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

On Fri, Mar 27, 2009 at 9:51 PM, Jason Dusek wrote:

2009/03/27 John Lato :

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From: Jules Bean

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wren ng thornton wrote:

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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.