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copied!<p>Generally speaking, <code>Any</code> types aren't very useful. Consider: If you make a polymorphic list that can hold anything, what can you do with the types in the list? The answer, of course, is nothing - you have no guarantee that there is any operation common to these elements.</p> <p>What one will typically do is either:</p> <ol> <li><p>Use <a href="http://en.wikibooks.org/wiki/Haskell/GADT" rel="noreferrer">GADTs</a> to make a list that can contain elements of a specific typeclass, as in:</p> <pre><code>data FooWrap where FooWrap :: Foo a => a -> FooWrap type FooList = [FooWrap] </code></pre> <p>With this approach, you don't know the concrete type of the elements, but you know they can be manipulated using elements of the <code>Foo</code> typeclass.</p></li> <li><p>Create a type to switch between specific concrete types contained in the list:</p> <pre><code>data FooElem = ElemFoo Foo | ElemBar Bar type FooList = [FooElem] </code></pre> <p>This can be combined with approach 1 to create a list that can hold elements that are of one of a fixed set of typeclasses.</p></li> <li><p>In some cases, it can be helpful to build a list of manipulation functions:</p> <pre><code>type FooList = [Int -> IO ()] </code></pre> <p>This is useful for things like event notification systems. At the time of adding an element to the list, you bind it up in a function that performs whatever manipulation you'll later want to do.</p></li> <li><p>Use <code>Data.Dynamic</code> (not recommended!) as a cheat. However, this provides no guarantee that a specific element can be manipulated at all, and so the above approaches should be preferred.</p></li> </ol>

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