Trying functional programming in cairo. Thanks https://github.com/onlydustxyz/cairo-streams for the inspiration

fold_left.cairo

%builtins output range_check

from starkware.cairo.common.registers import get_label_location
from starkware.cairo.common.serialize import serialize_word
from starkware.cairo.common.alloc import alloc

func main{output_ptr : felt*, range_check_ptr}():
    alloc_locals

    let (arr : felt*) = alloc()
    assert arr[0] = 1
    assert arr[1] = 2
    assert arr[2] = 3
    assert arr[3] = 4
    let (resp) = fold_left(sum, 0, 4, arr)
    serialize_word(resp)

    return ()
end

func sum{range_check_ptr}(acc : felt, value : felt) -> (output : felt):
    return (acc + value)
end

func fold_left{range_check_ptr}(
    function : codeoffset, acc : felt, args_len : felt, args : felt*
) -> (acc : felt):
    let (func_pc) = get_label_location(function)
    return _fold_left(func_pc, acc, args_len, args)
end

func _fold_left{range_check_ptr}(func_pc : felt, acc : felt, args_len : felt, args : felt*) -> (
    acc : felt
):
    if args_len == 0:
        return (acc)
    end
    [ap] = [args]; ap++
    [ap] = acc; ap++
    call abs func_pc
    return _fold_left(func_pc, [ap - 1], args_len - 1, args + 1)
end