I want to have a class that always evaluates to false in boolean expressions. Is there a way to extend Ruby so that it evaluates an object of a class to false like false and nil?
Example
class AlwaysFalse
end
should_be_false = AlwaysFalse.new
if should_be_false
#I don't want this to ever run
else
#will always run
end
The use case for this is to have the concept of false but have it contain instance variables. You can't stick instance variables on FalseClass because it is a singleton and every use of false will overwrite the previous instance variable values.
a = false
a.name = "fish"
b = false
b.name = "ocean"
#at this point in time a.name is equal to "ocean" and not "fish" like I want.
This example may seem like an awful design decision but in the program I am writing it is important to have an interface like this to meet the requirements I have to code against.
Is achieving this behavior as simple as redefining some operator methods like == or is this sort of behavior not possible in Ruby?
Thank you,
Matt Margolis
I want to have a class that always evaluates to false in boolean
expressions. Is there a way to extend Ruby so that it evaluates an
object of a class to false like false and nil?
Example
class AlwaysFalse
end
should_be_false = AlwaysFalse.new
if should_be_false
#I don't want this to ever run
else
#will always run
end
The use case for this is to have the concept of false but have it
contain instance variables. You can't stick instance variables on
FalseClass because it is a singleton and every use of false will
overwrite the previous instance variable values.
a = false
a.name = "fish"
b = false
b.name = "ocean"
#at this point in time a.name is equal to "ocean" and not "fish" like I
want.
This example may seem like an awful design decision but in the program I
am writing it is important to have an interface like this to meet the
requirements I have to code against.
Is achieving this behavior as simple as redefining some operator methods
like == or is this sort of behavior not possible in Ruby?
The latter.
OTOH:
class Object
def false?
false
end
end
class FalseClass
def false?
true
end
end
class YourClass
def false?
true
end
end
if condition.false?
...
end
Kind of annoying if you forget the .false? though. A better thing to
do would be to figure out what you are trying to model by having
multiple "falsy" values with some state.
···
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
Thank you,
Matt Margolis
This is your design problem.
You can't change the language to do what it doesn't do.
What you want is probably not a class if it always returns false.
You want a constant value.
You can create a class with an instance or class variable that happens to be a constant. (no setter, only a getter)
so you might do
The problem is your if will go to else if the should_be_false is false. not clear logic in the code.
Just do it the Ruby way, create a method or function:
my_class.false?
or
false?( my_arg )
This way the logic is more obvious and clear like natural language.
···
On Oct 9, 2007, at 10:53 AM, Matt Margolis wrote:
I want to have a class that always evaluates to false in boolean expressions. Is there a way to extend Ruby so that it evaluates an object of a class to false like false and nil?
Example
class AlwaysFalse
end
should_be_false = AlwaysFalse.new
if should_be_false
#I don't want this to ever run
else
#will always run
end
Quoting myself. In other words, no you can't do it.
···
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
Logan Capaldo wrote:
> On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
>> Is achieving this behavior as simple as redefining some operator methods
>> like == or is this sort of behavior not possible in Ruby?
>>
>>
> The latter.
>
An alternative approach might be to use 'case' expressions instead of 'if' and implement #=== properly in that class.
Matt, what are you implementing? Why is this functionality crucial?
Kind regards
robert
···
On 09.10.2007 18:12, Logan Capaldo wrote:
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
Logan Capaldo wrote:
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
Is achieving this behavior as simple as redefining some operator methods
like == or is this sort of behavior not possible in Ruby?
The latter.
Quoting myself. In other words, no you can't do it.
Robert Klemme wrote:
Logan Capaldo wrote:
Is achieving this behavior as simple as redefining some operator methods
like == or is this sort of behavior not possible in Ruby?
The latter.
Quoting myself. In other words, no you can't do it.
An alternative approach might be to use 'case' expressions instead of 'if' and implement #=== properly in that class.
Matt, what are you implementing? Why is this functionality crucial?
Kind regards
robert
I can't really get into specifics but basically I am doing something like the following
module PrimitiveExtensions
attr_accessor :name
def to_extended_primitive(name)
@name = name
return self
end
I then mix this in to String, Fixnum, Float etc...
MyClass contains
def my_val=(obj)
obj.to_extended_primitive("MyVal")
end
def other_val=(obj)
obj.to_extended_primitive("OtherVal")
end
The purpose of all of this is to unify an interface so that I can store primitive values along with a name without having to go through a container class to get at the value.
a = MyClass.new.my_val = 7
and then later be able to go back and do something like
a.name and get back "MyVal"
This approach works for all the base types except for true and false since they are Singletons so I can't store a @name on them.
···
On 09.10.2007 18:12, Logan Capaldo wrote:
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
Robert Klemme wrote:
Logan Capaldo wrote:
Is achieving this behavior as simple as redefining some operator methods
like == or is this sort of behavior not possible in Ruby?
The latter.
Quoting myself. In other words, no you can't do it.
An alternative approach might be to use 'case' expressions instead of 'if' and implement #=== properly in that class.
Matt, what are you implementing? Why is this functionality crucial?
Kind regards
robert
I can't really get into specifics but basically I am doing something like the following
module PrimitiveExtensions
attr_accessor :name
def to_extended_primitive(name)
@name = name
return self
end
I then mix this in to String, Fixnum, Float etc...
MyClass contains
def my_val=(obj)
obj.to_extended_primitive("MyVal")
end
def other_val=(obj)
obj.to_extended_primitive("OtherVal")
end
This seems a strange way to use assignment since you do not modify the receiver but the sender. I'd say this will likely lead to hard to code that is hard to understand.
The purpose of all of this is to unify an interface so that I can store primitive values along with a name without having to go through a container class to get at the value.
a = MyClass.new.my_val = 7
and then later be able to go back and do something like
a.name and get back "MyVal"
This approach works for all the base types except for true and false since they are Singletons so I can't store a @name on them.
I doubt this approach works for all basic types since you will get aliasing effects with Fixnums, i.e. you set the name in one place and it will be visible on all other places as well as Fixnums are singletons per value.
I do not know why you are so eager to have the attribute stored in the primitive value. Considering what I have seen of your requirements I would probably do something like this
NamedValue = Struct.new :value, :name
Kind regards
robert
···
On 09.10.2007 19:41, Matt Margolis wrote:
On 09.10.2007 18:12, Logan Capaldo wrote:
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote:
On 10/9/07, Matt Margolis <matt@mattmargolis.net> wrote: