Debugging by Sesame Street:

One of these things is not like the others:

stdPattern :: [LRep] -> Maybe StgHalfWord
stdPattern reps
  = case reps of
        []  -> Just ARG_NONE    -- just void args, probably
        [N] -> Just ARG_N
        [P] -> Just ARG_N
        [F] -> Just ARG_F
        [D] -> Just ARG_D
        [L] -> Just ARG_L

Today's debugging session was brought to you by the letter P.