爲什麼預期類型單位的表達式？

Question

給定2010年到2019年的日期，該函數應該返回一個int 0到6，對應於週日到週六。該功能基於2010年1月1日是星期五的事實。

爲什麼else語句朝向末尾else if year = 2019引發其整個主體的錯誤：「此表達式的類型爲int，但表達式期望爲type unit」？

let day_of_week (year: int) (month: int) (day: int): int = 
    if year < 2010 || year > 2019 then failwith "year out of range" 
    else if month < 1 || month > 12 then failwith "invalid month" 
    else if day < 1 then failwith "invalid day" 
    else if is_leap_year year != true then (
      if month = 2 then (
        if day > 28 then failwith "invalid day" 
      ) 
    ) else if month = 2 then 
      if day > 29 then failwith "invalid day" 
    else if month = 4 || month = 6 || month = 9 || month = 11 then 
      if day > 30 then failwith "invalid day" 
    else if day > 31 then failwith "invalid day" 
    else if year = 2010 then (
      if month = 1 then ((day mod 7) + 4) mod 7 
      else if month = 2 then (((31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((304 + day) mod 7) + 4) mod 7 
      else (((334 + day) mod 7) + 4) mod 7 
    ) else if year = 2011 then (
      if month = 1 then (((365 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((365 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((365 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((365 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((365 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((365 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((365 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((365 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((365 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((365 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((365 + 304 + day) mod 7) + 4) mod 7 
      else (((365 + 334 + day) mod 7) + 4) mod 7 
    ) else if year = 2012 then (
      if month = 1 then (((730 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((730 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((730 + 60 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((730 + 91 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((730 + 121 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((730 + 152 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((730 + 182 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((730 + 213 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((730 + 244 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((730 + 274 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((730 + 305 + day) mod 7) + 4) mod 7 
      else (((730 + 335 + day) mod 7) + 4) mod 7 
    ) else if year = 2013 then (
      if month = 1 then (((1096 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((1096 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((1096 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((1096 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((1096 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((1096 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((1096 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((1096 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((1096 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((1096 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((1096 + 304 + day) mod 7) + 4) mod 7 
      else (((1096 + 334 + day) mod 7) + 4) mod 7 
    ) else if year = 2014 then (
      if month = 1 then (((1461 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((1461 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((1461 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((1461 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((1461 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((1461 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((1461 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((1461 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((1461 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((1461 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((1461 + 304 + day) mod 7) + 4) mod 7 
      else (((1461 + 334 + day) mod 7) + 4) mod 7 
    ) else if year = 2015 then (
      if month = 1 then (((1826 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((1826 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((1826 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((1826 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((1826 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((1826 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((1826 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((1826 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((1826 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((1826 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((1826 + 304 + day) mod 7) + 4) mod 7 
      else (((1826 + 334 + day) mod 7) + 4) mod 7 
    ) else if year = 2016 then (
      if month = 1 then (((2191 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((2191 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((2191 + 60 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((2191 + 91 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((2191 + 121 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((2191 + 152 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((2191 + 182 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((2191 + 213 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((2191 + 244 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((2191 + 274 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((2191 + 305 + day) mod 7) + 4) mod 7 
      else (((2191 + 335 + day) mod 7) + 4) mod 7 
    ) else if year = 2017 then (
      if month = 1 then (((2557 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((2557 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((2557 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((2557 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((2557 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((2557 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((2557 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((2557 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((2557 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((2557 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((2557 + 304 + day) mod 7) + 4) mod 7 
      else (((2557 + 334 + day) mod 7) + 4) mod 7 
    ) else if year = 2018 then (
      if month = 1 then (((2922 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((2922 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((2922 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((2922 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((2922 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((2922 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((2922 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((2922 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((2922 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((2922 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((2922 + 304 + day) mod 7) + 4) mod 7 
      else (((2922 + 334 + day) mod 7) + 4) mod 7 
    ) else if year = 2019 then (
      if month = 1 then (((3287 + day) mod 7) + 4) mod 7 
      else if month = 2 then (((3287 + 31 + day) mod 7) + 4) mod 7 
      else if month = 3 then (((3287 + 59 + day) mod 7) + 4) mod 7 
      else if month = 4 then (((3287 + 90 + day) mod 7) + 4) mod 7 
      else if month = 5 then (((3287 + 120 + day) mod 7) + 4) mod 7 
      else if month = 6 then (((3287 + 151 + day) mod 7) + 4) mod 7 
      else if month = 7 then (((3287 + 181 + day) mod 7) + 4) mod 7 
      else if month = 8 then (((3287 + 212 + day) mod 7) + 4) mod 7 
      else if month = 9 then (((3287 + 243 + day) mod 7) + 4) mod 7 
      else if month = 10 then (((3287 + 273 + day) mod 7) + 4) mod 7 
      else if month = 11 then (((3287 + 304 + day) mod 7) + 4) mod 7 
      else (((3287 + 334 + day) mod 7) + 4) mod 7 
    ) 

Answer 1

我相信你的問題是這一行：

if day > 28 then failwith "invalid day" 

有沒有else一部分。缺少其他部分相當於()，這導致打字問題。

實際上編譯器是正確的。事實上，這段代碼在2月29日以外的日子裏將返回（）非閏年，在我看來。

一個可能的解決將是一段代碼改成這樣：

else if is_leap_year year != true && month = 2 && day > 28 then failwith "invalid day" 
else . . . 

作爲邊評論，is_leap_year != true更清楚地表述爲在我看來not is_leap_year。

（作爲另一方評論，傑夫·梅爾卡多是正確的。在實際項目中你絕不會這樣寫代碼。有很多方法可以做到更好。）