root / src / Ganeti / BasicTypes.hs @ b74ebe44
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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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|
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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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|
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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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-} |
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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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, 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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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-- * 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 |