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Err is now an instance of MonadPlus.

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krijo
2005-03-14 22:45:36 +00:00
parent 0d0da43253
commit a061ad4ee0
1 changed files with 10 additions and 4 deletions
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14
src/GF/Data/Operations.hs
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@@ -5,9 +5,9 @@
-- Stability : (stable) -- Stability : (stable)
-- Portability : (portable) -- Portability : (portable)
-- --
-- > CVS $Date: 2005/02/24 11:46:35 $ -- > CVS $Date: 2005/03/14 23:45:36 $
-- > CVS $Author: peb $ -- > CVS $Author: krijo $
-- > CVS $Revision: 1.16 $ -- > CVS $Revision: 1.17 $
-- --
-- some auxiliary GF operations. AR 19\/6\/1998 -- 6\/2\/2001 -- some auxiliary GF operations. AR 19\/6\/1998 -- 6\/2\/2001
-- --
@@ -77,7 +77,7 @@ module Operations (-- * misc functions
import Char (isSpace, toUpper, isSpace, isDigit) import Char (isSpace, toUpper, isSpace, isDigit)
import List (nub, sortBy, sort, deleteBy, nubBy) import List (nub, sortBy, sort, deleteBy, nubBy)
import Monad (liftM2) import Monad (liftM2, MonadPlus, mzero, mplus)
infixr 5 +++ infixr 5 +++
infixr 5 ++- infixr 5 ++-
@@ -107,6 +107,12 @@ instance Functor Err where
fmap f (Ok a) = Ok (f a) fmap f (Ok a) = Ok (f a)
fmap f (Bad s) = Bad s fmap f (Bad s) = Bad s
-- | added by KJ
instance MonadPlus Err where
mzero = Bad "error (no reason given)"
mplus (Ok a) _ = Ok a
mplus (Bad s) b = b
-- | analogue of @maybe@ -- | analogue of @maybe@
err :: (String -> b) -> (a -> b) -> Err a -> b err :: (String -> b) -> (a -> b) -> Err a -> b
err d f e = case e of err d f e = case e of