Haskell newbie question

[Anonymous]

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"?

Archived topic from Iceteks, old topic ID:3451, old post ID:27888

[wtd]

For the first question, this should work.

Code: Select all

import IO

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

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

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

:)

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[code]

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[code]

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][code]

I would get in return:

[code]Just 3[code]

But if I run:

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

I get:

[code]Nothing[code]

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)[code] 

[color=#888888][size=85]Archived topic from Iceteks,  old topic ID:3451, old post ID:27898[/size][/color]

[Anonymous]

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: Select all

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

[color=#888888][size=85]Archived topic from Iceteks,  old topic ID:3451, old post ID:27913[/size][/color]

[wtd]

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

Code: Select all

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[code]

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

[color=#888888][size=85]Archived topic from Iceteks,  old topic ID:3451, old post ID:27940[/size][/color]

[Anonymous]

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.

Archived topic from Iceteks, old topic ID:3451, old post ID:28483

[wtd]

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?

Archived topic from Iceteks, old topic ID:3451, old post ID:28486

[Anonymous]

module Main where


Just like in the tutorial

Archived topic from Iceteks, old topic ID:3451, old post ID:28487

[wtd]

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

Archived topic from Iceteks, old topic ID:3451, old post ID:28488

[Anonymous]

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

Archived topic from Iceteks, old topic ID:3451, old post ID:28659

[wtd]

Odd.

On my Linux box:

Code: Select all

$ 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]
$[code] 

[color=#888888][size=85]Archived topic from Iceteks,  old topic ID:3451, old post ID:28664[/size][/color]

[Anonymous]

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

Archived topic from Iceteks, old topic ID:3451, old post ID:28675

[wtd]

You should try GHCi.

http://haskell.org/ghc/

Archived topic from Iceteks, old topic ID:3451, old post ID:28678

[Anonymous]

"i"?

I think I already have GHC correctly installed.

Archived topic from Iceteks, old topic ID:3451, old post ID:28679

[wtd]

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

Archived topic from Iceteks, old topic ID:3451, old post ID:28680