We look at the representation of DFA in Haskell (using lists to represent the sets of the formal definition).

In this video the code isDFA is defining what counts as a DFA when we use the black-hole convention: it has a single start state and at most one a-labelled transition from any state q.

In the tutorial exercise you are asked to define when an FSM is a DFA in the strict sense that for each symbol a and state q there is exactly one a-labelled transition from q.