Data.Tuple.Solo

Solo fills the tuple gap with a singleton tuple.

Solo does not support the usual parenthesized tuple syntax.

Solo

• has the expected laziness properties
• can be pattern-matched
• ships with instances for several standard type classes, including all those supported by H98-standard tuples
• requires no language extensions, except for hierarchical modules

Note: on GHC-9.0 getSolo is not a record selector.

Documentation

data Solo a

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

Solo a

Bundled Patterns

pattern MkSolo :: a -> Solo a

Instances

Foldable Solo

Since: base-4.15

Defined in Data.Foldable

Methods

fold :: Monoid m => Solo m -> m

foldMap :: Monoid m => (a -> m) -> Solo a -> m

foldMap' :: Monoid m => (a -> m) -> Solo a -> m

foldr :: (a -> b -> b) -> b -> Solo a -> b

foldr' :: (a -> b -> b) -> b -> Solo a -> b

foldl :: (b -> a -> b) -> b -> Solo a -> b

foldl' :: (b -> a -> b) -> b -> Solo a -> b

foldr1 :: (a -> a -> a) -> Solo a -> a

foldl1 :: (a -> a -> a) -> Solo a -> a

toList :: Solo a -> [a]

null :: Solo a -> Bool

length :: Solo a -> Int

elem :: Eq a => a -> Solo a -> Bool

maximum :: Ord a => Solo a -> a

minimum :: Ord a => Solo a -> a

sum :: Num a => Solo a -> a

product :: Num a => Solo a -> a

Traversable Solo

Since: base-4.15

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Solo a -> f (Solo b)

sequenceA :: Applicative f => Solo (f a) -> f (Solo a)

mapM :: Monad m => (a -> m b) -> Solo a -> m (Solo b)

sequence :: Monad m => Solo (m a) -> m (Solo a)

Applicative Solo

Since: base-4.15

Defined in GHC.Base

Methods

pure :: a -> Solo a

(<*>) :: Solo (a -> b) -> Solo a -> Solo b

liftA2 :: (a -> b -> c) -> Solo a -> Solo b -> Solo c

(*>) :: Solo a -> Solo b -> Solo b

(<*) :: Solo a -> Solo b -> Solo a

Functor Solo

Since: base-4.15

Defined in GHC.Base

Methods

fmap :: (a -> b) -> Solo a -> Solo b

(<$) :: a -> Solo b -> Solo a

Monad Solo

Since: base-4.15

Defined in GHC.Base

Methods

(>>=) :: Solo a -> (a -> Solo b) -> Solo b

(>>) :: Solo a -> Solo b -> Solo b

return :: a -> Solo a

Monoid a => Monoid (a)

Since: base-4.15

Defined in GHC.Base

Methods

mempty :: (a)

mappend :: (a) -> (a) -> (a)

mconcat :: [(a)] -> (a)

Semigroup a => Semigroup (a)

Since: base-4.15

Defined in GHC.Base

Methods

(<>) :: (a) -> (a) -> (a)

sconcat :: NonEmpty (a) -> (a)

stimes :: Integral b => b -> (a) -> (a)

Bounded a => Bounded (a)

Defined in GHC.Enum

Methods

minBound :: (a)

maxBound :: (a)

Enum a => Enum (a)

Defined in GHC.Enum

Methods

succ :: (a) -> (a)

pred :: (a) -> (a)

toEnum :: Int -> (a)

fromEnum :: (a) -> Int

enumFrom :: (a) -> [(a)]

enumFromThen :: (a) -> (a) -> [(a)]

enumFromTo :: (a) -> (a) -> [(a)]

enumFromThenTo :: (a) -> (a) -> (a) -> [(a)]

Read a => Read (a)

Since: base-4.15

Defined in GHC.Read

Methods

readsPrec :: Int -> ReadS (a)

readList :: ReadS [(a)]

readPrec :: ReadPrec (a)

readListPrec :: ReadPrec [(a)]

Show a => Show (a)

Since: base-4.15

Defined in GHC.Show

Methods

showsPrec :: Int -> (a) -> ShowS

show :: (a) -> String

showList :: [(a)] -> ShowS

Eq a => Eq (a)

Defined in GHC.Classes

Methods

(==) :: (a) -> (a) -> Bool

(/=) :: (a) -> (a) -> Bool

Ord a => Ord (a)

Defined in GHC.Classes

Methods

compare :: (a) -> (a) -> Ordering

(<) :: (a) -> (a) -> Bool

(<=) :: (a) -> (a) -> Bool

(>) :: (a) -> (a) -> Bool

(>=) :: (a) -> (a) -> Bool

max :: (a) -> (a) -> (a)

min :: (a) -> (a) -> (a)

getSolo :: (a) -> a