[Haskell-cafe] Re: Use unsafePerformIO to catch Exception?

[John Lato]

On Fri, Mar 27, 2009 at 9:51 PM, Jason Dusek <jason.dusek at gmail.com> wrote:
> 2009/03/27 John Lato <jwlato at gmail.com>:
>> From: Jules Bean <jules at jellybean.co.uk>
>> > wren ng thornton wrote:
>> > > The type of head should not be [a] -> a + Error, it should
>> > > be (a:[a]) -> a. With the latter type the compiler can
>> > > ensure the precondition will be proved before calling
>> > > head, thus eliminating erroneous calls.
>> >
>> > Yes, but you know and I know that's not haskell.
>> >
>> > I'm talking about haskell.
>> >
>> > In haskell - a language which does not fully support
>> > dependent types - head is both necessary and useful.
>>
>> I could follow the rest of this, but I don't understand why
>> 'head' is necessary.  Couldn't you always replace it with a
>> case statement, with undefined on [] if necessary?
>
>  How would that be any different from head?
>

That's what I'm asking.  It was claimed that 'head' is a necessary
function, but I don't see why.