And Behind Door Number 2 Is . . . A Leaky Abstraction

December 11, 2008

Liskov Substitution  Principle (LSP):

Let q(x) be a property provable about objects x of type T. Then q(y) should be true for objects y of type S where S is a subtype of T.

My Bogus was an attempt to abstract a method by which I could get access to a variable using a syntatic-sugar idiom. The abstraction leaks. Here is how it leaks:

       (with-property (value 24)
         (value :is (incf (value))))
       (assert-equal 25 (value)))

‘incf’ is a destructive operation.

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Abstraction of an encapsulted property in Lisp

December 9, 2008

Abstraction (computer science): is a mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time.

If you read Lisp Trick #1, hopefully, you saw a pretty good abstraction. What it was doing was abstracting out the setting and getting a value from a variable. As a bonus, because of the way &key parameter keyword can be used, we were able to differentiate whether you were getting the value or setting the variable. Another bonus is that the variable we are getting and setting is encapsulated.

Usually an abstraction is created with the use of macros. Now I’ve made an even larger abstraction of the setter/getter abstraction.

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