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I want to implement a notion of a category, monoidal category and braided monoidal category in haskell. And I'm not sure if [a] or Data.Set a is a correct notion for representing an objects in a category.

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Not in general. See wikipedia. – Rasmus Aug 25 '11 at 15:33
@Andrew: There are the class “Category” in “base” and the classes “Monoidal”, “Braided” in “category-extras”. Maybe this is not what you need, as I do not understand “implementing”. Haskell is for computing, not for reasoning. – beroal Aug 25 '11 at 16:00
here is nice implementation by Edward Kmett hackage.haskell.org/package/categories – max taldykin Aug 25 '11 at 16:06
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btw, it's possible to define categories without notion of object, with arrows only – max taldykin Aug 25 '11 at 16:08
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How to implement the notion of a category in Haskell depends on whether you want categories to be values, types, or something else entirely. If a category is a value, as you seem to be wanting, there's no good answer to your question, but I think it may make more sense to make the category be an entire type, rather than just a list or set. That said, I think this is really more of a Haskell question than a math question; I'd suggest posting it on stackoverflow.com for better answers. – Tanner Swett Aug 25 '11 at 19:52

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Most categories do not have a set of objects. When this does happen, you have what is called a small category. Despite this, many categories have what is known as a small skeleton, meaning that the category of objects up to isomorphism is small. Such a category is said to be essentially small, and examples of such categories include finite dimensional $k$-vector spaces (for some field $k$) and finite sets.

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“essentially small” or svelte – PseudoNeo Sep 2 '11 at 7:39

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