Why are there frozen constants everywhere?

Question

We can easily find such style from lots of famous repositories, like rack, rails, etc.

For example in rack:

PATH_INFO      = 'PATH_INFO'.freeze REQUEST_METHOD = 'REQUEST_METHOD'.freeze SCRIPT_NAME    = 'SCRIPT_NAME'.freeze QUERY_STRING   = 'QUERY_STRING'.freeze CACHE_CONTROL  = 'Cache-Control'.freeze CONTENT_LENGTH = 'Content-Length'.freeze CONTENT_TYPE   = 'Content-Type'.freeze 

Another examle in rails:

HTTP_IF_MODIFIED_SINCE = 'HTTP_IF_MODIFIED_SINCE'.freeze HTTP_IF_NONE_MATCH     = 'HTTP_IF_NONE_MATCH'.freeze HTTP_IF_NONE_MATCH     = 'HTTP_IF_NONE_MATCH'.freeze 

I wonder why these constant strings are frozen. Since they are all constants, there should be only one instance. Of course we can put "foo".freeze somewhere to reference the same singleton instance, however people usually write literal variable name like HTTP_IF_MODIFIED_SINCEinstead.

So in my opinion, it doesn't make any difference in spite of using #freeze, so why do people freeze constants?

Answer 1

It is correct that Ruby prints a warning when you re-assign a value to an already initialized constant:

> FOO = 'foo' > FOO = 'bar' # :2: warning: already initialized constant FOO # :1: warning: previous definition of FOO was here > FOO # => "bar" 

But there is no protection from changing the value in the constant. Example without freeze:

> FOO = 'foo' > FOO[1] = '-' > FOO # => "f-o" 

But freeze allows to protect the value of the constants from being changed. Example with freeze:

> FOO = 'foo'.freeze > FOO[1] = '-' # => RuntimeError: can't modify frozen String