Toggle, don't retype

Toggle, don't retype is the rule Curseur is built on. A shopping list is not a document you write each week and delete in the shop. It is a board of everything your household buys, each item with a state: needed, or not. Shopping is changing states. Nothing is typed twice, because nothing is typed at all after the first time.

The failure of every shared note

The shared note is where most households keep the shop, and it fails the same way each week. Someone types milk. Someone else types milk. In the shop, lines are deleted one by one until the note is empty, and next week starts from nothing. The note has the right people on it and the wrong shape: it is a list that empties. Every item is retyped, week after week, by whoever remembers first, and the remembering is the work the list was supposed to do.

A board that never empties

A pense-bête à curseurs, the enamel board Curseur is named for, solved this a century ago with a painted list and a sliding cursor beside each line. The list never changed; the cursors did. Curseur does the same on a phone. The board holds everything you ever buy, and each item has a curseur that you flick on when the thing is needed and off when it is in the trolley. Milk is typed once, the day the household starts the board, and never again.

What follows from it

• Categories are optional. A board is readable without them, so an item with no category is not an error: it sits under Unsorted until someone files it, if anyone ever does.
• Filing is a swipe, not a tap. The tap is the flick, and it means one thing. One gesture, one meaning.
• Nothing is deleted in the shop. Buying an item flicks it off, and the item stays for next time. An item you stop buying goes to the shelf, from where it can come back.
• Shop mode shows only what is needed, aisle by aisle, and a tap buys. Undo takes it back, because a thumb in a supermarket is not precise.
• The whole board lives on every phone. There is no signal in the basement of the supermarket, and the board does not care.
• The widget is the board on the home screen: the flick, without opening the app.

Why it matters

A list that empties has to be rebuilt from memory, and memory is exactly what a household does not hold in common. A board that keeps its items and only changes their state carries the memory for you, and two people can keep it without a message between them. That is the whole of the idea, and everything in Curseur is a consequence of it.