Reminds me of an HWN quote from a few weeks back.
Jan 9, 2007 HWN
"*Paul Johnson*: Mutable state is actually another form of manual memory
management: every time you over-write a value you are making a decision that
the old value is now garbage, regardless of what other part of the program
might have been using it."
On 1/31/07, Doaitse Swierstra
If you ask an "ordinary" programmer what is the effect of an assignment like:
x := <expr>
you will get an answer along the lines: the <expr> is evaluated and the result is stored in a box named "x".
If you ask him/her whether something else has happened the answer will usually be "no"; and this is the wrong answer, since one tends to overlook the fact that one has, by mentioning the variable "x", also indicated that the old value of "x" is no longer needed. As such imperative programming is "functional programming with explicit garbage collection".
This may have been a necessity in the old days, when memory was scarce and machines slow (in my first course on numeric algorithms we were taught to declare the variables x, y, z, n, m, i, and j, and the array a, and to solve our problems with those only; points would be deducted for making use of more variables, or not reusing whenever possible!). Nowadays I think everyone agrees that automatic garbage collection is a good thing; think about the amount of wasted programmer years of people figuring out where they have space leak, etc., and users of programs getting a disappointing feeling about computer science as a whole since apparently we keep delivering programs that stop to work at unpredictable moments.
So, one might wonder whether assignments are always bad, bringing us back to the OO discussion. Of course if we have the following expression:
digest.chew.eat.serve.cook.chop.pluck.kill $ chicken
we all have a definite feeling that after applying the functions, the original object is no longer available, and the FP view does not feel entirely natural.
Doaitse Swierstra
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