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Test-LectroTest-0.2005
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/ Test::LectroTest::Property

NAME

Test::LectroTest::Property - Specifications of properties that your software must hold

SYNOPSIS

use MyModule;  # provides my_function_to_test

use Test::LectroTest::Generator qw( :common );
use Test::LectroTest::Property qw( Test );
use Test::LectroTest::TestRunner;

my $prop_non_neg = Property {
    ##[ x <- Int, y <- Int ]##
    $t->label("negative") if $x < 0;
    $t->label("odd")      if $x % 2;
    $t->retry             if $y == 0;  # 0 can't be used in test
    my_function_to_test( $x, $y ) >= 0;
}, name => "my_function_to_test output is non-negative";

my $runner = Test::LectroTest::TestRunner->new();
$runner->run_suite(
    $prop_non_neg,
    # ... more properties here ...
);

DESCRIPTION

STOP! If you're just looking for an easy way to write and run unit tests, see Test::LectroTest.

This module allows you to define Properties that can be checked automatically by Test::LectroTest. A Property is a specification of your software's required behavior over a given set of conditions. The set of conditions is given by a generator-binding specification. The required behavior is defined implicitly by a block of code that tests your software's observed behavior for a given set of generated conditions against the expected behavior and either accepts or rejects the observed behavior.

This documentation serves as reference documentation for LectroTest Properties. If you don't understand the basics of Properties yet, see "OVERVIEW" in Test::LectroTest::Tutorial before continuing.

Two ways to create Properties

There are two ways to create a property:

  1. Use the Property function to promote a block of code that contains both a generator-binding specification and a behavior test into a Test::LectroTest::Property object. This is the preferred method. Example:

    my $prop1 = Property {
    ##[ x <- Int ]##
    thing_to_test($x) >= 0;
}, name => "thing_to_test is non-negative";

  2. Use the new method of Test::LectroTest::Property and provide it with the necessary ingredients via named parameters:

    my $prop2 = Test::LectroTest::Property->new(
    inputs => [ x => Int ],
    test   => sub { my ($tcon,$x) = @_; thing_to_test($x) >= 0 },
    name   => "thing_to_test is non-negative"
);

Both are equivalent, but the first is concise, easier to read, and lets LectroTest do some of the heavy lifting for you. The second is probably better, however, if you are constructing property specifications programmatically.

Generator-binding specification

The generator-binding specification declares that certain variables are to be bound to certain kinds of random-value generators during the tests of your software's behavior. The number and kind of generators define the "condition space" that is examined during property checks.

If you use the Property function to create your Test::LectroTest::Property objects, your generator-binding specification must come first in your code block, and you must use the following syntax:

##[ var1 <- gen1, var2 <- gen2, ... ]##

Comments are not allowed within the specification, but you may break it across multiple lines:

##[ var1 <- gen1,
    var2 <- gen2, ...
]##

or

##[
    var1 <- gen1,
    var2 <- gen2, ...
]##

Further, for better integration with syntax-highlighting IDEs, the terminating ]## delimiter may be preceded with a hash symbol # and optional whitespace to make it appear like a comment:

##[
    var1 <- gen1,
    var2 <- gen2, ...
# ]##

On the other hand, if you use Test::LectroTest::Property->new() to create your objects, the generator-binding specification takes the form of an array reference containing variable-generator pairs that is passed to new() as a named parameter:

inputs => [ var1 => gen1, var2 => gen2, ... ]

Normal Perl syntax applies.

Behavior test

The behavior test is a subroutine that accepts a test-controller object and a given set of input conditions, tests your software's observed behavior with respect to the input conditions against the required behavior, and returns true or false to indicate acceptance or rejection. If you are using the Property-function method to create your property objects, lexical variables are declared and loaded automatically per your input-generator specification, so you can just use them immediately:

my $prop = Property {
  ##[ i <- Int, delta <- Float(range=>[0,1]) ]##
  my $lo_val = my_thing_to_test($i);
  my $hi_val = my_thing_to_test($i + $delta);
  $lo_val == $hi_val;
}, name => "my_thing_to_test ignores fractions" ;

On the other hand, if you are using Test::LectroTest::Property->new(), you must declare and initialize these variables manually from Perl's @_ variable in lexically increasing order after receiving $tcon, the test controller object. (This inconvenience, by the way, is why the former method is preferred.) The hard way:

my $prop = Test::LectroTest::Property->new(
  inputs => [ i => Int, delta => Float(range=>[0,1]) ],
  test => sub {
      my ($tcon, $delta, $i) = @_;
      my $lo_val = my_thing_to_test($i);
      my $hi_val = my_thing_to_test($i + $delta);
      $lo_val == $hi_val
  },
  name => "my_thing_to_test ignores fractions"
) ;

Control logic, retries, and labeling

Inside the behavior test, you have access to a special variable $tcon that allows you to interact with the test controller. Through $tcon you can do the following:

  • retry the current trial with different inputs (if you don't like the inputs you were given at first)

  • add a label to the current trial for reporting purposes

For example, let's say that we have written a function my_sqrt that returns the square root of its input. In order to check whether our implementation fulfills the mathematical definition of square root, we might specify the following property:

my $epsilon = 0.000_001;

Property {
    ##[ x <- Float ]##
    return $tcon->retry if $x < 0;
    $tcon->label("less than one") if $x < 1;
    my $sx = my_sqrt( $x );
    abs($sx * $sx - $x) < $epsilon;
}, name => "my_sqrt satisfies defn of square root";

Because we don't want to deal with imaginary numbers, our square-root function is defined only over non-negative numbers. To make sure we don't accidentally check our property "at" a negative number, we use the following line to re-start the trial with a different input should the input we are given at first be negative:

return $tcon->retry if $x < 0;

An interesting fact is that for all values x between zero and one, the square root of x is larger than x itself. Perhaps our implementation treats such values as a special case. In order to be confident that we are checking this case, we added the following line:

$tcon->label("less than one") if $x < 1;

In the property-check output, we can see what percentage of the trials checked this case:

1..1
ok 1 - 'my_sqrt satisfies defn of square root' (1000 attempts)
#   1% less than one

Trivial cases

Random-input generators may create some inputs that are trivial and don't provide much testing value. To make it easy to label such cases, you can use the following from within your behavior tests:

$tcon->trivial if ... ;

The above is exactly equivalent to the following:

$tcon->label("trivial") if ... ;

CAVEAT

The special syntax used to specify generator bindings relies upon a source filter (see Filter::Util::Call). If you don't want to use the syntax, you can disable the filter like so:

use Test::LectroTest::Property qw( NO_FILTER );

LECTROTEST HOME

The LectroTest home is http://community.moertel.com/LectroTest. There you will find more documentation, presentations, a wiki, and other helpful LectroTest-related resources. It's also the best place to ask questions.

AUTHOR

Tom Moertel (tom@moertel.com)

INSPIRATION

The LectroTest project was inspired by Haskell's fabulous QuickCheck module by Koen Claessen and John Hughes: http://www.cs.chalmers.se/~rjmh/QuickCheck/.

COPYRIGHT and LICENSE

Copyright (c) 2004 by Thomas G Moertel. All rights reserved.

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