Coursera Course: Programming Languages

第一课作业

fun is_older (date1 : int*int*int, date2 : int*int*int) =    if #1 date1 <> #1 date2    then        #1 date1 < #1 date2    else        if #2 date1 <> #2 date2        then            #2 date1 < #2 date2        else            if #3 date1 <> #3 date2            then                #3 date1 < #3 date2            else                falsefun number_in_month (ld : (int*int*int) list, m : int) =    if null ld    then 0    else         let             val tl_ans = number_in_month(tl ld, m)        in            if #2 (hd ld) = m             then 1 + tl_ans            else tl_ans        endfun number_in_months (ld : (int*int*int) list, ms : int list) =    if null ms    then 0    else        number_in_month(ld, hd ms) + number_in_months(ld, tl ms)fun dates_in_month (ld : (int*int*int) list, m : int) =    if null ld    then []    else        let            val tl_ans = dates_in_month(tl ld, m)        in            if #2 (hd ld) = m            then (hd ld) :: tl_ans            else tl_ans        endfun dates_in_months (ld : (int*int*int) list, ms : int list) =    if null ms    then []    else        dates_in_month(ld, hd ms) @ dates_in_months(ld, tl ms)fun get_nth (slist : string list, n : int) =    if n = 1    then        hd slist    else        get_nth(tl slist, n-1)fun date_to_string (date : int*int*int) =    let         val months = ["January","February","March","April","May", "June","July","August","September","October","November","December"]        val year = Int.toString(#1 date)        val month = get_nth(months, #2 date)        val day = Int.toString(#3 date)    in        month ^ " " ^ day ^ ", " ^ year    endfun number_before_reaching_sum (sum : int, nums : int list) =    if sum <= 0    then        ~1    else        1 + number_before_reaching_sum(sum - (hd nums), tl nums)fun what_month (day : int) =    let        val days = [31,28,31,30,31,30,31,31,30,31,30,31]    in        number_before_reaching_sum(day, days) + 1    endfun month_range (day1 : int, day2 : int) =    if day1 > day2    then []    else        what_month(day1) :: month_range(day1+1, day2)fun oldest (dates : (int*int*int) list) =    if null dates    then NONE    else        let val tl_ans = oldest(tl dates)        in            if isSome tl_ans andalso is_older(valOf tl_ans, hd dates)            then tl_ans            else SOME (hd dates)        end