OK, I have to take one more look...

What can I say? I can sympathize with Lot's wife.

I have yet to show you the spiffy graphic you can get showing memory usage.

What does this tell us?

First, we can say that it looks like we're down about a megabyte from earlier. (Alas, they don't give the scale all the way up the vertical axis; if only they'd move the word "bytes" out of the way!) The other thing is, it looks like it's accumulating thunks until about halfway through when something finally demands evaluation and the RAM soufflé collapses.

(What's a thunk? The name goes back to the days when the world was new and people were figuring out how to generate code for Algol 60 programs. When the defining committee set up the rules for "call by name" they perhaps didn't realize the full consequences of their actions (vide Jensen's Device). Implementing it required generating a little function for each call by name parameter that returned a reference to it, and those little functions were called "thunks". In the context of Haskell and other lazy programming languages, a thunk represents a value that hasn't yet been evaluated.)

Real World Haskell has a chapter on optimization that we've leaned heavily on, and it has a section on memory usage,. thunks, and strictness issues. I will look more closely at that section and take one last shot at the palindrome program--after all, these issues will come up in the future as well. (No, really, this will be the last time...)


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