Constants can be declared at the top level or inside other types. They must start with a capital letter:
PI = 3.14
module Earth
RADIUS = 6_371_000
end
PI #=> 3.14
Earth::RADIUS #=> 6_371_000
Although not enforced by the compiler, constants are usually named with all capital letters and underscores to separate words.
A constant definition can invoke methods and have complex logic:
TEN = begin
a = 0
while a < 10
a += 1
end
a
end
TEN #=> 10
Crystal provides a few pseudo-constants which provide reflective data about the source code being executed.
__LINE__
is the current line number in the currently executing crystal file. When __LINE__
is declared as the default value to a method parameter, it represents the line number at the location of the method call.
__END_LINE__
is the line number of the end
of the calling block. Can only be used as a default value to a method parameter.
__FILE__
references the full path to the currently executing crystal file.
__DIR__
references the full path to the directory where the currently executing crystal file is located.
# Assuming this example code is saved at: /crystal_code/pseudo_constants.cr
#
def pseudo_constants(caller_line = __LINE__, end_of_caller = __END_LINE__)
puts "Called from line number: #{caller_line}"
puts "Currently at line number: #{__LINE__}"
puts "End of caller block is at: #{end_of_caller}"
puts "File path is: #{__FILE__}"
puts "Directory file is in: #{__DIR__}"
end
begin
pseudo_constants
end
# Program prints:
# Called from line number: 13
# Currently at line number: 5
# End of caller block is at: 14
# File path is: /crystal_code/pseudo_constants.cr
# Directory file is in: /crystal_code