Sizing guidance

The purpose of sizing is to allow LectroTest to generate simple values at first and then, as testing progresses, to slowly ramp up the complexity. In this way, counterexamples for obvious problems will be easier for you to understand.

Generators

Each generator has a "generate" method that you can call to extract a new, random value from the generator.

Int

    my $gen = Int( range=>[0,9], sized=>0 );
    

Creates a generator for integer values, by default in the range [-32768,32767], inclusive, but this can be changed via the optional range modifier.

Float

    my $gen = Float( range=>[-2.0,2.0], sized=>1 );
    

Creates a generator for floating-point values, by default in the range [-32768.0,32768.0), but this can be changed via the optional range modifier. By default Float generators are sized.

Bool

    my $gen = Bool;
    

Creates a generator for boolean values: 0 for false, 1 for true. The generator ignores sizing guidance.

Char

    my $gen = Char( charset=>"A-Za-z0-9_" );
    

Creates a generator for characters. By default the characters are in the ASCII range [0,127], inclusive, but this behavior can be changed with the charset modifier:

List(elemgen)

    my $gen = List( Bool, length=>[1,10] );
    

Creates a generator for lists (which are returned as array refs). The elements of the lists are generated by the generator given as elemgen. The lengths of the generated lists are constrained by sizing guidance at the time of generation. You can override the default sizing behavior using the optional length modifier:

When the list generator calls the element generator, it divides the sizing guidance by the length of the list. For example, if the list being generated will have 7 elements, when the list generator calls the element generator to generate each element, it will scale the sizing guidance by 1/7. In this way the sizing guidance provides a rough constraint on the total number of elements produced, regardless of the depth of the list structure being generated.

Hash(keygen, valgen)

    my $gen = Hash( String( charset=>"A-Z", length=>3 ),
                    Float( range=>[0.0, 100.0] );
    

Creates a generator for hashes (which are returned as hash refs). The keys of the hash are generated by the generator given as keygen, and the values are generated by the generator valgen.

The Hash generator takes an optional length modifier that specifies the desired hash length (= number of keys):

String

    my $gen = String( length=>[3,], charset=>"A-Z" );
    

Creates a generator for strings. By default the strings will be drawn from the ASCII character set (0 through 127) and be of length constrained by the sizing factor. Both defaults can be changed using modifiers:

Elements(e1, e2, ...)

    my $gen = Elements( "alpha", "beta", "gamma" );
    

Creates a generator that chooses among the given elements e1, e2, ... with equal probability. Each call to the "generate" method will return one of the element values. Sizing guidance has no effect on this generator.

Note: This generator builder does not accept modifiers. If you pass any, they will be interpreted as elements to be added to the pool from which the generator randomly selects, which is probably not what you want.

Unit(e)

    my $gen = Unit( "alpha" );
    

Creates a generator that always returns the value e. Not too useful on its own but can be handy as a building block for combinators to chew on. Naturally, sizing guidance has no effect on this generator.

Note: This generator builder does not accept modifiers.

Generator combinators

Paste(gens..., glue=>str)

    my $gen = Paste( (String(charset=>"0-9",length=>4)) x 4,
                     glue => " " );
    # gens credit-card numbers like "4592 9459 9023 1369"

    my $lgen = Paste( List( String(charset=>"0-9",length=>4)
                          , length=>4 ), glue => " " );
    # another way of doing the same
    

Creates a combined generator that generates values by joining the values generated by each of the supplied sub-generators gens. (Generated list values will have their elements "flattened" into the rest of the generated results before joining.) The resulting string is returned.

The values are joined using the given glue string str. If no glue modifier is provided, the default glue is the empty string.

The sizing guidance given to the combined generator will be passed unchanged to each of the sub-generators.

OneOf(gens...)

    my $gen = OneOf( Unit(0), List(Int,length=>3) );
    # generates scalar 0 or a 3-element list of integers
    

Creates a combined generator that generates each value by selecting at random (with equal probability) one of the sub-generators in gens and using that generator to generate the output value.

The sizing guidance given to the combined generator will be passed unchanged to the selected sub-generator.

Note: This combinator does not accept modifiers.

