Copyright	(c) Edward Kmett 2018
License	BSD3
Maintainer	ekmett@gmail.com
Stability	stable
Portability	portable
Safe Haskell	Safe
Language	Haskell2010

Data.Perhaps

Contents

Maybe with an undisclosed error

Description

Synopsis

data Perhaps a
- = Can a
- | Can't Void
believe :: Perhaps a -> a
mayhap :: Perhaps a -> Maybe a
excuse :: (MonadPerhaps m, Exception e) => e -> m a

Maybe with an undisclosed error

data Perhaps a #

This monad occupies the middle ground between Maybe and Either in that you can get out an informative error but aren't able to care about its contents, except via bottoms.

Since bottoms are indistinguishable in pure code, one can view this as morally the same as Maybe, except when things go wrong, you can pass along a complaint, rather than take what you'd get from fromJust.

>>> import Control.Exception
>>> let x = excuse Overflow :: Perhaps ()

Attempting to Show a Perhaps value is hazardous, as it will contain an embedded exception.

>>> x
Can't *** Exception: arithmetic overflow

Recovery is possible as 'Can\'t' isn't strict in its argument.

>>> x <|> Can ()
Can ()

>>> x `seq` ()
()

Constructors

Can a
Can't Void

Instances

Monad Perhaps #
Instance details Methods (>>=) :: Perhaps a -> (a -> Perhaps b) -> Perhaps b # (>>) :: Perhaps a -> Perhaps b -> Perhaps b # return :: a -> Perhaps a # fail :: String -> Perhaps a #
Functor Perhaps #
Instance details Methods fmap :: (a -> b) -> Perhaps a -> Perhaps b # (<$) :: a -> Perhaps b -> Perhaps a #
MonadFix Perhaps #
Instance details Methods mfix :: (a -> Perhaps a) -> Perhaps a #
MonadFail Perhaps #
Instance details Methods fail :: String -> Perhaps a #
Applicative Perhaps #
Instance details Methods pure :: a -> Perhaps a # (<>) :: Perhaps (a -> b) -> Perhaps a -> Perhaps b # liftA2 :: (a -> b -> c) -> Perhaps a -> Perhaps b -> Perhaps c # (>) :: Perhaps a -> Perhaps b -> Perhaps b # (<*) :: Perhaps a -> Perhaps b -> Perhaps a #
Foldable Perhaps #
Instance details Methods fold :: Monoid m => Perhaps m -> m # foldMap :: Monoid m => (a -> m) -> Perhaps a -> m # foldr :: (a -> b -> b) -> b -> Perhaps a -> b # foldr' :: (a -> b -> b) -> b -> Perhaps a -> b # foldl :: (b -> a -> b) -> b -> Perhaps a -> b # foldl' :: (b -> a -> b) -> b -> Perhaps a -> b # foldr1 :: (a -> a -> a) -> Perhaps a -> a # foldl1 :: (a -> a -> a) -> Perhaps a -> a # toList :: Perhaps a -> [a] # null :: Perhaps a -> Bool # length :: Perhaps a -> Int # elem :: Eq a => a -> Perhaps a -> Bool # maximum :: Ord a => Perhaps a -> a # minimum :: Ord a => Perhaps a -> a # sum :: Num a => Perhaps a -> a # product :: Num a => Perhaps a -> a #
Traversable Perhaps #
Instance details Methods traverse :: Applicative f => (a -> f b) -> Perhaps a -> f (Perhaps b) # sequenceA :: Applicative f => Perhaps (f a) -> f (Perhaps a) # mapM :: Monad m => (a -> m b) -> Perhaps a -> m (Perhaps b) # sequence :: Monad m => Perhaps (m a) -> m (Perhaps a) #
MonadZip Perhaps #
Instance details Methods mzip :: Perhaps a -> Perhaps b -> Perhaps (a, b) # mzipWith :: (a -> b -> c) -> Perhaps a -> Perhaps b -> Perhaps c # munzip :: Perhaps (a, b) -> (Perhaps a, Perhaps b) #
Alternative Perhaps #
Instance details Methods empty :: Perhaps a # (<\|>) :: Perhaps a -> Perhaps a -> Perhaps a # some :: Perhaps a -> Perhaps [a] # many :: Perhaps a -> Perhaps [a] #
MonadPlus Perhaps #
Instance details Methods mzero :: Perhaps a # mplus :: Perhaps a -> Perhaps a -> Perhaps a #
MonadPerhaps Perhaps #
