fromJust Nothing problem

[Serge D. Mechveliani]

On Tue, Nov 14, 2006 at 12:08:07PM +0000, Jon Fairbairn wrote:

> >[..]
> > 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?

1) I do not see why the latter is tidier. It uses two extra functions.
2) By "needs to write" I meant "needs to write something like".

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