Frequency([freq1, gen1], [freq2, gen2], ...)

    my $gen = Frequency( [50, Unit("common"     )],
                         [35, Unit("less common")],
                         [15, Unit("uncommon"   )] );
    # generates one of "common", "less common", or
    # "uncommon" with respective probabilities
    # 50%, 35%, and 15%.
    

Creates a combined generator that generates each value by selecting at random one of the generators gen1 or gen2 or ... and using that generator to generate the output value. Each generator is selected with probability proportional to its associated frequency. (If all of the given frequencies are the same, the Frequency combinator effectively becomes OneOf.) The frequencies can be any non-negative numerical values you want and will be normalized to a 0-to-1 scale internally. At least one frequency must be greater than zero.

The sizing guidance given to the combined generator will be passed unchanged to the selected sub-generator.

Note: This combinator does not accept modifiers.

Each(gens...)

    my $gen = Each( Unit(1), Unit("X") );
    # always generates [ 1, "X" ]
    

Creates a generator that returns a list (array ref) whose successive elements are the successive values generated by the given generators gens.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: This combinator does not accept modifiers.

(Note for technical buffs: "Each(...)" is exactly equivalent to "List(..., length=>1)").

Apply(fn, gens...)

    my $gen = Apply( sub { $_[0] x $_[1] }
                   , Unit("X"), Unit(4) );
    # always generates "XXXX"
    

Creates a generator that applies the given function fn to arguments generated from each of the given sub-generators gens and returns the resulting value. Each sub-generator contributes one value, and the values are passed to fn as arguments in the same order as the sub-generators were given to Apply.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: The function fn is always evaluated in scalar context. If you need to generate an array, return it as an array reference.

Note: This combinator does not accept modifiers.

Map(fn, gens...)

    my $gen = Map( sub { "X" x $_[0] }
                 , Unit(4), Unit(3), Unit(0) );
    # always generates [ "XXXX", "XXX", "" ]
    

Creates a generator that applies the given function fn to the values generated by the given generators gen one at a time and returns a list (array ref) whose elements are each of the successive results.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: The function fn is always evaluated in scalar context. If you need to generate an array, return it as an array reference.

Note: This combinator does not accept modifiers.

Concat(gens...)

    my $gen = Concat( List( Unit(1),   length=>3 )
                    , List( Unit("x"), length=>1 ) );
    # always generates [1, 1, 1, "x"]
    

Creates a generator that concatenates the values generated by each of its sub-generators, resulting in a list (which is returned as a array reference). The values returned by the sub-generators are expected to be lists (array refs). If a sub-generator returns a scalar value, it will be treated like a single-element list that contains the value.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: If a sub-generator returns something other than a list or scalar, you will get a run-time error.

Note: This combinator does not accept modifiers.

Flatten(gens...)

    my $gen = Flatten( Unit( [[[[[[ 1 ]]]]]] ) );
    # generates [1]
    

Flatten is just like Concat except that it recursively flattens any sublists generated by the generators gen and then concatenates them to generate a final a list of depth one, regardless of the depth of any sublists.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: If a sub-generator returns something other than a list or scalar, you will get a run-time error.

Note: This combinator does not accept modifiers.

ConcatMap(fn, gens)

    sub take_odds { my $x = shift;
                    $x % 2 ? [$x] : [] }
    my $gen = ConcatMap( \&take_odds
                       , Unit(1), Unit(2), Unit(3) );
    # generates [1, 3]
    

Creates a generator that applies the function fn to each of the values generated by the given generators gen in turn, and then concatenates the results.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: The function fn is always evaluated in scalar context. If you need to generate an array, return it as an array reference.

Note: If a sub-generator returns something other than a list or scalar, you will get a run-time error.

Note: This combinator does not accept modifiers.

FlattenMap(fn, gens)

    my $gen = FlattenMap( sub { [ ($_[0]) x 3 ] }
                        , Unit([1]), Unit([[2]]) );
    # generates [1, 1, 1, 2, 2, 2]
    

Creates a generator that applies the function fn to each of the values generated by the given generators gen in turn, and then flattens and concatenates the results.

The sizing guidance given to the combined generator will be passed unchanged to each sub-generator.

Note: The function fn is always evaluated in scalar context. If you need to generate an array, return it as an array reference.

