# Talk:Type theory

*Question - does type theory have anything to do with category theory in mathematics?*

- Not really. A
*type*is a set of related things, together with a specification of the operations you can do to those things. A*category*is a set of related things, together with the maps (or arrows, or homomorphisms) between those things. AxelBoldt

But, aren't those "operations" on types very similar to "maps" between things in categories? For instance executing "push" on a stack must not change the fundamental nature of the stack, hence is not the operation "push" a mapping (or homomorphism) between stacks?

- Ok, I suppose you are right, for some types. So if we take the type "stack", we could manufacture a category out of it as follows: objects are the stacks, two stacks being considered different if they contain different things, and the morphisms between two stacks are all the sequences of push/pop operations that can get you from the first stack to the second stack.

- However, this works only for type operations that take one stack and return one stack. For instance, the "stack" ADT typically has an empty? boolean operation, and a top operation which returns the top element of the stack. Those two can't be modeled in category theory. AxelBoldt