| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | stable |
| Portability | portable |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
Data.Tuple
Description
Functions associated with the tuple data types.
Documentation
Solo is the canonical lifted 1-tuple, just like (,) is the canonical
lifted 2-tuple (pair) and (,,) is the canonical lifted 3-tuple (triple).
The most important feature of Solo is that it is possible to force its
"outside" (usually by pattern matching) without forcing its "inside",
because it is defined as a datatype rather than a newtype. One situation
where this can be useful is when writing a function to extract a value from
a data structure. Suppose you write an implementation of arrays and offer
only this function to index into them:
index :: Array a -> Int -> a
Now imagine that someone wants to extract a value from an array and store it in a lazy-valued finite map/dictionary:
insert "hello" (arr index 12) m
This can actually lead to a space leak. The value is not actually extracted from the array until that value (now buried in a map) is forced. That means the entire array may be kept live by just that value! Often, the solution is to use a strict map, or to force the value before storing it, but for some purposes that's undesirable.
One common solution is to include an indexing function that can produce its
result in an arbitrary Applicative context:
indexA :: Applicative f => Array a -> Int -> f a
When using indexA in a pure context, Solo serves as a handy
Applicative functor to hold the result. You could write a non-leaky
version of the above example thus:
case arr indexA 12 of
Solo a -> insert "hello" a m
While such simple extraction functions are the most common uses for unary tuples, they can also be useful for fine-grained control of strict-spined data structure traversals, and for unifying the implementations of lazy and strict mapping functions.
Constructors
| MkSolo a |
Instances
| MonadFix Solo Source # | Since: base-4.15 | ||||
| MonadZip Solo Source # | Since: base-4.15.0.0 | ||||
| Foldable Solo Source # | Since: base-4.15 | ||||
Defined in Data.Foldable Methods fold :: Monoid m => Solo m -> m Source # foldMap :: Monoid m => (a -> m) -> Solo a -> m Source # foldMap' :: Monoid m => (a -> m) -> Solo a -> m Source # foldr :: (a -> b -> b) -> b -> Solo a -> b Source # foldr' :: (a -> b -> b) -> b -> Solo a -> b Source # foldl :: (b -> a -> b) -> b -> Solo a -> b Source # foldl' :: (b -> a -> b) -> b -> Solo a -> b Source # foldr1 :: (a -> a -> a) -> Solo a -> a Source # foldl1 :: (a -> a -> a) -> Solo a -> a Source # toList :: Solo a -> [a] Source # null :: Solo a -> Bool Source # length :: Solo a -> Int Source # elem :: Eq a => a -> Solo a -> Bool Source # maximum :: Ord a => Solo a -> a Source # minimum :: Ord a => Solo a -> a Source # | |||||
| Foldable1 Solo Source # | Since: base-4.18.0.0 | ||||
Defined in Data.Foldable1 Methods fold1 :: Semigroup m => Solo m -> m Source # foldMap1 :: Semigroup m => (a -> m) -> Solo a -> m Source # foldMap1' :: Semigroup m => (a -> m) -> Solo a -> m Source # toNonEmpty :: Solo a -> NonEmpty a Source # maximum :: Ord a => Solo a -> a Source # minimum :: Ord a => Solo a -> a Source # foldrMap1 :: (a -> b) -> (a -> b -> b) -> Solo a -> b Source # foldlMap1' :: (a -> b) -> (b -> a -> b) -> Solo a -> b Source # foldlMap1 :: (a -> b) -> (b -> a -> b) -> Solo a -> b Source # foldrMap1' :: (a -> b) -> (a -> b -> b) -> Solo a -> b Source # | |||||
| Eq1 Solo Source # | Since: base-4.15 | ||||
| Ord1 Solo Source # | Since: base-4.15 | ||||
Defined in Data.Functor.Classes | |||||
| Read1 Solo Source # | Since: base-4.15 | ||||
Defined in Data.Functor.Classes Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Solo a) Source # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Solo a] Source # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Solo a) Source # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Solo a] Source # | |||||
| Show1 Solo Source # | Since: base-4.15 | ||||
| Traversable Solo Source # | Since: base-4.15 | ||||
| Applicative Solo Source # | Since: base-4.15 | ||||
| Functor Solo Source # | Since: base-4.15 | ||||
| Monad Solo Source # | Since: base-4.15 | ||||
| Generic1 Solo Source # | |||||
Defined in GHC.Generics Associated Types
| |||||
| Data a => Data (Solo a) Source # | Since: base-4.15 | ||||
Defined in Data.Data Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Solo a -> c (Solo a) Source # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Solo a) Source # toConstr :: Solo a -> Constr Source # dataTypeOf :: Solo a -> DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Solo a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Solo a)) Source # gmapT :: (forall b. Data b => b -> b) -> Solo a -> Solo a Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Solo a -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Solo a -> r Source # gmapQ :: (forall d. Data d => d -> u) -> Solo a -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> Solo a -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Solo a -> m (Solo a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Solo a -> m (Solo a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Solo a -> m (Solo a) Source # | |||||
| Monoid a => Monoid (Solo a) Source # | Since: base-4.15 | ||||
| Semigroup a => Semigroup (Solo a) Source # | Since: base-4.15 | ||||
| Bounded a => Bounded (Solo a) Source # | |||||
| Enum a => Enum (Solo a) Source # | |||||
Defined in GHC.Enum Methods succ :: Solo a -> Solo a Source # pred :: Solo a -> Solo a Source # toEnum :: Int -> Solo a Source # fromEnum :: Solo a -> Int Source # enumFrom :: Solo a -> [Solo a] Source # enumFromThen :: Solo a -> Solo a -> [Solo a] Source # enumFromTo :: Solo a -> Solo a -> [Solo a] Source # enumFromThenTo :: Solo a -> Solo a -> Solo a -> [Solo a] Source # | |||||
| Generic (Solo a) Source # | |||||
Defined in GHC.Generics Associated Types
| |||||
| Ix a => Ix (Solo a) Source # | |||||
| Read a => Read (Solo a) Source # | Since: base-4.15 | ||||
| Show a => Show (Solo a) Source # | Since: base-4.15 | ||||
| Eq a => Eq (Solo a) | |||||
| Ord a => Ord (Solo a) | |||||
| type Rep1 Solo Source # | Since: base-4.15 | ||||
Defined in GHC.Generics | |||||
| type Rep (Solo a) Source # | Since: base-4.15 | ||||
Defined in GHC.Generics | |||||
getSolo :: Solo a -> a Source #
Extract the value from a Solo. Very often, values should be extracted
directly using pattern matching, to control just what gets evaluated when.
getSolo is for convenience in situations where that is not the case:
When the result is passed to a strict function, it makes no difference whether the pattern matching is done on the "outside" or on the "inside":
Data.Set.insert (getSolo sol) set === case sol of Solo v -> Data.Set.insert v set
A traversal may be performed in Solo in order to control evaluation
internally, while using getSolo to extract the final result. A strict
mapping function, for example, could be defined
map' :: Traversable t => (a -> b) -> t a -> t b map' f = getSolo . traverse ((Solo $!) . f)