rule candidate: restkeep — unread String suffix stored again while walking it

[noah-emp]

Category

suspicious

Shape

A recursive String stepper puts the unread suffix into a returned constructor field and also passes that same suffix as the walk argument (or stores it again in the next state). Every step then keeps an extra copy of the rest of the input.

Prefer returning a mode that means "resume on the suffix already in hand" without embedding that suffix a second time.

Bad

type Mode is Data:
  MDone{saved: String}

def step(rest: String) -> Mode:
  match rest:
    case SNil{}:
      MDone{rest}
    case SCon{_', t}:
      step(t)

Good

type Mode is Data:
  MRest{}

def step(rest: String) -> (Mode & String):
  match rest:
    case SNil{}:
      (MRest{}, rest)
    case SCon{_', t}:
      step(t)

(or thread rest only as the walk argument and leave it out of Mode)