Marco Russo and Alberto Ferrari talk in hypotheticals:
DAX is a functional language. This means that – no matter how complicated it is – a measure is just ONE function call. Then, functions call other functions, creating the intricacies of a sophisticated DAX expression. However, there is always just one function at the top level. This is, at the same time, beautiful and painful, elegant and complex to understand. It is fair to say that being functional is what makes DAX so fascinating.
However, when a DAX formula is executed, it loses its functional nature. Indeed, in the end it needs to be transformed into a set of simpler queries executed by one of the engines of DAX: either the storage engine or the formula engine. During this step, the function execution is transformed, and it becomes much simpler.
Click through to see how the IF() function works in such a world.