[Haskell-cafe] One question...

Donald Bruce Stewart dons at cse.unsw.edu.au
Sat Dec 9 12:17:31 EST 2006

> Hello to everyone!
> Two days ago I have found Haskell in Internet. It sounds very nice.
> I have read some articles, few examples, ... yes it sounds nice.

Great! Welcome to Haskell.
> Now my problem is connected with the "non-update" object feature.
> I can't write "variable" instead object because - from the meaning of the word 
> ( variable ) - it has the possibility to CHANGE its value. 
> Yes that my problem :-(

It's not a problem :)

> Let's assume that I've got to write the application that
> should works as follows: 
> 1. At the beginning the user should write her/his name (for instance as the 
> application parameter).
> 2. Let's assume that the application has many various functions defined 
> inside, and - after printing each of the outputs - the name has to be 
> printout.
> How should I think "properly" in Haskell to get described action?

Forget about updating values in place: its not necessary, and makes your
programs more flexible, and simpler to understand.

Here's a short program to read a user's name, and then apply various
functions to that name, printing the result.

    import System.IO
    import Data.Char
    import System.Console.Readline

    main = do
        name <- do m <- readline "What is your name> "
                   return (maybe "Nobody" id m)
        print (reverse name)
        print (length name)
        let n = map ord name
        print n
        print (sum n)
        print name

We can run this:

    $ runhaskell A.hs
    What is your name> lambdaman

Just dive in (start on http://haskell.org). 

-- Don

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