# Definition of opposite category

From page 29 of The algebra of programming :

For any category C the opposite category $C^{op}$ is defined to have the same objects and arrows as C, but the source and target operators are interchanged and composition is defined by swapping arguments:

$$f \cdot g \text{ in } C^{op} = g \cdot f \text{ in } C.$$

This doesn't makes sense. Suppose C is the categories of all functions, and we have $f: A \leftarrow B$ and $g: B \leftarrow C$, then $g \cdot f$ doesn't even exist. Or, suppose $f: A \leftarrow B$ and $g: B \leftarrow A$, then $f \cdot g : A \leftarrow A$, and $g \cdot f : B \leftarrow B$, this is not interchanging source and target, this is changing both!

• Your arrows are backwards compared to what would normally be written, do you mean \rightarrow: $\rightarrow$? Mar 8 '15 at 0:26
• The author of that book insists that it should be this way. I tend to agree with him.
– qed
Mar 8 '15 at 0:30
• Fair enough. I'll switch to this format for this question :). Mar 8 '15 at 0:34

The problem in your first example is that the arrows don't compose. In the first, you have $f: A \leftarrow B$ and $g: C \leftarrow B$, so $f\circ g$ is not in $C$, hence in $C^{op}$ $f : B \leftarrow A$ and $g : B \leftarrow C$, and they still don't compose.

If you had $f: A \leftarrow B$ and $g: B \leftarrow C$, then they compose to $f\circ g =h : A \leftarrow C$ in $C$. In $C^{op}$ you have $f : B \leftarrow A$ and $g : C \leftarrow B$ and get $g\circ f = h : C \leftarrow A$ (remember in $C^{op}$ you want every thing running backwards) .

Your second example is fine, you just have $f : A \leftarrow B$ and $g : B \leftarrow A$ in $C$ and hence $f\circ g : A\leftarrow A$ and $g\circ f:B\leftarrow B$ in $C$. In $C^{op}$, they switch: $f :B \leftarrow A$, $g : A \leftarrow B$ are in $C^{op}$ and so are $g \circ f : A \leftarrow A$ and $f\circ g : B\leftarrow B$.

I wonder perhaps if the phrasing used by Bird is causing a problem. The rule is better stated:

If $h = g \circ f$ in $C$ , then $h = f \circ g$ in $C^{op}$.

There's also a kind of name overloading going on here, from a programmer's perspective, reusing the same names for the opposite versions is a bit odd, it might make more sense to think along the lines (restating one of the rules):

If $f : X \leftarrow Y$ is in $C$, then $f^{op} : Y \leftarrow X$ is in $C^{op}$.