[Haskell-beginners] scope for variables

David Virebayre dav.vire+haskell at gmail.com
Thu Dec 10 03:17:47 EST 2009

Forgot to post the list :(

On Wed, Dec 9, 2009 at 10:40 PM, legajid <legajid at free.fr> wrote:

> Hello,
> i wrote the following (very simplified) program :
> word :: [Char]
> word="initial"
> letters=['a'..'z']
> myloop = do
>  myloop2
> myloop2 = do
>  putStrLn word
>  putStrLn letters
> main = do
>      putStrLn "Enter the word"
>      word <- enter_word
>      myloop
> enter_word= do
>   return("abcdefghij")

Remind that a haskell function depends only on its parameters. Unless you
use an unsafe trick, often frowned upon, mutable global variables don't

So normally you have to modify your functions to pass the string as a
parameter. While it may seem tedious at first, perhaps you will realise that
having the type signature mention that you need a string is a precious
information. Also, start by modifying only myloop2, ghc will complain about
every function that use the new myloop2 the wrong way, so refactoring is not
that hard.

In the case you have a more complicated program where state is an important
part, you can use the State monad, or more specifically in your example a
State transformer that uses IO as the underlying monad. Your program would
look like : ( you can copy and paste this in a source file, then play with
it in ghci )

module Main where

import Control.Monad.State

word :: [Char]

myloop = do

myloop2 =  do
  word <- get  -- récupère l'état.
  liftIO $ do
    putStrLn word
    putStrLn letters

main = runStateT mafonction word

mafonction = do
     liftIO $ putStrLn "Enter the word"
     w <- enter_word
     put w           -- enregistre l'état


enter_word= do

Note how the myloop function wasn't modified. On the other hand, have a look
at the type signatures using ghci. Your function now is in the StateT String
IO monad. So that's why every time you use IO you have to use liftIO to
reach the underlying IO monad.  So there's still some rewriting.

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