Question

{-# LANGUAGE TypeFamilies, QuasiQuotes, TemplateHaskell, MultiParamTypeClasses, OverloadedStrings #-}
import Yesod

data Entry = Entry
    { entryTitle :: String
    , entrySlug :: String -- ^ used in the URL
    , entryContent :: String
    }

loadEntries :: IO [Entry]
loadEntries = return
    [ Entry "Entry 1" "entry-1" "My first entry"
    , Entry "Entry 2" "entry-2" "My second entry"
    , Entry "Entry 3" "entry-3" "My third entry"
    ]

data Blog = Blog { blogEntries :: [Entry] }

type Handler = GHandler Blog Blog

mkYesod "Blog" [parseRoutes|
/               HomeR   GET
/entry/#String  EntryR  GET
|]

instance Yesod Blog where approot _ = ""

getHomeR :: Handler ()
getHomeR = do
    Blog entries <- getYesod
    let newest = last entries
    redirect RedirectTemporary $ EntryR $ entrySlug newest

data TemplateArgs = TemplateArgs
    { templateTitle :: Html
    , templateContent :: Html
    , templateNavbar :: [Nav]
    }

data Nav = Nav
    { navUrl :: Route Blog
    , navTitle :: Html
    }

entryTemplate :: TemplateArgs -> Hamlet (Route Blog)
entryTemplate args = [hamlet|
    !!!

    <html>
        <head>
            <title>#{templateTitle args}
        <body>
            <h1>Yesod Sample Blog
            <h2>#{templateTitle args}
            <ul id="nav">
                $forall nav <- templateNavbar args
                    <li>
                        <a href="@{navUrl nav}">#{navTitle nav}
            <div id="content">
                \#{templateContent args}
    |]

getEntryR :: String -> Handler RepHtml
getEntryR slug = do
    Blog entries <- getYesod
    case filter (\e -> entrySlug e == slug) entries of
         [] -> notFound
         (entry:_) -> do
            let nav = reverse $ map toNav entries
            let tempArgs = TemplateArgs
                    { templateTitle = toHtml $ entryTitle entry
                    , templateContent = toHtml $ entryContent entry
                    , templateNavbar = nav
                    }
            hamletToRepHtml $ entryTemplate tempArgs
  where
    toNav :: Entry -> Nav
    toNav e = Nav
        { navUrl = EntryR $ entrySlug e
        , navTitle = toHtml $ entryTitle e
        }

main :: IO ()
main = do
    entries <- loadEntries
    warpDebug 3000 $ Blog entries

$ runghc Blog.hs 

Blog.hs:21:1:
    Multiple declarations of `Handler'
    Declared at: Blog.hs:19:6
                 Blog.hs:21:1

Answer 1

最新的Yesod现在自动声明Handler类型别名。只需删除type Handler = ...行即可。我会更新这本书，我想我的检查脚本还没有处理Literate Haskell代码。

编译yesod博客示例的错误

1 个答案: