## root / src / Ganeti / BasicTypes.hs @ b1c772fd

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{-# LANGUAGE FlexibleInstances #-} |
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{- |

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Copyright (C) 2009, 2010, 2011, 2012 Google Inc. |

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This program is free software; you can redistribute it and/or modify |

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it under the terms of the GNU General Public License as published by |

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the Free Software Foundation; either version 2 of the License, or |

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(at your option) any later version. |

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This program is distributed in the hope that it will be useful, but |

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WITHOUT ANY WARRANTY; without even the implied warranty of |

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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |

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General Public License for more details. |

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You should have received a copy of the GNU General Public License |

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along with this program; if not, write to the Free Software |

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Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA |

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02110-1301, USA. |

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-} |

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module Ganeti.BasicTypes |

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( GenericResult(..) |

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, genericResult |

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, Result |

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, ResultT(..) |

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, resultT |

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, FromString(..) |

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, isOk |

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, isBad |

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, eitherToResult |

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, annotateResult |

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, iterateOk |

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, select |

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, LookupResult(..) |

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, MatchPriority(..) |

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, lookupName |

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, goodLookupResult |

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, goodMatchPriority |

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, prefixMatch |

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, compareNameComponent |

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) where |

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import Control.Applicative |

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import Control.Monad |

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import Control.Monad.Trans |

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import Data.Function |

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import Data.List |

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-- | Generic monad for our error handling mechanisms. |

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data GenericResult a b |

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= Bad a |

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| Ok b |

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deriving (Show, Eq) |

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-- | Sum type structure of GenericResult. |

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genericResult :: (a -> c) -> (b -> c) -> GenericResult a b -> c |

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genericResult f _ (Bad a) = f a |

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genericResult _ g (Ok b) = g b |

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-- | Type alias for a string Result. |

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type Result = GenericResult String |

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-- | Type class for things that can be built from strings. |

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class FromString a where |

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mkFromString :: String -> a |

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-- | Trivial 'String' instance; requires FlexibleInstances extension |

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-- though. |

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instance FromString [Char] where |

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mkFromString = id |

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-- | 'Monad' instance for 'GenericResult'. |

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instance (FromString a) => Monad (GenericResult a) where |

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(>>=) (Bad x) _ = Bad x |

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(>>=) (Ok x) fn = fn x |

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return = Ok |

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fail = Bad . mkFromString |

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instance Functor (GenericResult a) where |

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fmap _ (Bad msg) = Bad msg |

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fmap fn (Ok val) = Ok (fn val) |

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instance MonadPlus (GenericResult String) where |

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mzero = Bad "zero Result when used as MonadPlus" |

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-- for mplus, when we 'add' two Bad values, we concatenate their |

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-- error descriptions |

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(Bad x) `mplus` (Bad y) = Bad (x ++ "; " ++ y) |

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(Bad _) `mplus` x = x |

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x@(Ok _) `mplus` _ = x |

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instance Applicative (GenericResult a) where |

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pure = Ok |

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(Bad f) <*> _ = Bad f |

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_ <*> (Bad x) = Bad x |

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(Ok f) <*> (Ok x) = Ok $ f x |

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-- | This is a monad transformation for Result. It's implementation is |

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-- based on the implementations of MaybeT and ErrorT. |

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newtype ResultT a m b = ResultT {runResultT :: m (GenericResult a b)} |

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instance (Monad m, FromString a) => Monad (ResultT a m) where |

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fail err = ResultT (return . Bad $ mkFromString err) |

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return = lift . return |

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x >>= f = ResultT $ do |

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a <- runResultT x |

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case a of |

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Ok val -> runResultT $ f val |

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Bad err -> return $ Bad err |

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instance MonadTrans (ResultT a) where |

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lift x = ResultT (liftM Ok x) |

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instance (MonadIO m, FromString a) => MonadIO (ResultT a m) where |

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liftIO = lift . liftIO |

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-- | Lift a `Result` value to a `ResultT`. |

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resultT :: Monad m => GenericResult a b -> ResultT a m b |

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resultT = ResultT . return |

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-- | Simple checker for whether a 'GenericResult' is OK. |

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isOk :: GenericResult a b -> Bool |

