Catamorphism

editable example

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Using reduce

ghul

use IO.Std.write_line

use Ghul.Pipes

let numbers = [1, 2, 3, 4, 5]

let largest =

(left: int, right: int) => if left > right then left else right fi

let sum = numbers |> reduce(0, (total, value) => total + value)

let product = numbers |> reduce(1, (total, value) => total * value)

write_line("sum: {sum}")

write_line("product: {product}")

write_line("largest: {numbers |> reduce(numbers[0], largest)}")

sum:     15
product: 120
largest: 5

Writing fold

ghul

use IO.Std.write_line

use Collections.Iterable

use Collections.List

fold_left[T, A](values: Iterable[T], seed: A, combine: (A, T) -> A) -> A is

let running mut = seed

for value in values do

running = combine(running, value)

od

return running

si

fold_right[T, A](values: List[T], seed: A, combine: (T, A) -> A) -> A is

let running mut = seed

for i in 0..values.count do

running = combine(values[values.count - 1 - i], running)

od

return running

si

let numbers = [1, 2, 3, 4, 5]

let bracket_left = (running: string, value: int) => "({running} {value})"

let bracket_right = (value: int, running: string) => "({value} {running})"

let left_sum = fold_left(numbers, 0, (total, value) => total + value)

let right_sum = fold_right(numbers, 0, (value, total) => value + total)

write_line("left sum: {left_sum}")

write_line("right sum: {right_sum}")

write_line("left tree: {fold_left(numbers, "nil", bracket_left)}")

write_line("right tree: {fold_right(numbers, "nil", bracket_right)}")

left  sum:  15
right sum:  15
left  tree: (((((nil 1) 2) 3) 4) 5)
right tree: (1 (2 (3 (4 (5 nil)))))