't know it. Let's say that value is 4. The above reduces to this:

    if ( ( 6 != 4 ) == ( 6 != 4 ) ) {

Since 6 != 4 is true, we get this:

    if ( 1 == 1 ) {

Ah, but what if <$value1>'s hidden value was actually 6? Then we get this:

    if ( ( 6 != 6 ) == ( 6 != 6 ) ) {

Since 6 != 6 is false, we get this:

    if ( 0 == 0 ) {

In other words, there's a lot of interesting things we could do, but this would likely involve a fair amount of work breaking out the code for each and every operator and ensuring that it's handled correctly.

Of course, this would eat up both memory and performance and certainly be filled with fiddly bugs.


Curtis "Ovid" Poe <>


This software is copyright (c) 2021 by Curtis "Ovid" Poe.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.