Haskell/Solutions/Category theory

Which law implies transitivity?

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Formally, transitivity of a partial order is defined as: for any  ,  , and   if   and   then  . In the category defined by a partial order this translates to: for any objects  ,  , and   if there exist morphisms   and   then there exists a morphism  . This is guaranteed by the second law of categories, i.e., that they are closed under composition. Indeed, the last morphism is a composition of the first two.

Why does adding an arrow break category laws?

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Let's consider what must be the result of the compositions   and  . Notice that since   and   then   and given that there is only one morphism from   to  , namely  , it follows that  . Using similar reasoning we can show that  .

Now, let us consider the composition  ; substituting   into it yields  , which can be simplified to   using the third law of categories. However, the first law of categories states that this composition should be equal to  , yet  .