Typing Haskell
I've recently begun learning Haskell. This is my first foray into functional programming, and I must say it's fascinating. However, I had some trouble getting a good grasp on Haskell's typing system. The differences between types, type classes, type instances, type constructors, and data constructors, when to use each, and even just a good idea of what each of them are, did not want to sink in. I went through A Gentle Introduction to Haskell as my basic intro to the language, and was satisfied with the things I could do right away. But finding a good, easy to understand tutorial or explanation on the different aspects of the type system was eluding me. Then I found a link to "Tour of the Haskell Syntax" under the "Learning Haskell" page. It's written as a quick reference on the actual language syntax, with brief explanations of each rule. The section labelled "Data Types" gives a nice simple and straightfoward description of how data types are setup and used, and helped me get the glimmer of insight I needed.
Edited 2005-01-09
Added 2004-12-13
JAPHs
s iir ejkucsath alnroetph eri;split$+;print$_[$-++]until($-++>$#_);pr
+int$_[$---]until($---<1);
s iir ejkucsath alnroetph eri;split$+;print$_[$-++]while($-++-$#_);pr
+int$_[$---]while($----())
s oohaa ctkseurjogis;
s ii rpeehrtlo nisom;
+$ -= + split $ +; ;
print + splice @ ## ;
_ => + $ ---, $ ++ 1
while + $ ---- $ ++ @
_
Odd Ball Challenge
http://rec-puzzles.org/new/sol.pl/logic/weighing/balance
{-
Given the number of weighings, prints instructions to solve the foll
+owing
riddle:
You have (3^N - 3) / 2 identical looking objects, one of which is
+lighter
or heavier than the others (but you don't know which). In N weighi
+ngs,
determine which is the odd one, and whether it is lighter or heavi
+er.
-}
module Main where
import IO
import System.Environment
main :: IO ()
main = do
args <- getArgs
let w :: Int
w = read $ head args -- number of weighings
l = labels w -- list of object labels
n = length l -- number of objects you can use
putStr $
" You can find the odd one out of " ++ show n ++ " objects wit
+h " ++
show w ++ " weighings, as follows:\n\n"++
" 1. Assign the following \"magic number\" labels to the objec
+ts:\n" ++
" " ++ unwords l ++ "\n\n" ++
" 2. Keep track of two strings of numbers, LIGHT and HEAVY.\n\
+n" ++
" 3. Record weighing results as follows:\n" ++
" If pans balance, append 1 to both LIGHT and HEAVY.\n" ++
" If left side is lighter, append 0 to LIGHT and 2 to HEAVY.\
+n" ++
" If right side is lighter, append 2 to LIGHT and 0 to HEAVY.
+\n\n" ++
" 4. Perform weighings in this order:\n" ++
weighingOrder 1 w ++ "\n" ++
" 5. After " ++ show w ++ " weighings, either LIGHT or HEAVY w
+ill " ++
"match one of the object labels. That object is the odd one, a
+nd " ++
"its fault is shown by whether LIGHT or HEAVY matched.\n"
weighingOrder :: Int -> Int -> String
weighingOrder current total =
" #" ++ show current ++ ":\n" ++
" Left : " ++ unwords (weighing current total '0') ++ "\n" ++
" Right: " ++ unwords (weighing current total '2') ++ "\n" ++
if current == total then "" else weighingOrder (current+1) total
weighing i w lr =
filter (\l -> l !! (i-1) == lr) $ labels w
labels =
filter (isValidLabel) . labelsOfLength
-- if entire string is the same character, or it's a bad sequence,
-- this label is not valid
isValidLabel l =
not $ or [
allSame l,
badSequence l
]
allSame l@(x:_) =
l == (replicate (length l) x)
badSequence [] = True
badSequence (x:[]) = False
badSequence (x:y:ys) =
if x == y
then badSequence (y:ys)
else badChange x y
where
badChange '0' '1' = False
badChange '1' '2' = False
badChange '2' '0' = False
badChange _ _ = True
-- list of all strings of length n consisting of '0', '1', and '2'
labelsOfLength n =
foldr ($) [""] ( replicate n $ cartesia "012" )
-- Gives the cartesion product of a list and a list of lists
cartesia :: [a] -> [[a]] -> [[a]]
cartesia xs =
concat .
