Data.Monoid.First

First' n is a generalization of the First exported by Data.Monoid: whereas Data.Monoid.First returns up to one value, First' n returns up to n values.

Data.Monoid.First    a ≡
            First' 1 a ≡
            First    a

This library also provides an API-compatible type synonym First and function getFirst allowing you to use it as a drop-in replacement.

First

type First a = First' 1 a

A type isomorphic to Data.Monoid.First

getFirst :: First a -> Maybe a

Get the first value of type a, if any.

>>> getFirst (foldMap pure [])
Nothing

>>> getFirst (foldMap pure [1,2,3,4])
Just 1

First'

data First' n a

A generalized version of Data.Monoid.First

Instances

Functor (First' n)

KnownNat n => Applicative (First' n)

Foldable (First' n)

Traversable (First' n)

Generic1 (First' n)

KnownNat n => Alternative (First' n)

Eq a => Eq (First' n a)

(Data a, Typeable Nat n) => Data (First' n a)

Ord a => Ord (First' n a)

Read a => Read (First' n a)

Show a => Show (First' n a)

KnownNat n => IsString (First' n Char)

Generic (First' n a)

KnownNat n => Monoid (First' n a)

type Rep1 (First' n)

type Rep (First' n a)

getFirst' :: First' n a -> [a]

Get the first n values or fewer of type a.

>>> getFirst' (foldMap pure [1,2,3,4] :: First' 0 Int)
[]

>>> getFirst' (foldMap pure [1,2,3,4] :: First' 1 Int)
[1]

>>> getFirst' (foldMap pure [1,2,3,4] :: First' 2 Int)
[1,2]