标签: haskelldb

我有一个有6种不同类型实体的模式,但它们都有很多共同点.我想我可能在类型级别抽象了很多这种共性,但我遇到了HaskellDB和重叠实例的问题.这是我开始使用的代码,工作正常:

import Database.HaskellDB
import Database.HaskellDB.DBLayout

data Revision a = Revision deriving Eq
data Book = Book

instance FieldTag (Revision a) where
  fieldName _ = "rev_id"

revIdField :: Attr (Revision Book) (Revision Book)
revIdField = mkAttr undefined

branch :: Table (RecCons (Revision Book) (Expr (Revision Book)) RecNil)
branch = baseTable "branch" $ hdbMakeEntry undefined
bookRevision :: Table (RecCons (Revision Book) (Expr (Revision Book)) RecNil)
bookRevision = baseTable "book_revision" $ hdbMakeEntry undefined

masterHead :: Query (Rel (RecCons (Revision Book) (Expr (Revision Book)) …

制作一个三元逻辑表,我想为我将调用的运算符创建自己的函数<=>.

所以,例如,我想这样做,但那是不对的.这样做的正确方法是什么？

data Ternary = T | F | M
deriving (Eq,  Show, Ord)

<=> :: Ternary -> Ternary -> Ternary
<=> T F = F
<=> T T = T
<=> T M = M
<=> F F = T
<=> F T = F
<=> F M = M
<=> M F = M
<=> M T = M
<=> M M = T

假设我有以下(PostgreSQL)表定义:

CREATE TABLE books (
    id serial NOT NULL,
    title character varying NOT NULL,

    PRIMARY KEY (id)
);

以下记录定义:

data Book = 
  { id :: Int
  , title :: String
  }

查询数据库中所有书籍的"取消映射"功能的基本示例是什么allBooks :: Database -> IO [Book]？