map
(
\ys ->
map ( \x -> ys ++ [x] ) xs
)
<html>
<head>
<style type="text/css">
body {
position: relative;
font-family: arial;
background: #ddd;
}
.header {
border: solid 0.05em #000;
background: #888;
color: #ffd;
padding: 1em;
margin-bottom: 1em;
}
.main {
position: relative;
}
.content {
position: relative;
border: solid 0.05em #448;
background: #ddf;
padding: 1em;
margin-left: 8em;
}
.menu {
position: absolute;
top: 0;
left: 0;
border: solid 0.05em #844;
background: #fdd;
padding: 1em;
width: 5em;
height: 5em;
}
.menu a {
display: block;
}
.corner {
position: relative;
float: left;
top: -1.05em;
left: -1.05em;
border-left-color: transparent;
border-top-color: transparent;
border-right-color: inherit;
border-bottom-color: inherit;
border-style: solid;
border-width: inherit;
background: #ddd;
width: 3em;
height: 3em;
}
</style>
</head>
<body>
<div class="header">
<h1>Website Title</h1>
</div>
<div class="main">
<div class="content">
<div class="corner"></div>
<p>
Lorem ipsum dolor sit amet, consectetuer adipiscing el
+it. Phasellus ultricies nonummy nunc. Maecenas sagittis tellus vitae
+diam. Nam bibendum augue non nulla. Nam vehicula urna vel arcu. Nulla
+ in enim. Donec neque magna, pellentesque non, consequat sit amet, pl
+acerat quis, mauris. Integer nec elit ac metus pulvinar tincidunt. Ma
+uris vel elit. Etiam feugiat. Etiam posuere. Integer volutpat nulla e
+u leo. Cum sociis natoque penatibus et magnis dis parturient montes,
+nascetur ridiculus mus. Curabitur eleifend, dolor rhoncus molestie pe
+llentesque, nisl magna eleifend felis, scelerisque sodales leo turpis
+ at enim. Duis sapien nisi, feugiat in, facilisis sed, congue eu, orc
+i. Ut ac sem. Maecenas vel dolor eget eros euismod rutrum. Curabitur
+sed libero a lectus imperdiet fermentum.
</p>
<p>
Integer eget mi ut ipsum elementum ultrices. Pellentes
+que habitant morbi tristique senectus et netus et malesuada fames ac
+turpis egestas. Vivamus ac augue. Aenean dapibus pretium sapien. Viva
+mus orci lorem, tincidunt ut, fringilla ut, placerat viverra, lorem.
+Cras a augue. Aenean eu lacus eget lectus feugiat lobortis. Nulla ull
+amcorper rutrum nunc. Phasellus ullamcorper, elit quis sodales facili
+sis, lectus tortor pharetra turpis, eu luctus risus nisi eu sem. Sed
+eget neque. Sed viverra. Etiam dapibus, justo sed aliquam tempor, dia
+m sem pulvinar nulla, eu malesuada metus sem ut nibh. Suspendisse ac
+justo ac neque placerat pharetra. Duis mattis laoreet magna. Etiam me
+tus nulla, interdum sed, elementum in, luctus et, ante.
</p>
<p>
In sodales bibendum risus. Morbi quis diam. Aliquam no
+n lectus. Integer vitae mi. Suspendisse sagittis, ligula at consectet
+uer ultricies, pede mi rhoncus est, in iaculis mi eros eget felis. Do
+nec dignissim, nisl mollis vehicula tristique, eros mauris sagittis v
+elit, sit amet gravida lacus enim suscipit libero. Donec interdum tem
+por nulla. Sed aliquet nulla ut massa. In vel augue. Nunc leo nulla,
+tristique eget, commodo ac, gravida ac, leo. Donec velit nulla, ornar
+e et, varius et, dictum eget, felis. Etiam aliquet odio tincidunt pur
+us. Cum sociis natoque penatibus et magnis dis parturient montes, nas
+cetur ridiculus mus. Cras tincidunt. Etiam tortor purus, tristique a,
+ rutrum eu, vestibulum eu, arcu. Quisque sit amet felis non ligula da
+pibus feugiat.
</p>
<p>
Curabitur at erat nec lacus tempus condimentum. Sed si
+t amet ipsum. Praesent velit risus, sagittis a, vulputate eu, blandit
+ venenatis, quam. Aenean quis nibh id risus commodo aliquam. Suspendi
+sse nec tellus eu mauris hendrerit pretium. Nullam vitae libero. Etia
+m mi risus, lobortis a, eleifend id, venenatis nec, felis. Ut blandit
+ vestibulum libero. Nullam ultricies condimentum turpis. Nulla facili
+si. Suspendisse tincidunt sagittis arcu. Donec ultrices sem ac felis
+venenatis egestas. Proin faucibus lectus pharetra ante. Donec egestas
+, arcu sed luctus egestas, dolor nunc pharetra odio, in laoreet pede
+lectus eu quam. Quisque vitae dui at leo rutrum sagittis. Nam pretium
+ enim sit amet sem. Maecenas quis nisl.
</p>
</div>
<div class="menu">
<a href="">Link One</a>
<a href="">Link Two</a>
<a href="">Link Three</a>
<a href="">Link Four</a>
</div>
</div>
</body>
</html>
|
