#Monoid Source

class (Semigroup m) <= Monoid m  where

A Monoid is a Semigroup with a value mempty, which is both a left and right unit for the associative operation <>:

forall x. mempty <> x = x <> mempty = x

Monoids are commonly used as the result of fold operations, where <> is used to combine individual results, and mempty gives the result of folding an empty collection of elements.



#power Source

power :: forall m. Monoid m => m -> Int -> m

Append a value to itself a certain number of times. For the Multiplicative type, and for a non-negative power, this is the same as normal number exponentiation.

If the second argument is negative this function will return mempty (unlike normal number exponentiation). The Monoid constraint alone is not enough to write a power function with the property that power x n cancels with power x (-n), i.e. power x n <> power x (-n) = mempty. For that, we would additionally need the ability to invert elements, i.e. a Group.

#guard Source

guard :: forall m. Monoid m => Boolean -> m -> m

Allow or "truncate" a Monoid to its mempty value based on a condition.

#MonoidRecord Source

class (SemigroupRecord rowlist row subrow) <= MonoidRecord rowlist row subrow | rowlist -> row subrow where

A class for records where all fields have Monoid instances, used to implement the Monoid instance for records.



Re-exports from Data.Semigroup

#Semigroup Source

class Semigroup a 

The Semigroup type class identifies an associative operation on a type.

Instances are required to satisfy the following law:

  • Associativity: (x <> y) <> z = x <> (y <> z)

One example of a Semigroup is String, with (<>) defined as string concatenation.


#SemigroupRecord Source

class SemigroupRecord rowlist row subrow | rowlist -> subrow

A class for records where all fields have Semigroup instances, used to implement the Semigroup instance for records.


#(<>) Source

Operator alias for Data.Semigroup.append (right-associative / precedence 5)