Interpreting Function Notation, Graphs, and Context

In Macroeconomics, Gross Domestic Product (GDP) is defined as $$\text{GDP}=C+I+G+NX$$ Where $$\begin{align} C&=\text{Consumption}\\ I&=\text{Investment}\\ G&=\text{Government spending}\\ NX&=\text{Net Exports}\end{align}$$

How would this be written in function notation?