Errors Across the Seam

Get, Update, and Delete all reduce to one translation: "the row isn't there" becomes ErrNotFound, and everything else is a generic wrapped error. That covers those three methods completely — but Create can fail for a reason neither of those categories fits. Give tasks.title a UNIQUE constraint and Create("write lesson", 2) twice, and the second call fails not because anything is missing, but because something already exists. An HTTP handler above this store needs to tell that apart from "the database is unreachable": one is a 409 Conflict the client should see and maybe retry differently; the other is a 500 and an alert.

Naming a second failure

var ErrDuplicate = errors.New("task title already exists")

Detecting it means going one level below database/sql, into what the driver actually returns:

var sqliteErr *sqlite.Error
if errors.As(err, &sqliteErr) && sqliteErr.Code() == sqliteConstraintUnique {
	return Task{}, ErrDuplicate
}

modernc.org/sqlite doesn't export a friendly ErrConstraintUnique — you get a *sqlite.Error with an integer Code(), 2067, which is SQLite's own extended result code for SQLITE_CONSTRAINT_UNIQUE. This is normal, not a gap in the driver: database/sql is deliberately driver-agnostic, so anything database-specific — including "which integer means duplicate" — necessarily lives one layer down, in a driver-specific error type you reach with errors.As. Every SQL driver you'll use professionally has some version of this: a typed error, an inconvenient integer, and a comment explaining what it means because the constant isn't exported either.

Translate what you have a plan for, and nothing else

It would be easy to reach for a switch over every sqlite.Error code and map each to some sentinel. Resist it. ErrNotFound and ErrDuplicate exist because the layer above has different, specific behavior for each — a 404 and a 409. A sqliteErr.Code() you have no distinct response for isn't a sentinel candidate; it's just an error, and it should stay one, wrapped with %w so whoever reads the log still sees SQLite's actual message. Translating everything "for consistency" throws that message away and replaces it with whichever sentinel the switch happened to fall into — which, for a genuinely unexpected failure, is actively worse than not translating it at all: it tells the next person debugging a real outage the wrong thing with total confidence.

Try it

Update Create to return ErrDuplicate when the title already exists, leaving every other failure — including ones you haven't seen — as a generic wrapped error.