id package:incipit-base

Identity function.

id x = x

This function might seem useless at first glance, but it can be very useful in a higher order context.

Examples

>>> length $ filter id [True, True, False, True]
3

>>> Just (Just 3) >>= id
Just 3

>>> foldr id 0 [(^3), (*5), (+2)]
1000

class Semigroup a => Monoid a

incipit-base Incipit.Base

The class of monoids (types with an associative binary operation that has an identity). Instances should satisfy the following:

Right identity x <> mempty = x
Left identity mempty <> x = x
Associativity x <> (y <> z) = (x <> y) <> z (Semigroup law)
Concatenation mconcat = foldr (<>) mempty

You can alternatively define mconcat instead of mempty, in which case the laws are:

Unit mconcat (pure x) = x
Multiplication mconcat (join xss) = mconcat (fmap mconcat xss)
Subclass mconcat (toList xs) = sconcat xs

The method names refer to the monoid of lists under concatenation, but there are many other instances. Some types can be viewed as a monoid in more than one way, e.g. both addition and multiplication on numbers. In such cases we often define newtypes and make those instances of Monoid, e.g. Sum and Product. NOTE: Semigroup is a superclass of Monoid since base-4.11.0.0.

data Void

incipit-base Incipit.Base

Uninhabited data type

decideChar :: forall (a :: Char) (b :: Char) proxy1 proxy2 . (KnownChar a, KnownChar b) => proxy1 a -> proxy2 b -> Either ((a :~: b) -> Void) (a :~: b)

incipit-base Incipit.Base

We either get evidence that this function was instantiated with the same type-level characters, or that the type-level characters are distinct.

decideNat :: forall (a :: Nat) (b :: Nat) proxy1 proxy2 . (KnownNat a, KnownNat b) => proxy1 a -> proxy2 b -> Either ((a :~: b) -> Void) (a :~: b)

incipit-base Incipit.Base

We either get evidence that this function was instantiated with the same type-level numbers, or that the type-level numbers are distinct.

decideSymbol :: forall (a :: Symbol) (b :: Symbol) proxy1 proxy2 . (KnownSymbol a, KnownSymbol b) => proxy1 a -> proxy2 b -> Either ((a :~: b) -> Void) (a :~: b)

incipit-base Incipit.Base

We either get evidence that this function was instantiated with the same type-level symbols, or that the type-level symbols are distinct.

void :: Functor f => f a -> f ()

incipit-base Incipit.Base

void value discards or ignores the result of evaluation, such as the return value of an IO action.

Examples

Replace the contents of a Maybe Int with unit:

>>> void Nothing
Nothing

>>> void (Just 3)
Just ()

Replace the contents of an Either Int Int with unit, resulting in an Either Int ():

>>> void (Left 8675309)
Left 8675309

>>> void (Right 8675309)
Right ()

Replace every element of a list with unit:

>>> void [1,2,3]
[(),(),()]

Replace the second element of a pair with unit:

>>> void (1,2)
(1,())

Discard the result of an IO action:

>>> mapM print [1,2]
1
2
[(),()]

>>> void $ mapM print [1,2]
1
2