YesodDB，如何正确放置类型

Question

我已经创建了一个函数来加载一些 Entities 但我在理解如何放置它的类型声明时遇到了一些麻烦，所以我采用了这个 Yesod book chapter 关于 Yesod monads，为了更好地理解它，我来到了这个片段：

-- 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'

我认为类型声明中的 Int 需要以某种方式进行转换，更具体地说，我收到此错误：

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) }

Answer 1

我记得有一个chapter about joins in the great Yesod book，所以我重新阅读并找到了解决方案，也许我的代码可以帮助别人，所以就这样：

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)

类型声明处的

Value Distance就是答案

YesodDB，如何正确放置类型

YesodDB, how to put types correctly

haskell

yesod

esqueleto