This is a quick post about the new RowLacks type class that was added to purescript-typelevel-prelude. This can be used to require that a row does not contain a specific key. The type class has the following signature:

class RowLacks (key :: Symbol)
               (row :: # Type)

There is a single instance defined for this type class. Using RowCons, Union, and instance resolution in interesting ways such that an instance isn’t found if row contains the label key.

So the following will compile:

eg0 :: Unit
eg0 = unit :: RowLacks "foo" (bar :: Unit) => Unit

But this program fails because row contains a label key:

eg1 :: Unit
eg1 = unit :: RowLacks "foo" (foo :: Unit, bar :: Unit) => Unit

-- compile error:
-- [1/1 NoInstanceFound]
--       eg1 = unit :: RowLacks "foo" (foo :: Unit, bar :: Unit) => Unit
--             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
--   No type class instance was found for
--     Type.Row.RowLacking Entry "foo" Unit (foo :: Unit, bar :: Unit)


Here’s the full implementation:

module Type.Row
  ( class RowLacks
  , class RowLacking
  ) where

-- Must not be exported
foreign import data Entry :: Type

-- | If you get "No type class instance was found" for this class, then your
-- | `row` shouldn't contain the label `key`.
class RowLacking (entry :: Type)
                 (key :: Symbol)
                 (typ :: Type)
                 (row :: # Type) |
                 entry typ -> key row

instance rowLacking :: RowLacking entry key entry row

-- | Encodes the constraint that a given row does not contain a specific key.
class RowLacks (key :: Symbol)
               (row :: # Type)

-- Append `Entry` at label `key` to the right of `row` then lookup `key` on the
-- left - if we check via instance solving that the `typ` we get back is
-- `Entry`, then `row` lacks `key`.  In the case that `row` doesn't lack
-- `key`, we get a "No type class instance found" error for:
-- `RowLacking Entry key typ row`.
instance rowLacks
  :: ( RowCons key Entry () keyEntry
     , Union row keyEntry rowKeyEntry
     , RowCons key typ ignored rowKeyEntry
     , RowLacking Entry key typ row )
  => RowLacks key row