programming_languages/sml/week4/hw3.sml

use "operators.sml";
use "list.sml";

exception NoAnswer

fun first str = String.sub (str, 0)
val only_capitals = filter (first >> Char.isUpper)

val size = String.size
fun choose f a b  = if f a b then a else b
fun biggest a b = size a > size b
fun bigger_or_equal a b = size a >= size b

val longest_string1 = foldl (choose biggest) ""

val longest_string2 = foldl (choose bigger_or_equal) ""

fun longest_string_helper f =
    let
        val longest_string3 = longest_string1
        val longest_string4 = longest_string2
    in
        if f (0, 0)
        then longest_string4
        else longest_string3
    end

val longest_capitalized = only_capitals >> longest_string2

val rev_string = String.explode >> List.rev >> String.implode

fun first_answer f lst =
    case lst of
       [] => raise NoAnswer
     | head :: tail => 
        case f head of
        NONE => first_answer f tail
        | SOME answer => answer

fun all_answers (f: ('a -> 'b list option)) lst: 'b list option =
    let
        fun collect lst acc =
            case lst of
            [] => SOME acc
            | head :: tail =>
                case f head of
                NONE => NONE
                | SOME lst => collect tail (acc @ lst)
    in
        collect lst []
    end