在wxHaskell中顯示數據庫內容

Question

使用來自wxHaskell的教程並希望在網格中顯示錶格電影的內容。 這裏是我的代碼：

{-------------------------------------------------------------------------------- 
    Test Grid. 
--------------------------------------------------------------------------------} 

module Main where 

import Graphics.UI.WX 
import Graphics.UI.WXCore hiding (Event) 

import Database.HDBC.Sqlite3 (connectSqlite3) 
import Database.HDBC 


main 
    = start gui 

gui :: IO() 
gui 
    = do f <- frame [text := "Grid test", visible := False] 

     -- grids 
     g <- gridCtrl f [] 
     gridSetGridLineColour g (colorSystem Color3DFace) 
     gridSetCellHighlightColour g black 
     appendColumns g (movies) -- Here is error: 

--  Couldn't match expected type `[String]' 
--  against inferred type `IO [[String]]' 


     --appendRows g (map show [1..length (tail movies)]) 
     --mapM_ (setRow g) (zip [0..] (tail movies)) 
     gridAutoSize g 

     -- layout 
     set f [layout := column 5 [fill (dynamic (widget g))] 
      ]  
     focusOn g 
     set f [visible := True] -- reduce flicker at startup. 
     return() 
    where 

    movies = do 
     conn <- connectSqlite3 "Spop.db" 
     r <- quickQuery' conn "SELECT id, title, year, description from Movie where id = 1" [] 
     let myResult = map convRow r 
     return myResult 

setRow g (row,values) 
    = mapM_ (\(col,value) -> gridSetCellValue g row col value) (zip [0..] values) 



{-------------------------------------------------------------------------------- 
    Library?f 
--------------------------------------------------------------------------------} 

gridCtrl :: Window a -> [Prop (Grid())] -> IO (Grid()) 
gridCtrl parent props 
    = feed2 props 0 $ 
    initialWindow $ \id rect -> \props flags -> 
    do g <- gridCreate parent id rect flags 
     gridCreateGrid g 0 0 0 
     set g props 
     return g 

appendColumns :: Grid a -> [String] -> IO() 
appendColumns g [] 
    = return() 
appendColumns g labels 
    = do n <- gridGetNumberCols g 
     gridAppendCols g (length labels) True 
     mapM_ (\(i,label) -> gridSetColLabelValue g i label) (zip [n..] labels) 

appendRows :: Grid a -> [String] -> IO() 
appendRows g [] 
    = return() 
appendRows g labels 
    = do n <- gridGetNumberRows g 
     gridAppendRows g (length labels) True 
     mapM_ (\(i,label) -> gridSetRowLabelValue g i label) (zip [n..] labels) 

convRow :: [SqlValue] -> [String] 
convRow [sqlId, sqlTitle, sqlYear, sqlDescription] = [intid, title, year, description] 
       where intid = (fromSql sqlId)::String 
        title = (fromSql sqlTitle)::String 
        year = (fromSql sqlYear)::String 
        description = (fromSql sqlDescription)::String 

我應該怎麼做才能在代碼中的錯誤上面（24日線）

Answer 1

該錯誤消息說，appendColumns g預計[String]類型的值的RIF，但movies是類型IO [[String]]。

所以，你需要解決兩件事情：

1. movies是一個IO動作返回一個值，但你需要它返回的值。

   更換

   appendColumns g (movies) 
   

   與

   movieList <- movies 
   appendColumns g movieList 
   

   （順便說一下，在第一行的括號呢？他們沒有做任何事情。）

2. 你需要養活appendColumns g列表的字符串，但是你想給它一個字符串列表的列表。您需要將列表列表轉換爲字符串列表，或者您需要依次給每個字符串列表輸入appendColumns g。

   我猜你想要後者，所以你會得到一行屏幕上的數據庫中的每一行。 （我不熟悉wxhaskell，所以我可能誤解了什麼appendColumns一樣。）

   movieList <- movies 
   mapM_ (appendColumns g) movieList