Design Pattern: Depth-first Search

(defn search [state] ; returns path to goal
         (if (goal? state)
         '() ; no ops needed to get to goal
         (if (failure? state)
         nil
         (some
         (fn [op]
         (let
         [path (search
         (applyop op state ))]
         (and path (cons op path)) ))
         (operators state)) )))

Complications:

• It may not be possible to apply op.

• Applying op might violate a constraint.

• We could get into a loop applying op and its inverse, or going in a circle back to the same state.