halkyon/docs/adr/0041-learn-subjects.md

ADR 0041: Learn's subjects: one per studyset, from a fixed list

• Status: decided 9 and 10 Nov 2026 by the developer: D1 to D4 and D6 are their own choices, D5 and D7 to D9 and D8's details the agent's recommendations, which they confirmed. D1 to D9 are built; D10 comes later.
• Builds on: ADR 0002 (payloads and their schemas), ADR 0005 (publishing), ADR 0031 (the palette's twenty colours).

Context

A library of a hundred studysets, most of them computing books and language courses, shows them by title alone. Vaia files every studyset under a subject from a fixed list, shown on its card with a stripe in the subject's colour, and lets the library be narrowed to one.

Decisions

• D1. One subject per studyset, from a fixed list of about Vaia's breadth, with Other / not sure last.
• D2. Each subject has a Lucide icon and a default colour from the palette. Choosing one gives the studyset that colour, which stays the studyset's to change.
• D3. Languages are one subject, Languages. The language itself comes from the studyset's front language, which every course already gives.
• D4. Computing is five subjects: Computer science, Software engineering, Data science and AI, IT and operations, Security and privacy. Most of the library is computing, and a single subject would narrow nothing.
• D5. The subject sits on the top studyset alone, which its sub-studysets show as theirs (subjectOf); their sheet offers none. It is kept in the studyset's sealed payload as subject, a subject's ID (schema 8), and so goes into archives and is published with the studyset, for Explore to filter by later.
• D6. The list is the agent's draft of 10 Nov 2026, taken as it is (below).
• D7. IDs are never renamed. An ID this app version doesn't know (a newer one's) is kept as it is and reads as no subject until another is picked.
• D8. The library, as Vaia's: a dropdown of the subjects in use, cards naming the subject under the title with a stripe in its colour, sections by recency (the last 7 days, the last month, older), the archive last. Decided on 10 Nov 2026:
  • recent by the later of a card's last rating in the studyset (or inside it) and the studyset's own last change; one from before changes were recorded counts from when it was made, as its UUID v7 says (lastUsedOf, shelvesOf);
  • the starred on a shelf of their own on top, however long ago they were used;
  • the subject chosen kept on the device (DeviceSettings.librarySubject), all subjects where none is chosen or the one kept is no longer in use; a studyset without a subject, or with one this version doesn't know, is under Other;
  • the stripe is the studyset's own colour, the card's bar on top, which the subject sets when picked; the subject's line shows the subject's icon, and a language course its language beside (Languages · Japanese).
• D9. Existing studysets: the agent writes a subject into each of the developer's 72 archives not yet imported; those already in the library get one by hand.
• D10. Explore filters by subject later; its releases don't keep the subject in a column yet (it is in each release's repository).

The list (D6)

ID, English and German name, Lucide icon, default colour (Subject, subject.dart).

Group	ID	English	German	Icon	Colour
Sciences and health	biology	Biology	Biologie	dna	fern
	chemistry	Chemistry	Chemie	flask-conical	reef
	physics	Physics	Physik	atom	slateblue
	astronomy	Astronomy	Astronomie	telescope	enzian
	earth-sciences	Earth sciences	Geowissenschaften	mountain	sahara
	environmental-science	Environmental science	Umweltwissenschaften	leaf	forest
	medicine	Medicine	Medizin	stethoscope	cerise
	nursing	Nursing	Pflege	heart-pulse	strawberry
	pharmacy	Pharmacy	Pharmazie	pill	pink
	nutrition	Nutrition	Ernährung	apple	carrot
	psychology	Psychology	Psychologie	brain	plum
	sports-science	Sports science	Sportwissenschaft	dumbbell	copper
Mathematics and computing	mathematics	Mathematics	Mathematik	sigma	cobalt
	statistics	Statistics	Statistik	chart-column	ocean
	computer-science	Computer science	Informatik	cpu	slateblue
	software-engineering	Software engineering	Softwareentwicklung	code	enzian
	data-science-ai	Data science and AI	Data Science und KI	brain-circuit	purple
	it-operations	IT and operations	IT und Betrieb	server	pine
	security-privacy	Security and privacy	Sicherheit und Datenschutz	shield-check	forest
	engineering	Engineering	Ingenieurwesen	cog	soil
	architecture	Architecture	Architektur	building-2	sahara
Society	history	History	Geschichte	landmark	soil
	geography	Geography	Geografie	globe	pacific
	politics	Politics	Politik	vote	cobalt
	law	Law	Recht	scale	pickle
	economics	Economics	Volkswirtschaft	trending-up	olive
	business	Business	Betriebswirtschaft	briefcase	copper
	marketing	Marketing	Marketing	megaphone	carrot
	accounting-finance	Accounting and finance	Rechnungswesen und Finanzen	calculator	pine
	sociology	Sociology	Soziologie	users	plum
	anthropology	Anthropology	Anthropologie	footprints	sahara
	education	Education	Pädagogik	school	ocean
	media-studies	Media studies	Medienwissenschaft	tv	pink
	philosophy	Philosophy	Philosophie	lightbulb	purple
	religious-studies	Religious studies	Religionswissenschaft	book-open	olive
Languages and arts	languages	Languages	Sprachen	languages	cerise
	literature	Literature	Literatur	book-marked	strawberry
	linguistics	Linguistics	Linguistik	speech	enzian
	art-design	Art and design	Kunst und Design	palette	pink
	music	Music	Musik	music	plum
	theatre-film	Theatre and film	Theater und Film	clapperboard	cerise
Everyday	cooking-hospitality	Cooking and hospitality	Kochen und Gastronomie	chef-hat	carrot
	driving-licences	Driving and licences	Führerschein und Lizenzen	car	soil
Other	other	Other / not sure	Sonstiges / unsicher	circle-question-mark	none

Consequences

• A studyset saved by this version is in schema 8, which older versions refuse to read, as every schema before (ADR 0002): an archive with subjects imports from this version on.
• Adding a subject is cheap; renaming an ID isn't (D7), so names change, IDs don't.