Re: fromJust Nothing problem

14 Nov 2006

      On 2006-11-14 at 13:57+0300 "Serge D. Mechveliani" wrote:
...
Thanks to people who discussed the question of 
"who said `fromJust Nothing'"  and  -xc  option.
My last impression is that instead of using  -xc  it is better to write 
programs in a debug-friendly style. For example, let  g x  must
return (Just _), but the programmer is not 100% sure that  g x  is free
of bugs. Then, instead of
                          f x = h $ fromJust $ g x
one needs too write
f x = case  g x
        of
        Just y -> h y
        _      -> error $ concat 
                  ["Foo.f ", shows x "\n:\nImpossible happened:\n",
                   "g ", shows x " = Nothing.\n"
                  ]
wouldn't 

f x = maybe mistake h (g x)

      where mistake = error $ ("Foo.f "++).
                              shows x.
                              (":\nThis shouldn't happen:\ng "++).
                              shows x
			    $ " = Nothing.\n"

be slightly tidier?

 Jón

-- 
Jón Fairbairn                              Jon.Fairbairn at cl.cam.ac.uk

Re: fromJust Nothing problem

Jon Fairbairn