結合可能作爲單子線成更普通的東西

Question

我正在寫一些mysql-simple數據庫代碼，並且讓我的QueryResults類型類的數據類型成員很容易讓它們變得不自在。

但我結束了很多功能，如：

data FullName { 
    first_name :: String, 
    middle_name :: String, 
    last_name :: String 
} deriving Show 

newtype UID = UID Integer deriving Show 

go :: [(String, String)] -> Maybe FullName 
go fvs = do 
    first <- lookup "first_name" fvs 
    middle <- lookup "middle_name" fvs 
    last <- lookup "last_name" fvs 
    return $ FullName first middle last 

go :: [(String, String)] -> Maybe UID 
go fvs = do 
    uid <- lookup "uid" fvs 
    return $ UID (read uid) 

哪裏查找僅返回（也許）。其中一些數據類型在其中有十幾列，所以這會變得單調乏味。

所以也有很多這樣的數據類型的，我希望能夠寫一個函數我會叫這樣的：

go RealName ["first_name","middle_name","last_name"] fvs 
go UID ["uid"] fvs 

，但我不知道這樣的事情的類型應該是，或者我會怎樣去做。也許這是不可能的。

Answer 1

使用Template Haskell，我懷疑你可以使用如下代碼結束：

makeGo ''FullName 

一個可擴展到

goFullName :: [(String, String)] -> Maybe FullName 
goFullName fvs = do 
    first <- lookup "first_name" fvs 
    middle <- lookup "middle_name" fvs 
    last <- lookup "last_name" fvs 
    return $ FullName first middle last 

---

旁白：如果你import Control.Applicative，goFullName可以寫更多簡潔如此：

goFullName :: [(String, String)] -> Maybe FullName 
goFullName fvs 
    = FullName <$> lookup "first_name" fvs 
       <*> lookup "middle_name" fvs 
       <*> lookup "last_name" fvs 

Answer 2

如果你的目標是與QueryResults情況下結束了，我會寫這個功能

import Control.Applicative 

go :: [(String, String)] -> Maybe FullName 
go fvs = FullName <$> l "first_name" <*> l "middle_name" <*> l "last_name" 
    where l field = lookup field fvs 

Answer 3

，爲什麼不這樣做：

{-# LANGUAGE GeneralizedNewtypeDeriving,OverloadedStrings #-} 

Module Main where 

import Database.MySQL.Simple 
import Database.MySQL.Simple.QueryResults 
import Database.MySQL.Simple.Result 
import Database.MySQL.Simple.Param 

data FullName = 
    FullName { 
     first_name :: String 
    , middle_name :: String 
    , last_name :: String 
    } deriving Show 

newtype UID = UID Integer deriving (Show,Param) 

instance QueryResults FullName where 
    convertResults fs@[_,_,_] vs@[_,_,_] = FullName first middle last 
    where [first,middle,last] = zipWith convert fs vs 
    convertResults fs vs = convertError fs vs 3 

instance QueryResults UID where 
    convertResults fs@[fuid] vs@[vuid] = UID (convert fuid vuid) 
    convertResults fs vs = convertError fs vs 1 

uidToFullName :: UID -> IO FullName 
uidToFullName (UID uid) = 
    do conn <- connect defaultConnectInfo 
    [fn] <- query conn 
     "SELECT first_name,middle_name,last_name FROM user WHERE uid = ?" [uid] 
    return fn 

一個簡單的會話：

λ> :load Main.sh 
λ> uidToFullName (UID 23) 
FullName {first_name = "John", middle_name = "Horatio", last_name = "Smith"} 

以下是隨附的SQL：

CREATE TABLE user (
    uid Integer PRIMARY KEY, 
    email Text, 
    first_name Text, 
    middle_name Text, 
    last_name Text 
); 

INSERT INTO user VALUES (23,'john.horation@smith.com','John','Horatio','Smith');