Ship

Lexicon · Row subtraction

Row subtraction

Remove tags from a row with - after the source row. Everything after - is subtracted from the type on the left.

type WorkStatus = [ :ready | :waiting | :paused ]
type NotWaiting = [ WorkStatus - :waiting ]
type NotWaiting = [ WorkStatus - | :waiting ]   // optional | after -

type OnlyPaused = [ WorkStatus - :ready | :waiting ]
type Lower = [> WorkStatus - :ready ]
type Upper = [< WorkStatus - :ready ]

  • In match, a named rest arm binds the subtracted residual row (see ../01-syntax.md).

  • On the exception row, handled error :tag arms subtract tags; inferred signatures may show exn [ 'row - :tag ] (see ../08-errors.md).

  • Payload tags must match exactly; subtracting a missing or mismatched tag is a compile error.

    Pair with row-extension. See crates/ship-types/tests/golden/check-types-row-subtraction/.