module Stdlib.Trait.Foldable.Monomorphic;

import Stdlib.Function open;
import Stdlib.Trait.Foldable.Polymorphic as Poly;

--- A trait for combining elements into a single result, processing one element at a time.
{-# specialize: true #-}
trait
type Foldable (Container Elem : Type) :=
  mkFoldable@{
    syntax iterator for {init := 1; range := 1};
    for : {B : Type} -> (B -> Elem -> B) -> B -> Container -> B;

    syntax iterator rfor {init := 1; range := 1};
    rfor : {B : Type} -> (B -> Elem -> B) -> B -> Container -> B;
  };

open Foldable public;

--- Make a monomorphic ;Foldable; instance from a polymorphic one.
--- All polymorphic types that are an instance of ;Poly.Foldable; should use this function to create their monomorphic ;Foldable; instance.
{-# inline: always #-}
fromPolymorphicFoldable
  {F : Type -> Type}
  {{foldable : Poly.Foldable F}}
  {Elem}
  : Foldable (F Elem) Elem :=
  mkFoldable@{
    for := Poly.for;
    rfor := Poly.rfor;
  };

{-# inline: true #-}
foldl
  {Container Elem}
  {{Foldable Container Elem}}
  {Acc : Type}
  (fun : Acc -> Elem -> Acc)
  (initialValue : Acc)
  (container : Container)
  : Acc :=
  for (acc := initialValue) (x in container) {
    fun acc x
  };

--- Combine the elements of the type using the provided function starting with the element in the right-most position.
{-# inline: 2 #-}
foldr
  {Container Elem : Type}
  {{Foldable Container Elem}}
  {Acc : Type}
  (fun : Elem -> Acc -> Acc)
  (initialValue : Acc)
  (container : Container)
  : Acc := foldl (flip fun) initialValue container;