LSP (was Re: Function.prototype.bind)

Graydon Hoare graydon at mozilla.com
Wed Sep 24 11:23:26 PDT 2008


Waldemar Horwat wrote:
> Graydon Hoare wrote:
>> Quoting Liskov[1]:
>>
>>    "What is wanted here is something like the following substitution
>>     property: If for each object o1 of type S there is an object o2 of
>>     type T such that for all programs P defined in terms of T, the
>>     behavior of P is unchanged when o1 is substituted for o2, then S
>>     is a subtype of T."
>>
>> Think it over. Imagine what you'd have to delete from any language you 
>> use, for its "subtype" relation to conform to that definition.
> 
> What?  For example, what would you have to delete from Java, ignoring the parts where a program can examine itself like reflection?

The big one is "method overriding in subtypes", though as you say there 
are various other cases such as reflection. If I have a program that 
uses some type:

   class Mul {
     int f(int x, int y) { return x * y; }
   }

I can form a subtype such that substituting the subtype for the 
supertype in some program will likely change the behavior of the program:

   class NotTheMulYoureLookingFor extends Mul {
     int f(int x, int y) { return x + y; }
   }

There was once a line of work on "ADT" languages that aimed to formulate 
subtyping in more restrictive terms. Liskov's work was generally in that 
lineage. Ours is not.

-Graydon



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