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Comment by yaroslavvb | Hacker News Reader

Well, that looks easy...so why are people saying that writing memoization in Haskell is PhD-thesis level task?

Some people seem to think "a task described in a phD thesis" implies "a phD-level task". "Oh, crap, dude! This expects me to read a phD thesis! I'd better wait until somebody on reddit summarizes a blog post about a blog post about it."

By that logic, implementing Lisp is "a phD-level task", too. (http://repository.readscheme.org/ftp/papers/orbit-thesis.pdf) (http://www.cs.indiana.edu/~dyb/papers/3imp.pdf) (http://repository.readscheme.org/ftp/papers/ai-lab-pubs/AITR...) *

* The third is actually a masters thesis, but whatever.

Because memoization is best implemented in the compiler and runtime, and the person on the Clojure list wanted to implement it in his application.

I've seen people get exasperated trying to implement "if" in Prolog, too. The whole language is already an 'if' / pattern matching engine!

("How do you implement lazy evaluation in Haskell? No, like, in an application...")

Yeah, lazy evaluation can be surprising (in a good way). Once you get in the mindset of thinking "this is going to be hard", you start programming Haskell like you would C, and while it works, you feel stupid when you realize you just rewrote the Haskell compiler. Poorly.

(Personally, I have done this a number of times. I remember writing some dependency evaluation system, with a central data structure that looked something like:

   data Dependency a e = Thunk (e -> a) | Resolved a

with a bunch of code to turn a set of Thunks into Resolved when necessary.

But of course, Haskell always does this anyway!)

I was also surprised when I was on Windows and needed the "head" utility to look at the first line of a file, but didn't have it installed. Since I had a ghci session going, I just wrote:

   readFile "foo.csv" >>= putStrLn . unlines . take 10 . lines

Half-expecting it to error out because the file didn't fit into memory. Nope! Problem solved!

(And yes, I know unsafeInterleaveIO is evil.)

I suspect because writing memoization means controlling memoization, and controlling memoization in Haskell means either hacking the compiler, or dealing with state, and state means monads, and nobody likes monads. (Except those people who love monads, of course).

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