[Haskell] lazy printing

[Serge D. Mechveliani]

Dear Haskellers,

Can you tell me how to force Haskell to output the results in a
`lazy' way? 
Consider, for example, the program

    main = putStr (concat ["\n min1 = ", show min1,
                           "\n min2 = ", show min2,
                           "\n"
                          ]
                  )
           where
           min1 = minimum [1 .. 10     ]  :: Integer
           min2 = minimum [1 .. (10^10)]  :: Integer

I do not know whether the effect is due to implementation.
And I `make'-run the program under  GHC-6.2.2-pre  by commanding

    ghc -O --make Main
    ./a.out 

This prints       min1 = 1

and hangs silently -- probably, because  min2  takes long to compute.
But why does not it print immediately
                  
                  min1 = 1
                  min2 = 
?  

This is for the first time in my practice that a `lazy' printing is
essential.
The above example is contrived.
The real example is a program that searches for the proof and 
accumulates a certain
                      ptrace :: ProofTrace,
                      type ProofTrace = [Step]

The first steps are performed easily, and also they are informative. 
So, the user should see their results immediately -- when one applies
(show ptrace).
The whole proof may take long, the last steps in  ptrace  may be 
ready after a very long time.
And Haskell just holds until the  whole ptrace  is ready, which is
annoying.

Copy, please, the answer to   mechvel at botik.ru

With kind regards,

-----------------
Serge Mechveliani
mechvel at botik.ru