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    <p>If I'm understanding this right, your problem boils down to "I want to stuff things of unknown type into a container, and then at run-time I want to pull them out and run a different function based on their types".</p> <p>There are a couple of possible ways to attack this problem, with varying degrees of complexity.</p> <p>First of all, Haskell has <em>type erasure</em>. When you compile a program, after the compiler checks that all the types are OK, it <em>erases</em> them. So it's impossible, at run-time, to tell what type anything has. This only works because it's impossible, at compile-time, to not know the type of something. This is a verbose way of saying "Haskell is statically-typed".</p> <p>Having just said that, "Haskell with a few extensions" can be made to do this. However, I suspect that your problem may have a much simpler solution...</p> <p>OK, so you want to store different types of stuff. But what do you actually want to <em>do</em> to this stuff once you've stored it? If you just want to execute some function on the data (depending on its type), then why not <em>just store the function itself?</em></p> <p>Think about it. Instead of a container with lots of different types inside it, it would simply contain lots of functions with identical types. You fetch the function you want, and you run it. Done.</p> <p>What's that? You need to be able to run <em>several</em> different functions on your data? Well, in that case, try storing a data structure containing all the functions you need.</p>
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    1. POInvoke a function dynamically based on the type of one parameter (was "Zope in Haskell")
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    1. COI was going to suggest this too. In particular, instead of creating a context, the routing table calls a handler for an URL, where the handler understands the context implicitly.
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    2. COThat's an interesting approach, but wouldn't this be wasteful? For example, there's only one `addQuestion` view/function for all records of type `FAQ`. So if there are five FAQs in the tree, there should be five copies of the same function. And, if someone decide to add a new view, then he should visit all nodes on the tree and add a copy of the new function on each node of the right type.
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