[Haskell-cafe] Re: exceptions vs. Either

[André Pang]

On 04/08/2004, at 12:28 AM, MR K P SCHUPKE wrote:

>> f (case xs of (x:_) -> x; [] -> error "whoops")  -- direct style
>
> Yup, this is how I do it... I never use head!
>
> I like to pass failures back up to the level where some kind of 
> sensible
> error message can be generated. In your example the error is no
> better than with 'head' - the point is a Nothing can be 'caught'
> outside of an IO monad.
>
> I would suggest using the type system as I said earlier so:
>
> toNonEmptyList :: [a] -> Maybe (NonEmpty a)
> toNonEmptyList (a0:_) = Just (NonEmpty a)
> toNonEmptyList _ = Nothing
>
> Then redefine head:
>
> head :: NonEmpty a -> a
> head (NonEmpty (a0:_)) = a0

There's an interesting discussion going on at Lambda the Ultimate right 
now, about this very topic:

     http://lambda-the-ultimate.org/node/view/157#comment

There are plenty of noteworthy comments there, but one which quite 
nicely expresses my point of view is:

     "Using Maybe for this is like saying - let's turn this partial 
function into a total one by lifting its range to include Nothing. It 
became total by obtaining permission to return something I have no use 
of. I do not say monads are not useful, or Maybe is not useful."

And, of course, there's the type wizardry post by Oleg:

     http://lambda-the-ultimate.org/node/view/157#comment-1043

     "I'd like to point out that it is possible in Haskell98 to write 
non-trivial list-processing programs that are statically assured of 
never throwing a `null list' exception. That is, tail and head are 
guaranteed by the type system to be applied only to non-empty lists. 
Again, the guarantee is static, and it is available in Haskell98. 
Because of that guarantee, one may use implementations of head and tail 
that don't do any checks. Therefore, it is possible to achieve both 
safety and efficiency. Please see the second half of the following 
message:

     http://www.haskell.org/pipermail/haskell/2004-June/014271.html


-- 
% Andre Pang : trust.in.love.to.save