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{-# LANGUAGE TypeFamilies #-}
module T1897b where
import Control.Monad
import Data.Maybe
class Bug s where
type Depend s
next :: s -> Depend s -> Maybe s
start :: s
-- isValid :: (Bug s) => [Depend s] -> Bool
-- Inferred type should be rejected as ambiguous
isValid ds = isJust $ foldM next start ds
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