Haskell for the Evil Genius

http://www.yellosoft.us/evilgenius/

by vimes656 • 15 years ago

47 12 15 years ago

Cute but the claimed example of memoization is just plain old recursion. Its a nicely styled tutorial with good flow that could perhaps use some time getting fact checked by the nice folks on the Haskell mailIng list

flatline• 15 years ago

  main = let fib 30 = 832040 in do
	putStrLn (show (fib 30))

This is not what I would normally consider 'lazy' or 'memoization'...

Not to be too pedantic but that 'sieve' is also not really the sieve of Eratosthenes either, as it continually recomputes each slot:

http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf

ricree2• 15 years ago

Worse, I think it actually overwrites the fib he just defined with a function that simply pattern matches on 30 and returns 832040.

In GHCI, trying that example with fib 29 results in a non exhaustive pattern error.

ramchip• 15 years ago

Exactly; just like

    let 2+2=5 in 2+2

amusingly returns 5.

Hemospectrum• 15 years ago

This seems very much a work in progress despite the apparent degree of polish. I wonder if the author asked for feedback in #haskell and someone decided to run around posting it here and there without reading it carefully? That happens a lot with Haskell...

vimes656• 15 years ago

I found the post interesting, that's why I submitted HN. I can't be blamed for other people upvoting it.

I didn't notice the errors but having people at HN showing me what is wrong helped me learn more about Haskell. I hope that pointing those errors helps make the post more robust, too.

shasta• 15 years ago

I recommend rotating the pitchfork to make it look like a lambda.

mhitza• 15 years ago

For the use made out for records there was no reason to derive Eq, Ord and Show.

ricree2• 15 years ago

Out of curiosity, is there a particular downside to deriving them if the type permits it?

rbxbx• 15 years ago

I'd imagine you're unnecessarily polluting the type with unused functionality, no?

(not sure if this is true, perhaps GHC is smart about it).

If nothing else, superfluous derives would seem to distract/obfuscate your intent.

Haskell for the Evil Genius

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