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isOk (Ok _) = True |

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isOk _ = False |

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-- | Simple checker for whether a 'GenericResult' is a failure. |

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isBad :: GenericResult a b -> Bool |

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isBad = not . isOk |

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-- | Converter from Either to 'GenericResult'. |

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eitherToResult :: Either a b -> GenericResult a b |

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eitherToResult (Left s) = Bad s |

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eitherToResult (Right v) = Ok v |

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-- | Annotate a Result with an ownership information. |

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annotateResult :: String -> Result a -> Result a |

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annotateResult owner (Bad s) = Bad $ owner ++ ": " ++ s |

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annotateResult _ v = v |

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-- | Iterate while Ok. |

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iterateOk :: (a -> GenericResult b a) -> a -> [a] |

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iterateOk f a = genericResult (const []) ((:) a . iterateOk f) (f a) |

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-- * Misc functionality |

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-- | Return the first result with a True condition, or the default otherwise. |

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select :: a -- ^ default result |

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-> [(Bool, a)] -- ^ list of \"condition, result\" |

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-> a -- ^ first result which has a True condition, or default |

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select def = maybe def snd . find fst |

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-- * Lookup of partial names functionality |

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-- | The priority of a match in a lookup result. |

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data MatchPriority = ExactMatch |

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| MultipleMatch |

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| PartialMatch |

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| FailMatch |

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deriving (Show, Enum, Eq, Ord) |

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-- | The result of a name lookup in a list. |

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data LookupResult = LookupResult |

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{ lrMatchPriority :: MatchPriority -- ^ The result type |

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-- | Matching value (for ExactMatch, PartialMatch), Lookup string otherwise |

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, lrContent :: String |

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} deriving (Show) |

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-- | Lookup results have an absolute preference ordering. |

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instance Eq LookupResult where |

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(==) = (==) `on` lrMatchPriority |

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instance Ord LookupResult where |

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compare = compare `on` lrMatchPriority |

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-- | Check for prefix matches in names. |

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-- Implemented in Ganeti core utils.text.MatchNameComponent |

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-- as the regexp r"^%s(\..*)?$" % re.escape(key) |

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prefixMatch :: String -- ^ Lookup |

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-> String -- ^ Full name |

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-> Bool -- ^ Whether there is a prefix match |

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prefixMatch = isPrefixOf . (++ ".") |

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-- | Is the lookup priority a "good" one? |

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goodMatchPriority :: MatchPriority -> Bool |

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goodMatchPriority ExactMatch = True |

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goodMatchPriority PartialMatch = True |

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goodMatchPriority _ = False |

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-- | Is the lookup result an actual match? |

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goodLookupResult :: LookupResult -> Bool |

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goodLookupResult = goodMatchPriority . lrMatchPriority |

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-- | Compares a canonical name and a lookup string. |

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compareNameComponent :: String -- ^ Canonical (target) name |

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-> String -- ^ Partial (lookup) name |

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-> LookupResult -- ^ Result of the lookup |

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compareNameComponent cnl lkp = |

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select (LookupResult FailMatch lkp) |

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[ (cnl == lkp , LookupResult ExactMatch cnl) |

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, (prefixMatch lkp cnl , LookupResult PartialMatch cnl) |

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] |

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-- | Lookup a string and choose the best result. |

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chooseLookupResult :: String -- ^ Lookup key |

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-> String -- ^ String to compare to the lookup key |

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-> LookupResult -- ^ Previous result |

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-> LookupResult -- ^ New result |

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chooseLookupResult lkp cstr old = |

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-- default: use class order to pick the minimum result |

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select (min new old) |

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-- special cases: |

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-- short circuit if the new result is an exact match |

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[ (lrMatchPriority new == ExactMatch, new) |

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-- if both are partial matches generate a multiple match |

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, (partial2, LookupResult MultipleMatch lkp) |

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] where new = compareNameComponent cstr lkp |

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partial2 = all ((PartialMatch==) . lrMatchPriority) [old, new] |

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-- | Find the canonical name for a lookup string in a list of names. |

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lookupName :: [String] -- ^ List of keys |

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-> String -- ^ Lookup string |

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-> LookupResult -- ^ Result of the lookup |

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lookupName l s = foldr (chooseLookupResult s) |

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(LookupResult FailMatch s) l |