Note: If a sub-generator returns something other than a list or scalar, you will get a run-time error.

Note: This combinator does not accept modifiers.

Sized(fn, gen)

    my $gen = Sized { 2 * $_[0] } List(Int);
        # ^ magnify sizing guidance by factor of two
    my $gen2 = Sized { 10 } Int;
        # ^ use constant guidance of 10
    

Creates a generator that adjusts sizing guidance by passing it through the function fn. Then it calls the generator gen with the adjusted guidance and returns the result.

Note: This combinator does not accept modifiers.

Rolling your own generators

The easiest way to create a generator is by using the magic function "Gen". It promotes a block of code into a generator. For example, here's a home-brew generator for times in ctime(3) format that is built on top of an Int generator:

  use Test::LectroTest::Generator qw( :common Gen );

  my $time_gen = Int(range=>[0, 2_147_483_647], sized=>0);
  my $ctime_gen = Gen {
      scalar localtime $time_gen->generate( @_ );
  };

  print($ctime_gen->generate($_), "\n") for 1..5;
  # Fri Jun  2 18:13:21 1978
  # Thu Mar 28 00:55:51 1974
  # Wed Mar 26 06:41:09 2025
  # Sun Sep 11 15:39:44 2016
  # Fri Dec 26 00:39:31 1975

Alternatively, we could build the generator using the Apply combinator:

  my $ctime_gen2 = Apply { localtime $_[0] } $time_gen;

Note: "Gen" is not exported into your code's namespace by default. If you want to use it, you must import it by name or import ":all" when you use this module.

Simple examples

 use strict;
 use Test::LectroTest::Generator qw(:common);

 show("Ints (sized by default)", Int);

 show("Floats (sized by default)", Float);

 show("Percentages (unsized)",
      Int( range=>[0,100], sized=>0 ));

 show("Lists (sized by default) of Ints (unsized) in [0,10]",
      List( Int( sized=>0, range=>[0,10] ) ));

 show("Uppercase-alpha identifiers at least 3 chars long",
      String( length=>[3,], charset=>"A-Z" ));


 show("Hashes (sized by default) of form AAA=>Digit",
      Hash( String( length=>3, charset=>"A-Z" ),
            Int( sized=>0, range=>[0,9] ) ));

 sub show {
     print "\n", shift(), "\n";
     my ($gen) = @_;
     for (1..10) {
         my $val = $gen->generate($_);
         printf "Size %2d:  ", $_;
         if (ref $val eq "HASH") {
             my @pairs = map {"$_=>$val->{$_}"} keys %$val;
             print "{ @pairs }";
         }
         elsif (ref $val eq "ARRAY") {
             print "[ @$val ]"
         }
         else {
             print $val;
         }
         print "\n";
     }
 }

Advanced examples

    use Data::Dumper;
    use Test::LectroTest::Generator qw(:common Gen);

First, here's a recipe for building a list of lists of integers:

    my $loloi_gen = List( List( Int(sized=>0) ) );
    print Dumper($loloi_gen->generate(10));

You may want to run the example several times to get a feel for the distribution of the generated output.

Now, a more complicated example. Here we build sized trees of random depth using a recursive set of generators.

    my $tree_gen = do {
        my $density = 0.5;
        my $leaf_gen = Int( sized=>0 );
        my $tree_helper = \1;
        my $branch_gen = List( Gen { $$tree_helper->generate(@_) } );
        $tree_helper = \Gen {
            my ($size) = @_;
            return rand($size) < $density
                ? $leaf_gen->generate($size)
                : $branch_gen->generate($size + 1);
        };
        $$tree_helper;
    };

    print Dumper($tree_gen->generate(30));

We define a tree as either a leaf or a branch, and we randomly decide between the two at each node in the growing tree. Leaves are just integers and become more likely when the sizing guidance diminishes (which happens as we go deeper). The code uses $density as a control knob for leaf density. (Try re-running the above code after changing the value of $density. Try 0, 1, and 2.) Branches, on the other hand, are lists of trees. Because branches generate trees, and trees generate branches, we use a reference trick to set up the mutually recursive relationship. This we encapsulate within a do block for tidiness.