On Saturday 18 November 2006 00:37, Neil Mitchell wrote:
> Hi
>
> > > How controversial would a proposal to {-# DEPRECATE fromJust #-} be, in
> > > favour of:
> > >
> > > Just _ = x -- which will give you the precise line number
> > >
> > > It seems to me this is one cause of mysterious newbie errors we
> > > could easily discourage, with little harm.
> >
> > Btw, I'm not seriously suggesting removing it ;)
> > It could be discouraged ever so slightly in the haddocks though.
>
> I strongly disagree. If we are removing things that confuse newbies
> why not start with higher rank types, MPTC's and GADT's ;)
>
> fromJust is simple, useful and clear. What you mean is that
> implementations aren't very good at debugging this. It seems unfair to
> blame partial functions for the lack of a debugger. If a call stack
> was automatically output every time a fromJust failed would this even
> be something people complained about?
Well, I strongly disagree. :)
I suspect I would be classified as a newbie relative to most posters on this
list but here's my thoughts anyway...
I chose to learn haskell largely because I thought the static type safety
would help eliminate bugs, I've never once been happy when I've needed to
fire up a debugger or add trace statements and I hoped they would become
things of the past.
One of my initial responses to haskell was disappointment upon seeing head,
fromJust and the like. 'Those look nasty', I sez to meself.
From day one I've tried to avoid using them. Very occasionally I do use them
in the heat of the moment, but it makes me feel unclean and I end up having
to take my keyboard into the bathroom for a good scrubbing with the sandsoap.
The disappointment has pretty much changed to frustration since reading Oleg,
and others, type-hackery work. I realise the problems are just poor/limited
usage of haskell, not inherent in the language. Unfortunately I can
understand the 'right' solutions but still have trouble formulating them
myself. I'm hoping that if I hang out in the type-hackery 'hot zone' long
enough I'll start emitting enough picoOlegs myself to solve simple
problems. ;)
My view is that if I can reason about my program and know a static property I
want to be able to stamp this into the types and have my reasoning validated
during compilation.
But existing library code with weak typesafety is a large inertial mass, when
trying to do something 'right' the interfacing is a daunting prospect.
In general I'd rather have effort go into promoting 'right' solutions, and
making those solutions easier to express, than going into debuggers
for 'wrong' solutions.
Daniel
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