Instance details Methods perhaps :: Perhaps a -> Perhaps a # excuse :: Exception e => e -> Perhaps a #
Eq a => Eq (Perhaps a) #
Instance details Methods (==) :: Perhaps a -> Perhaps a -> Bool # (/=) :: Perhaps a -> Perhaps a -> Bool #
Data a => Data (Perhaps a) #
Instance details Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Perhaps a -> c (Perhaps a) # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Perhaps a) # toConstr :: Perhaps a -> Constr # dataTypeOf :: Perhaps a -> DataType # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Perhaps a)) # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Perhaps a)) # gmapT :: (forall b. Data b => b -> b) -> Perhaps a -> Perhaps a # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Perhaps a -> r # gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Perhaps a -> r # gmapQ :: (forall d. Data d => d -> u) -> Perhaps a -> [u] # gmapQi :: Int -> (forall d. Data d => d -> u) -> Perhaps a -> u # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Perhaps a -> m (Perhaps a) # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Perhaps a -> m (Perhaps a) # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Perhaps a -> m (Perhaps a) #
Ord a => Ord (Perhaps a) #
Instance details Methods compare :: Perhaps a -> Perhaps a -> Ordering # (<) :: Perhaps a -> Perhaps a -> Bool # (<=) :: Perhaps a -> Perhaps a -> Bool # (>) :: Perhaps a -> Perhaps a -> Bool # (>=) :: Perhaps a -> Perhaps a -> Bool # max :: Perhaps a -> Perhaps a -> Perhaps a # min :: Perhaps a -> Perhaps a -> Perhaps a #
Read a => Read (Perhaps a) #
Instance details Methods readsPrec :: Int -> ReadS (Perhaps a) # readList :: ReadS [Perhaps a] # readPrec :: ReadPrec (Perhaps a) # readListPrec :: ReadPrec [Perhaps a] #
Show a => Show (Perhaps a) #
Instance details Methods showsPrec :: Int -> Perhaps a -> ShowS # show :: Perhaps a -> String # showList :: [Perhaps a] -> ShowS #
Generic (Perhaps a) #
Instance details Associated Types type Rep (Perhaps a) :: * -> * # Methods from :: Perhaps a -> Rep (Perhaps a) x # to :: Rep (Perhaps a) x -> Perhaps a #
Semigroup a => Semigroup (Perhaps a) #
Instance details Methods (<>) :: Perhaps a -> Perhaps a -> Perhaps a # sconcat :: NonEmpty (Perhaps a) -> Perhaps a # stimes :: Integral b => b -> Perhaps a -> Perhaps a #
Semigroup a => Monoid (Perhaps a) #
Instance details Methods mempty :: Perhaps a # mappend :: Perhaps a -> Perhaps a -> Perhaps a # mconcat :: [Perhaps a] -> Perhaps a #
Generic1 Perhaps #
Instance details Associated Types type Rep1 Perhaps :: k -> * # Methods from1 :: Perhaps a -> Rep1 Perhaps a # to1 :: Rep1 Perhaps a -> Perhaps a #
type Rep (Perhaps a) #
Instance details type Rep (Perhaps a) = D1 (MetaData "Perhaps" "Control.Monad.Perhaps" "perhaps-0-inplace" False) (C1 (MetaCons "Can" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 a)) :+: C1 (MetaCons "Can't" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 Void)))
type Rep1 Perhaps #
Instance details type Rep1 Perhaps = D1 (MetaData "Perhaps" "Control.Monad.Perhaps" "perhaps-0-inplace" False) (C1 (MetaCons "Can" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) Par1) :+: C1 (MetaCons "Can't" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 Void)))

believe :: Perhaps a -> a #

This partial function can be used like fromJust, but throws the user error.

mayhap :: Perhaps a -> Maybe a #

excuse :: (MonadPerhaps m, Exception e) => e -> m a #

Fail with an exception as an excuse instead of just a string.