Snipplr

(Haskell) Simultaneous filter and map - deepsoul

Thu, 06 Oct 2011 07:53:01 GMT

The `filtermap` functional below allows to perform the standard `map` and `filter` operations in one go. The mapping function has to return a `Maybe` monad, and values of `Nothing` are filtered out.

(Haskell) Haskell 99 Problems - Problem 23 - rtperson

Sat, 10 Sep 2011 07:27:49 GMT

problem 23, Extract a given number of randomly selected elements from a list. Example: Prelude System.Random>rnd_select "abcdefgh" 3 >>= putStrLn "eda" Two problems: 1) How to return a list, and 2) how to sample without duplication

(Haskell) Haskell 99 Problems, numbers 1 through 9 - rtperson

Thu, 08 Sep 2011 00:23:54 GMT

I had originally started these problems from #10 (Run-length encoding). I went back and did 1-8 for completeness.

(Haskell) Haskell 99 Problems - Number 20, Arrowed! - rtperson

Thu, 25 Aug 2011 01:54:30 GMT

problem 20, (*) Remove the K'th element from a list *Main> removeAt 1 "abcd" "acd" Trivial using a pure function. A bit more challenging if you use this problem to work up your Arrow-fu.

(Haskell) Haskell 99 Problems - Number 18 and 19 - rtperson

Sun, 21 Aug 2011 01:58:02 GMT

Problem 18: Extract a slice from a list. Given two indices, i and k, the slice is the list containing the elements between the i'th and k'th element of the original list (both limits included). Start counting the elements with 1. Example: *Main> slice ['a','b','c','d','e','f','g','h','i','k'] 3 7 "cdefg 9 Problem 19 - Rotate a list N places to the left. *Main> rotate ['a','b','c','d','e','f','g','h'] 3 "defghabc" *Main> rotate ['a','b','c','d','e','f','g','h'] (-2) "ghabcdef" (This one is so easy it feels like cheating...)

(Haskell) Haskell 99 Problems - Number 17 - rtperson

Thu, 18 Aug 2011 06:50:09 GMT

problem 17, Split a list into two parts; the length of the first part is given. Do not use any predefined predicates. (Meaning no splitAt or take or drop) *Main> split "abcdefghik" 3 ("abc", "defghik")

(Haskell) Haskell 99 Problems - Numbers 15 and 16 - rtperson

Thu, 18 Aug 2011 03:59:30 GMT

problem 15, Replicate the elements of a list a given number of times. > repli "abc" 3 "aaabbbccc" problem 16, Drop every N'th element from a list. *Main> dropEvery "abcdefghik" 3 "abdeghk"

(Haskell) Haskell 99 Problems - Number 14 - rtperson

Thu, 18 Aug 2011 01:04:04 GMT

problem 14, Duplicate the elements of a list > dupli [1, 2, 3] [1,1,2,2,3,3]

(Haskell) Haskell 99 Problems - Solutions to 11 and 12 - rtperson

Fri, 29 Jul 2011 08:03:55 GMT

problem 11, modified run-length encoding ======================================== If an element has no duplicates, mark it as such. Example: encodeModified "aaaabccaadeeee" [Multiple 4 'a',Single 'b',Multiple 2 'c', Multiple 2 'a',Single 'd',Multiple 4 'e'] problem 12, Decode a run-length encoded list. ============================================= Given a run-length code list generated as specified in problem 11. Construct its uncompressed version. Example: decodeModified [Multiple 4 'a',Single 'b',Multiple 2 'c', Multiple 2 'a',Single 'd',Multiple 4 'e'] "aaaabccaadeeee"

(Haskell) Run-Length Encoding in Haskell - rtperson

Fri, 29 Jul 2011 06:08:49 GMT

Problem 10 of the famous 99 Problems. I got 99 problems, but a Lisp ain't one.

(Haskell) Towers of Hanoi by Bit Twiddling - rtperson

Thu, 07 Jul 2011 01:45:07 GMT

Credit where it's due: I got this from udpn over at codereview.stackexchange.com. I'm just putting it here for further study.

(Haskell) Towers of Hanoi in Haskell - rtperson

Thu, 07 Jul 2011 01:05:06 GMT

(Haskell) Real World Haskell Exercise - Ch 4 - rtperson

Wed, 01 Jun 2011 01:00:47 GMT

The first fold exercise from RWH, Chapter 4

(Haskell) Real World Haskell Exercise - Ch 4, Ex 2-4 - rtperson

Wed, 01 Jun 2011 00:59:27 GMT

I'm posting the rest of these, mostly because I'm still patting myself on the back for my point-free implementation of exercise 3, and because I give myself about two months before I entirely forget how I did it...

(Haskell) Fibonacci List One-Liner in Haskell - rtperson

Wed, 01 Jun 2011 00:44:37 GMT

I didn't invent this, but it's too cool a snippet not to capture. Here's a one-liner that will generate a list of Fibonacci numbers in linear time. EXPLANATION: the colon operator is used to add elements to a list, so it starts out adding on the two base cases, zero and one. After you have these two, you can mathematically generate the rest of the list. The zipWith function takes two lists and combines them using a function (in this case, addition). The two lists in question are fiblist (that is, the list you're currently generating) and "tail fiblist" (that is, every element of the list after the first element). The only reason this works is because Haskell's laziness gives it the ability to define infinite lists. Haskell will never even attempt to calculate the nth Fibonacci number until it is asked to do so.

(Haskell) Haskell Radix Conversion using ByteStrings - rtperson

Wed, 01 Jun 2011 00:30:19 GMT

Another radix conversion, when you need to zippy greatness of Haskell's ByteString.(Haskell strings are slooooow!)

(Haskell) Haskell Radix Conversion - rtperson

Wed, 01 Jun 2011 00:26:01 GMT

Converts an integer from base 10 to a string of base x (where 0 > x >= 20). The only thing I'd improve is getting rid of all the calls to error. Perhaps hand back a Maybe String that simply hands back Nothing if unable to convert the number.

(Haskell) FizzBuzz in Haskell - rtperson

Wed, 01 Jun 2011 00:14:43 GMT

The dread FizzBuzz question -- really, a test if your average programmer knows his or her FOR loops. The spec is that you're counting from 1 to 100. Your program should print out "fizz" if the index is divisible by three, and "buzz" if it's divisible by five, and "fizzbuzz" if it's divisible by both. I have a couple different versions in this code: first, a few functions just fizzing or buzzing. Second, a generalization which allows any standard message against any divisor. Third, a purely functional version that zips two lists together (giving us free concatenation for "fizzbuzz"). Fourth, a list comprehension and lastly, a monadic version that calls a pure function that uses guards.

(Haskell) Euler Problem 4 in Haskell - rtperson

Wed, 01 Jun 2011 00:11:10 GMT

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 * 99. Find the largest palindrome made from the product of two 3-digit numbers. There are shorter ways to do this, and I cheat a bit by counting down to 900 rather than 100, but this one short-circuits when it finds the answer, and so is very efficient.

(Haskell) takeAllMVars - CopperKoo

Tue, 31 May 2011 06:18:29 GMT

Collecting all values in mvar boxes, assuming all mvars are filled at some point. This function does not block on every element of the MVar list, but proceeds with other elements of the list.