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爲什麼在範圍無法加載一個正常的文件,有錯誤:`S.elementOf'不在範圍:`S.elementOf '
import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Error
import qualified Data.Set as S
....
freshen s = do
names <- get
let s' = head [ns|suf<-suffixes
,let ns = s ++ show suf
,not (ns `S.elementOf` names)]
suffixes = [1..]
put (S.insert s' names)
return s'