Perl::CritiPerl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers(3)NAMEPerl::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 v5.14.Perl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers(3)
