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POHow to use MultiPiece
 

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  1. POHow to use MultiPiece
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    <p>I'm completely new to Yesod (and not very experienced in haskell) and I'm trying to build my first handler. I scraffolded my app using default parameters (I'm using Yesod 0.9.4.1 version and choose postgresql in scraffolding) and now I'm trying to retrieve some data from a table using selectList. I defined a new table (let's call it Foo) in models config file:</p> <pre><code> Foo xStart Int yStart Int </code></pre> <p>and want to pass a list of FooId's and some other Foo attributes so I defined a route: </p> <p><code>/foos/#Int/#Int/*FooId FoosReturnR GET</code></p> <p>and a handler:</p> <pre><code> module Handler.FoosReturn where import Import selectWindowSize :: Int selectWindowSize = 10000 getFoosReturnR :: Int -&gt; Int -&gt; [FooId] -&gt; Handler RepPlain getFoosReturnR x y withoutIds = do foos &lt;- runDB $ selectList [FooId /&lt;-. withoutIds, FooXStart &lt;. x + selectWindowSize, FooXStart &gt;=. x - selectWindowSize, FooYStart &lt;. y + selectWindowSize, FooYStart &gt;=. y - selectWindowSize] [] return $ RepPlain $ toContent $ show foos </code></pre> <p>I imported the handler in Application.hs and added it to cabal file and now when I'm trying to run it I receive an error saying that FooId is not an instance of MultiPiece - but when I try to make it an instance there is an error saying that FooId is a type synonym and cannot be an instance of MultiPiece - how to resolve this problem?</p> <hr> <p>EDIT: Daniel: well, actually I don't know what exactly is FooId - it's a part of Yesod's magic which I don't fully understand so far - it's generated automatically from the table definition - but it's a some kind of a number.</p> <p>Because I don't know how to use MultiPiece I switched to simpler solution and modified:</p> <p>route: <code>/foos/#Int/#Int/#String FoosReturnR GET</code></p> <p>handler: [added also some logging]</p> <pre><code> module Handler.FoosReturn where import Import import Data.List.Split import qualified Data.Text.Lazy as TL selectWindowSize :: Int selectWindowSize = 10000 getFoosReturnR :: Int -&gt; Int -&gt; String -&gt; Handler RepPlain getFoosReturnR x y withoutIds = do app &lt;- getYesod liftIO $ logLazyText (getLogger app) ("getFoosReturnR('" `TL.append` (TL.pack $ (show x) ++ "', '" ++ (show y) ++ "', '" ++ withoutIds ++ "') ")) foos &lt;- runDB $ selectList [FooId /&lt;-. (map (\a -&gt; read a :: FooId) $ splitOn "," withoutIds), FooXStart &lt;. x + selectWindowSize, FooXStart &gt;=. x - selectWindowSize, FooYStart &lt;. y + selectWindowSize, FooYStart &gt;=. y - selectWindowSize] [] return $ RepPlain $ toContent $ show foos </code></pre> <p>and now it is compiling but when I browse to: <a href="http://localhost:3000/sectors/1/1/1,2" rel="nofollow">http://localhost:3000/sectors/1/1/1,2</a> I get a page containing only: Internal Server Error Prelude.read: no parse</p> <p>Well, I don't fully understand what is FooId here - how to create such a list of FooId's from list of strings containing numbers?</p> <p>And of course a solution of how to make the FooId an instance of MultiPiece is most wanted.</p> <hr> <p>EDIT: </p> <p>Daniel and svachalek, thanks for your posts - I tried your (Daniel's) solution but then I was receiving errors saying that [FooId] is expected (as in the handler function declaration) but FooId type was given and this lead me to the following solution:</p> <pre><code> data FooIds = FooIds [FooId] deriving (Show, Read, Eq) instance MultiPiece FooIds where toMultiPiece (FooIds fooList) = map (Data.Text.pack . show) fooList fromMultiPiece texts = if length (filter isNothing listOfMaybeFooId) &gt; 0 then Nothing else Just $ FooIds $ map fromJust listOfMaybeFooId where listOfMaybeFooId = map constructMaybeFooId texts constructMaybeFooId :: Text -&gt; Maybe FooId constructMaybeFooId x = case reads (Data.Text.unpack x) :: [(FooId,String)] of [(foo,_)] -&gt; Just foo _ -&gt; Nothing </code></pre> <p>of course I changed the route to: <code>/foos/#Int/#Int/*FooIds FoosReturnR GET</code></p> <p>and the handler to: </p> <pre><code> getFoosReturnR :: Int -&gt; Int -&gt; FooIds -&gt; Handler RepPlain getFoosReturnR coordX coordY (FooIds withoutIds) = do </code></pre> <p>and now I don't get any errors during compilation nor runtime, and the only not satisfying thing is that I always receive Not Found as a result, even if I supply parameters that should give me some results - so now I have to figure out how to determine what SQL was exactly sent to the database</p> <hr> <p>EDIT:</p> <p>Now I see that the "Not Found" is connected to the problem and that the above edit is not a solution - when I browse to localhost:3000/foos/4930000/3360000 then I get the results (but then the FooIds is empty) - but when I add something like: localhost:3000/sectors/4930000/3360000/1 then I always get "Not Found" - so it's still not working..</p>
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    1. USPiotr Suszyński
    1. USPiotr Suszyński
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      1. POHow to use MultiPiece
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    1. COYou can write instances for type synonyms with the `TypeSynonymInstances` language extension (may also need `FlexibleInstances`). What is `FooId` a synonym for?
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      1. POHow to use MultiPiece
      1. USDaniel Fischer
    2. CODaniel: I replied in an edit. Could you post some code how to make the FooId an instance of MultiPiece?
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      1. USPiotr Suszyński
 

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