ity :: MatchGroup id body -> Arity
-- Precondition: MatchGroup is non-empty
-- This is called before type checking, when mg_arg_tys is not set
matchGroupArity (MG { mg_alts = alts })
  | L _ (alt1:_) <- alts = length (hsLMatchPats alt1)
  | otherwise        = panic "matchGroupArity"

hsLMatchPa
matchGroupArity :: MatchGroup id body -> Arity
matchGroupArity (MG { mg_alts = alts })
  | L _ (alt1:_) <- alts = length (hsLMatchPats alt1)
  | otherwise        = panic "matchGroupArity"
matchGroupArity (MG { mg_alts = alts })
  | L _ (alt1:_) <- alts = length (hsLMatchPats alt1)
  | otherwise        = panic "matchGroupArity"