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I have created a function for loading some Entities but I am having some trouble to understand how I should put its type declaration, so I took this Yesod book chapter about Yesod monads, to understand it better, and I came to this snippet:

-- type Distance = Int
worksByNhood :: AdrNhoodId -> Int64 -> Int64 -> YesodDB App [(Entity Work, Int, [Entity WImage])] 
worksByNhood nId offset' limit' = do
  works <- select $ from $ \(w `InnerJoin` d) -> do
    on $ d ^. AdrDistanceNhood2 ==. w ^. WorkNhood
    where_ (d ^. AdrDistanceNhood1 ==. val nId)
    orderBy [asc (d ^. AdrDistanceDistance)]
    offset offset'
    limit limit'
    return (w, d ^. AdrDistanceDistance)
  works' <- forM works $ \(w@(Entity wId _), d) -> do
    images <- select $ from $ \wi -> do
      where_ (wi ^. WImageWork ==. val wId)
      return wi
    return (w, d, images);
  return works'

I think this Int in the type declaration needs to be converted someway, to be more specific, I am getting this error:

Select.hs:20:12:
    Couldn't match type ‘Database.Esqueleto.Value Distance’ with ‘Int’
    Expected type: (Entity Work, Distance, [Entity WImage])
      Actual type: (Entity Work,
                    Database.Esqueleto.Value Distance,
                    [Entity WImage])
    In the first argument of ‘return’, namely ‘(w, d, images)’
    In a stmt of a 'do' block: return (w, d, images)
    In the expression:
      do { images <- select $ from $ \ wi -> do { ... };
           return (w, d, images) }
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1 回答 1

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我记得在伟大的 Yesod 书中有一章关于加入，所以我再次阅读并找到了解决方案，也许我的代码可以帮助其他人，所以这里是：

module Select where

import Import hiding((==.), on, Value)
import Database.Persist.Sql (toSqlKey) -- Useful when I want to explicitly use primary keys
import Database.Esqueleto

-- type Distance = Int
worksByNhood :: AdrNhoodId -> Int64 -> Int64 -> YesodDB App [(Entity Work, Value Distance, [Entity WImage])] 
worksByNhood nId offset' limit' = do
  works <- select $ from $ \(w `InnerJoin` d) -> do
    on $ d ^. AdrDistanceNhood2 ==. w ^. WorkNhood
    where_ (d ^. AdrDistanceNhood1 ==. val nId)
    orderBy [asc (d ^. AdrDistanceDistance)]
    offset offset'
    limit limit'
    return (w, d ^. AdrDistanceDistance)
  forM works $ \(w@(Entity wId _), d) -> do
    images <- select $ from $ \wi -> do
      where_ (wi ^. WImageWork ==. val wId)
      return wi
    return (w, d, images)

Value Distance在类型声明是答案

于 2016-04-18T19:04:48.460 回答