fromJust Nothing problem

[Jon Fairbairn]

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