February 2012 - Beginners

How memorize intermediate computation value[my brain is so hurt]?
by anyzhen 04 Feb '12

04 Feb '12

there have a lot of situation we need memorize intermediate computation value. and use in further. like this, compute nth fib question. So,how memorize it? i have an ugly solution.it used HashTable for memorization. fib n table | n==1 =1 | n==2 =1 | otherwise = case lookup table of Just v ->(v,table) Nothing ->let (v,table') = fib (n-1) table in let ( v',table'')= v + fib(n-2) table' in (v',table'') i am an beginner come from imperative programming language. so fib memorize version program hurt my brain ... particular in Nothing branches. so do you have some good idea

4 3

nested guards
by Thomas Engel 03 Feb '12

03 Feb '12

Hello, I'm new to haskell. I want to write a function for a s-curve acceleration in haskell. A solution in VBA is already working. Are there nested guards in haskell available or how can I rewrite this function? The function should calculate the acceleration for a given distance, max acceleration, velocity, jerk at time since start acc_skurve::Float->Float->Float->Float->Float->Float acc_skurve str acc v jerk taktuell | str <= str2jerk |taktuell <= (0.5 * str / jerk)**(1/3::Float) = jerk * taktuell |taktuell <= (4 * str / jerk)**(1/3::Float) = jerk**(2/3::Float) * 2**(2/3::Float) * str**(1/3::Float) - jerk * taktuell |taktuell <= (27/2 * str / jerk)**(1/3::Float) = - jerk * taktuell + 2**(2/3::Float) * str**(1/3::Float) * jerk**(2/3::Float) |taktuell <= (32 * str / jerk)**(1/3::Float) = -2 * 2**(2/3::Float) * str**(1/3::Float) * jerk**(2/3::Float) + jerk * taktuell | str > str2jerk && str < str2acc |taktuell <= acc / jerk = jerk * taktuell |taktuell <= - acc / 2 / jerk + sqrt(acc^3 + 4 * str * jerk^2) / (2 * jerk * sqrt(acc)) = acc |taktuell <= acc / 2 / jerk + sqrt(acc^3 + 4 * str * jerk^2) / (2 * jerk * sqrt(acc)) = acc / 2 - jerk * taktuell + sqrt(acc^3 + 4 * str * jerk^2) / (2 * sqrt(acc)) |taktuell <= 3 * acc / 2 / jerk + sqrt(acc^3 + 4 * str * jerk^2) / (2 * jerk * sqrt(acc)) = acc / 2 - jerk * taktuell + sqrt(acc^3 + 4 * str * jerk^2) / (2 * sqrt(acc)) |taktuell <= sqrt(acc^3 + 4 * str * jerk^2) / (sqrt(acc * jerk)) = - acc |taktuell <= acc / jerk + sqrt(acc^3 + 4 * str * jerk^2) / (sqrt(acc * jerk)) = - acc + jerk * taktuell - sqrt(acc^3 + 4 * str * jerk^2) / sqrt(acc) | str >= str2acc |taktuell <= acc / jerk = jerk * taktuell |taktuell <= v / acc = acc |taktuell <= acc / jerk + v / acc = acc - jerk * taktuell + v * jerk / acc |taktuell <= str / v = 0 |taktuell <= str / v + acc / jerk = jerk * str / v - jerk * taktuell |taktuell <= str / v + v / acc = - acc |taktuell <= str / v + acc / jerk + v / acc = -(str / v + acc / jerk + v / acc - taktuell) * jerk where str2jerk = 2 * acc^2 / jerk^2 str2acc = acc^2 * v + v^2 * jerk / (jerk * acc) best regards Thomas

3 2

(no subject)
by Thomas Engel 03 Feb '12

03 Feb '12

1 0

unchecked exceptions as side effects
by Ovidiu Deac 02 Feb '12

02 Feb '12

In this video Erik Meijer talks about side effects: https://www.youtube.com/watch?v=UuamC0T3hv8&t=6m and unchecked exceptions as side effects. I tried to read the paper "Unchecked Exceptions can be Strictly More Powerful than Call/CC" but I got lost pretty quick. Can somebody explain the matter nicely? Thanks, ovidiu

1 0

mailing list question
by Gregory Guthrie 01 Feb '12

01 Feb '12

I notice that the mailing list digest form is text only, so special symbols (Greek - alpha, beta, ..) all get mangled into "?". This is not so common on the Beginners list, but not infrequent on the Café list. To find the original version, one needs to go to the archives, but they are indexed by date, and the digest is by Vol, No, Topic. So, Is there any way to get an HTML version of the digest? And/or, to add some indexing to the archives? (an indexed page into the archives?) I presume the answer to these is no, since I did not see any such options on the mailman archive page; so likely this is just either a comment or suggestion, or both. :)

2 1