Provided by: libperl-critic-perl_1.121-1_all 
NAME
Perl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers - Don't use values that
don't explain themselves.
AFFILIATION
This Policy is part of the core Perl::Critic distribution.
DESCRIPTION
What is a "magic number"? A magic number is a number that appears in code without any
explanation; e.g. "$bank_account_balance *= 57.492;". You look at that number and have
to wonder where that number came from. Since you don't understand the significance of the
number, you don't understand the code.
In general, numeric literals other than 0 or 1 in should not be used. Use the constant
pragma or the Readonly or Const::Fast modules to give a descriptive name to the number.
There are, of course, exceptions to when this rule should be applied. One good example is
positioning of objects in some container like shapes on a blueprint or widgets in a user
interface. In these cases, the significance of a number can readily be determined by
context.
The maximum number of violations per document for this policy defaults to 10.
Ways in which this module applies this rule.
By default, this rule is relaxed in that 2 is permitted to allow for common things like
alternation, the STDERR file handle, etc..
Numeric literals are allowed in "use" and "require" statements to allow for things like
Perl version restrictions and Test::More plans. Declarations of $VERSION package
variables are permitted. Use of "Readonly", "Readonly::Scalar", "Readonly::Array", and
"Readonly::Hash" from the Readonly module are obviously valid, but use of
"Readonly::Scalar1", "Readonly::Array1", and "Readonly::Hash1" are specifically not
supported.
Use of binary, exponential, hexadecimal, octal, and version numbers, even for 0 and 1,
outside of "use"/"require"/"Readonly" statements aren't permitted (but you can change
this).
There is a special exemption for accessing the last element of an array, i.e. $x[-1].
$x = 0; # ok
$x = 0.0; # ok
$x = 1; # ok
$x = 1.0; # ok
$x = 1.5; # not ok
$x = 0b0 # not ok
$x = 0b1 # not ok
$x = 0x00 # not ok
$x = 0x01 # not ok
$x = 000 # not ok
$x = 001 # not ok
$x = 0e1 # not ok
$x = 1e1 # not ok
$frobnication_factor = 42; # not ok
use constant FROBNICATION_FACTOR => 42; # ok
use 5.6.1; # ok
use Test::More plan => 57; # ok
plan tests => 39; # ok
our $VERSION = 0.22; # ok
$x = $y[-1] # ok
$x = $y[-2] # not ok
foreach my $solid (1..5) { # not ok
...
}
use Readonly;
Readonly my $REGULAR_GEOMETRIC_SOLIDS => 5;
foreach my $solid (1..$REGULAR_GEOMETRIC_SOLIDS) { #ok
...
}
CONFIGURATION
This policy has four options: "allowed_values", "allowed_types",
"allow_to_the_right_of_a_fat_comma", and "constant_creator_subroutines".
"allowed_values"
The "allowed_values" parameter is a whitespace delimited set of permitted number values;
this does not affect the permitted formats for numbers. The defaults are equivalent to
having the following in your .perlcriticrc:
[ValuesAndExpressions::ProhibitMagicNumbers]
allowed_values = 0 1 2
Note that this policy forces the values 0 and 1 into the permitted values. Thus,
specifying no values,
allowed_values =
is the same as simply listing 0 and 1:
allowed_values = 0 1
The special "all_integers" value, not surprisingly, allows all integral values to pass,
subject to the restrictions on number types.
Ranges can be specified as two (possibly fractional) numbers separated by two periods,
optionally suffixed with an increment using the Perl 6 ":by()" syntax. E.g.
allowed_values = 7..10
will allow 0, 1, 7, 8, 9, and 10 as literal values. Using fractional values like so
allowed_values = -3.5..-0.5:by(0.5)
will permit -3.5, -3, -2.5, -2, -2.5, -1, -0.5, 0, and 1. Unsurprisingly, the increment
defaults to 1, which means that
allowed_values = -3.5..-0.5
will make -3.5, -2.5, -2.5, -0.5, 0, and 1 valid.
Ranges are not lazy, i.e. you'd better have a lot of memory available if you use a range
of "1..1000:by(0.01)". Also remember that all of this is done using floating-point math,
which means that "1..10:by(0.3333)" is probably not going to be very useful.
Specifying an upper limit that is less than the lower limit will result in no values being
produced by that range. Negative increments are not permitted.
Multiple ranges are permitted.
To put this all together, the following is a valid, though not likely to be used,
.perlcriticrc entry:
[ValuesAndExpressions::ProhibitMagicNumbers]
allowed_values = 3.1415269 82..103 -507.4..57.8:by(0.2) all_integers
"allowed_types"
The "allowed_types" parameter is a whitespace delimited set of subclasses of
PPI::Token::Number.
Decimal integers are always allowed. By default, floating-point numbers are also allowed.
For example, to allow hexadecimal literals, you could configure this policy like
[ValuesAndExpressions::ProhibitMagicNumbers]
allowed_types = Hex
but without specifying anything for "allowed_values", the allowed hexadecimal literals
will be 0x00, 0x01, and 0x02. Note, also, as soon as you specify a value for this
parameter, you must include "Float" in the list to continue to be able to use floating
point literals. This effect can be used to restrict literals to only decimal integers:
[ValuesAndExpressions::ProhibitMagicNumbers]
allowed_types =
If you permit exponential notation, you automatically also allow floating point values
because an exponential is a subclass of floating-point in PPI.
"allow_to_the_right_of_a_fat_comma"
If this is set, you can put any number to the right of a fat comma.
my %hash = ( a => 4512, b => 293 ); # ok
my $hash_ref = { a => 4512, b => 293 }; # ok
some_subroutine( a => 4512, b => 293 ); # ok
Currently, this only means directly to the right of the fat comma. By default, this value
is true.
"constant_creator_subroutines"
This parameter allows you to specify the names of subroutines that create constants, in
addition to "Readonly", "Const::Fast", and friends. For example, if you use a custom
"Const::Fast"-like module that supports a "create_constant" subroutine to create
constants, you could add something like the following to your .perlcriticrc:
[ValuesAndExpressions::ProhibitMagicNumbers]
constant_creator_subroutines = create_constant
If you have more than one name to add, separate them by whitespace.
The subroutine name should appear exactly as it is in your code. For example, if your
code does not import the creating subroutine subroutine, you would need to configure this
policy as something like
[ValuesAndExpressions::ProhibitMagicNumbers]
constant_creator_subroutines = create_constant Constant::Create::create_constant
BUGS
There is currently no way to permit version numbers in regular code, even if you include
them in the "allowed_types". Some may actually consider this a feature.
AUTHOR
Elliot Shank "<perl@galumph.com>"
COPYRIGHT
Copyright (c) 2006-2011 Elliot Shank.
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself. The full text of this license can be found in the LICENSE file
included with this module.
perl v5.18.1 Perl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers(3pm)