[Haskell-cafe] existential types

Jake McArthur jake.mcarthur at gmail.com
Wed Feb 13 11:41:03 EST 2008

Let's try a different example. Let's start with a list designed to  
hold numbers:

data NumericList a = Num a => [a]

Normally this would be fine. We can hold things like [1, 2, 3, 4, 5]  
and [1.7, 5.3, 2.0, 99.1]. But what if we wanted to be able to hold  
numbers with _different types_ in the list? That is, we won't know  
anything about them except that they are instances of Num. That is  
where existential types come in:

data NumericList = forall a . Num a => [a]

Now we can have NumericLists like [1 :: Int, 5.7 :: Double, 4%5 ::  
Rational]. The only useful thing you can do with the values, in this  
case, is apply functions of the Num type class, but there are times  
when this is useful. (There are actually ways to cast these values  
back to their original types elsewhere, but I'll ignore that because  
it is far out of the scope of your question).

Does this help?

- Jake

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