## /Prelude-v20.1.0/List/filter.dhall

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Only keep elements of the list where the supplied function returns `True`

### Examples

filter Natural Natural/even [ 2, 3, 5 ] ≡ [ 2 ]

filter Natural Natural/odd [ 2, 3, 5 ] ≡ [ 3, 5 ]

### Source

--| Only keep elements of the list where the supplied function returns `True`

let filter

: ∀(a : Type) → (a → Bool) → List a → List a

= λ(a : Type) →

λ(f : a → Bool) →

λ(xs : List a) →

List/build

a

( λ(list : Type) →

λ(cons : a → list → list) →

List/fold

a

xs

list

(λ(x : a) → λ(xs : list) → if f x then cons x xs else xs)

)

let example0 = assert : filter Natural Natural/even [ 2, 3, 5 ] ≡ [ 2 ]

let example1 = assert : filter Natural Natural/odd [ 2, 3, 5 ] ≡ [ 3, 5 ]

in filter