An engaging quote from Paul Johnson, from the January 9, 2007 issue of Haskell Weekly News:

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.

My little theory: there is no reason to ever change the value of a variable except for efficiency. Instead, make a new immutable variable based on the old value, which gives you freedom to name your new variable to better describe what it does. My C and C++ code is littered with const almost everywhere, and it helps enormously when you look back at the code a month later to figure out what it’s doing.

And, just to throw in a little more Haskell evangelism from someone else who’s merrily had their brain reconfigured to to see the awe of the lambda calculus:

So far I’ve written about 300 LOC replacing about 1500 LOC of a system written in Perl… The thing that totally, utterly blows my mind is that I’ve done all this without any loops and using only 3 conditional statements. (One if and two case, if it matters. And I don’t mean this in the cheap sense that Haskell doesn’t have any looping constructs and so I’m writing recursive functions instead, which everyone knows is the same thing. There is no explicit recursion in my code. I’m just flabbergasted.