/Prelude-v21.1.0/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 : alistlist) →
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