# Definition:Domain Functor

## Definition

Let $\mathbf C$ be a metacategory.

Let $\mathbf C^\to$ be its morphism category.

The domain functor is the functor $\operatorname{\mathbf{dom}}: \mathbf C^\to \to \mathbf C$ defined by:

 Object functor: $\ds \operatorname{\mathbf{dom} } f := \operatorname{dom} f$ Morphism functor: $\ds \operatorname{\mathbf{dom} } \left({g_1, g_2}\right) := g_1$

That it is in fact a functor is shown on Domain Functor is Functor.

The functor $\mathbf{dom}$ can be represented as follows:

$\begin{xy} <0em,2em>*+{A} = "A", <0em,-2em>*+{B} = "B", <4em,2em>*+{A'} = "A2", <4em,-2em>*+{B'} = "B2", "A";"B" **@{-} ?>*@{>} ?*!/^1em/{f}, "A";"A2" **@{-} ?>*@{>} ?*!/_1em/{g_1}, "A2";"B2" **@{-} ?>*@{>} ?*!/_1em/{f'}, "B";"B2" **@{-} ?>*@{>} ?*!/^1em/{g_2}, <6em,0em>;<10em,0em> **@{~} ?>*@2{>} ?*!/_1em/{\mathbf{dom}}, <12em,2em>*+{A} = "AA", <16em,2em>*+{A'} = "AA2", "AA";"AA2" **@{-} ?>*@{>} ?*!/_1em/{g_1}, \end{xy}$

It is thus seen to be an example of a forgetful functor.