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Programming -> Haskell newbie question

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wtd - Jul-25-2005 server time

GHCi is the interactive version of GHC. Quite like Hugs, but in my opinion, better.

Endow - Jul-25-2005 server time

"i"?

I think I already have GHC correctly installed.

wtd - Jul-25-2005 server time

You should try GHCi.

http://haskell.org/ghc/

Endow - Jul-25-2005 server time

The worst part is that WinHugs isn't working alright for me.

wtd - Jul-24-2005 server time

Odd.

On my Linux box:

CODE

$ cat > Main.hs
module Main where

main = putStrLn "Hello world"
$ hugs
__ __ __ __ ____ ___ _________________________________________
|| || || || || || ||__ Hugs 98: Based on the Haskell 98 standard
||___|| ||__|| ||__|| __|| Copyright (c) 1994-2003
||---|| ___|| World Wide Web: http://haskell.org/hugs
|| || Report bugs to: hugs-bugs@haskell.org
|| || Version: November 2003 _________________________________________

Haskell 98 mode: Restart with command line option -98 to enable extensions

Type :? for help
Prelude> :load Main
Main> main
Hello world

Main> :q
[Leaving Hugs]
$

Endow - Jul-24-2005 server time

Ah yes but it doens't have anything to do with the capitalized M.I checked.I still get the same error message.

wtd - Jul-23-2005 server time

Module "Main" should be in "Main.hs". If I indicated otherwise in the turorial, then I apologize.

Endow - Jul-23-2005 server time

module Main where

Just like in the tutorial

wtd - Jul-23-2005 server time

QUOTE (Endow @ Jul 23 2005, 12:23 PM)

Okay I'm new here and I'm also new to functional programming.But my problem isn't with the actual programming.I'm reading that evry helpful Haskell tutorial but when I'm in Hugs and I want to load a module and I do this :

:load main

this appears :

ERROR "main.hs":1 - Syntax error in input (unexpected backslash (lambda) )

The truth is I never even used the command prompt before but it doens't look liekt hat's the problem.

What's the first line of main.hs?

Endow - Jul-23-2005 server time

wtd - Jul-04-2005 server time

Using "div", which does integer math... (div 3 2 == 1, for instance)

CODE

pow _ 0 = 1
pow x 1 = x
pow x n =
if rem n 2 == 0
then pow x (div n 2) ^ 2
else x * pow x (div n 2) ^ 2

Oh, and you'll notice that I used a new definition of "pow" for an exponent of 1.

WhoDoo - Jul-03-2005 server time

Thank you

I have another question and I cant understand why it doesnt work. I wanna calculate x^n by using:

x^0 = 1
x^n = (x^(n/2))^2 if n is even
x^n = x * (x^(n/2))^2 if n is odd

I use this code

CODE

pow (x,0) = 1
pow (x, n) =
if rem n 2 == 0
then (pow (x, n/2))^2
else x * (pow (x, n/2))^2

but when trying it by typing pow (2,4) or something, I get an error

wtd - Jul-02-2005 server time

For the first question, this should work.

CODE

import IO

myfunc nbr =
do
putStr ("This is " ++ show nbr)
hFlush stdout
if nbr < 10 then myfunc (nbr + 1)
else return ()

However, Haskell already makes this pretty easy. You could simply write:

CODE

mapM_ (\nbr -> do { putStr ("This is " ++ show nbr); hFlush stdout }) [0..9]

As for your second question... the closest thing to nothing in Haskell is ().

However, it sounds more like you're thinking of the Maybe data type. Let's consider a fairly simple example. You want to find the position of an element in a list.

First, we know that nothing we could possibly be located anywhere in an empty list. So the item and the counter don't even matter.

CODE

findPos _ _ [] = Nothing

But it might be in a non-empty list.

CODE

findPos item counter (x:xs) =
if x == item then
Just counter
else
findPos item (counter + 1) xs

Here we used the Just constructor for the Maybe data type, whereas we had used the Nothing constructor previously.

Now if I were to run:

CODE

findPos 4 0 [1,2,3,4,5]

I would get in return:

CODE

Just 3

But if I run:

CODE

findPos 7 0 [1,2,3,4,5]

I get:

CODE

Nothing

I can use pattern matching to discern this at run-time.

CODE

case findPos n 0 [1..9] of
Nothing -> putStrLn (show n ++ " wasn't found.")
Just pos -> putStrLn (show n ++ " found at index " ++ show pos)

WhoDoo - Jul-01-2005 server time

I've just started learning Haskell and I have two questions. First of all, what if I want to do two things in a function? (like first I want to print something, then I would like to do a function call). I want to make a simple recursive "for-loop" like this

myfunc nbr =
putStr "This is " ++ nbr;
if nbr < 10
then myfunc(nbr+1)
else return

somethings like that, but the code above isnt working. In C it would be something like

void function(int nbr) {
cout<<"This is" << nbr;
if(nbr < 10)
function(nbr+1);
else
return;
}

my second question is, what if I dont want a function to return something? Mabye I have a function that should print something if I pass a certain argument. Since I need an else-statement, what should I put there in order to do "nothing"?

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