putStr

[Hal Daume III]

In general, you can't, not with these type signatures, since printing 
something is a side effect.

If this is only for debugging purposes, you might try using trace from 
IOExts.  You can use it in a nice fashion like:

> f1 :: Int -> Int
> f1 x 
>   | trace ("The initial value is " ++ show x) False = undefined
>   | otherwise = f2 x

In general, the 'trace ... False = undefined' thing is quite useful, but 
note that trace uses unsafePerformIO and very little is guarenteed about 
the output (especially the ordering might be different than you would 
expect...but it's usually fine for debugging purposes).   

On 13 Oct 2003, Jose Morais wrote:

> Hi,
> 
> 	I am trying to something like
> 
> f1 :: Int -> Int
> f1 x = f2 x
> 
> f2 :: Int -> Int
> f2 x = 2 * x
> 
> 
> 	but before f2 returns its result I'd like it to print something like
> "The initial value is " ++ show x.
> 
> 	How could I do this?
> 
> 
> 	Thank you
> 
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
> 

-- 
 Hal Daume III                                   | hdaume at isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume