Re: [Haskell-beginners] How to process a list that contains "undefined" values?

On Sunday 03 July 2011, 08:40:03, Mike Meyer wrote:

On Sat, 2 Jul 2011 16:32:37 +0200

Daniel Fischer wrote:

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On Saturday 02 July 2011, 16:09:05, Costello, Roger L. wrote:

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Hi Folks,

How would you find the maximum value in this list: [undefined, 10, undefined, 20]

Not. There is no proper maximum in the presence of undefined.

Doesn't that mean the maximum value for this list is also "undefined"?

Yes, and that way, a simple call to maximum does the right thing (note however, that it doesn't necessarily does the right thing for lists of Double/Float containing NaNs). I assumed the OP wanted the maximum of the defined values, in which case such a situation is best avoided by using Maybe or something serving similar purposes.

That matches my intuition, which is that undefined should be a "contagious" value, so that any value that depends on evaluating an undefined value is itself undefined.

Right. Except that in IO exceptions can be caught which muddies the waters (is foo = bar `Control.Exception.catch` (\(ErrorCall msg) -> if msg == "Prelude.undefined" then return 1 else return 2) legit?)