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.

uncurry converts a curried function to a function on pairs.

Examples

>>> uncurry (+) (1,2)
3

>>> uncurry ($) (show, 1)
"1"

>>> map (uncurry max) [(1,2), (3,4), (6,8)]
[2,4,8]

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)

Extract the first component of a pair.

Extract the second component of a pair.

curry converts an uncurried function to a curried function.

Examples

>>> curry fst 1 2
1

Swap the components of a pair.

Update the first component of a pair.

first succ (1,"test") == (2,"test")

Update the second component of a pair.

second reverse (1,"test") == (1,"tset")

Given two functions, apply one to the first component and one to the second. A specialised version of ***.

(succ *** reverse) (1,"test") == (2,"tset")

Given two functions, apply both to a single argument to form a pair. A specialised version of &&&.

(succ &&& pred) 1 == (2,0)

Duplicate a single value into a pair.

dupe 12 == (12, 12)

Apply a single function to both components of a pair.

both succ (1,2) == (2,3)

Update the first component of a pair.

firstM (\x -> [x-1, x+1]) (1,"test") == [(0,"test"),(2,"test")]

Update the second component of a pair.

secondM (\x -> [reverse x, x]) (1,"test") == [(1,"tset"),(1,"test")]

Extract the fst of a triple.

Extract the snd of a triple.

Extract the final element of a triple.

Update the first component of a triple.

first3 succ (1,1,1) == (2,1,1)

Update the second component of a triple.

second3 succ (1,1,1) == (1,2,1)

Update the third component of a triple.

third3 succ (1,1,1) == (1,1,2)

Converts an uncurried function to a curried function.

Converts a curried function to a function on a triple.