Custom errors with instance chains

Help
5

For posterity, the example can be solved by using a RowList like so:

import Prim.RowList (class RowToList, Cons, Nil, RowList)
import Prim.TypeError (class Fail, Text)
import Type.Proxy (Proxy(..))

class MustLackRow :: forall k. Symbol -> Row k -> Constraint
class MustLackRow key row

instance mustLack :: (RowToList row rowList, MustLackRowList key rowList) => MustLackRow key row

class MustLackRowList :: forall k. Symbol -> RowList k -> Constraint
class MustLackRowList key rowList

instance mustLackRowList_Fail :: Fail (Text "Already has key in row") => MustLackRowList key (Cons key value tail)
else instance mustLackRowList_Looking :: ( MustLackRowList key tail ) => MustLackRowList key (Cons otherKey otherValue tail)
else instance mustLackRowList_Nil :: MustLackRowList key Nil

check ::
  forall key value row.
  MustLackRow key row =>
  Proxy key -> value -> Record row -> Boolean
check _ _ _ = true

_foo_ = Proxy :: Proxy "foo"

r = check _foo_ 5 { foo: "Hello" }
1 Like