When applying events, for instance. In F#, you could do:

  match msg with

  | IncreaseCounter cnt ->
   { model with Count = model.Count + cnt }

  | DecreaseCounter cnt ->
   { model with Count = model.Count - cnt }

  | ResetCounter ->
   { model with Count = 0 }

  | ChangeRubric name ->
   { model with Rubric = name, Count = 0 }

The "with" says: copy the original record by value, but change these fields. For completeness' sake: F# also has implicit returns, so the bit between brackets is the function's return value.