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chusql-engine/src/ChuSQL/Storage/Memory.hs

@@ -3,23 +3,15 @@ module ChuSQL.Storage.Memory (MemoryStorage (runMemoryStorage)) where
 import ChuSQL.Model
 import ChuSQL.Storage
 
--- 内存实现：状态是 Database，错误通道是 Either String。
---
--- 它只实现"必须有"的那几个：批量插入、行级删除、索引、统计全走类的默认实现
--- （内存里数据本来就在手上，扫一遍就是最快的做法）。
+-- 内存实现：Database 上的状态，错误通道是 Either String。
 
--- * 实例
-
--- | 内存存储：Database 上的状态
 newtype MemoryStorage a = MemoryStorage {runMemoryStorage :: Database -> Either String (a, Database)}
 
--- | Functor：转发给底层状态
 instance Functor MemoryStorage where
     fmap f (MemoryStorage m) = MemoryStorage $ \db -> do
         (a, db') <- m db
         Right (f a, db')
 
--- | Applicative：转发 pure 和 <*>
 instance Applicative MemoryStorage where
     pure x = MemoryStorage $ \db -> Right (x, db)
     MemoryStorage mf <*> MemoryStorage ma = MemoryStorage $ \db -> do
@@ -27,13 +19,11 @@ instance Applicative MemoryStorage where
         (a, db2) <- ma db1
         Right (f a, db2)
 
--- | Monad：顺序执行
 instance Monad MemoryStorage where
     MemoryStorage m >>= k = MemoryStorage $ \db -> do
         (a, db') <- m db
         runMemoryStorage (k a) db'
 
--- | MonadStorage：直接改内存库
 instance MonadStorage MemoryStorage where
     -- \| 全表扫描
     scan t = MemoryStorage $ \db -> do
@@ -62,11 +52,28 @@ instance MonadStorage MemoryStorage where
             then Right (Right (), filter ((/= name) . fst) db)
             else Right (Left ("unknown table: " ++ name), db)
 
+    -- \| 删一列：表定义里去掉这一列，每一行里的那一格也一起去掉
+    dropColumn name col = MemoryStorage $ \db ->
+        case lookup name db of
+            Nothing -> Right (Left ("unknown table: " ++ name), db)
+            Just tbl ->
+                if col `notElem` map fst (tableCols tbl)
+                    then Right (Left ("unknown column: " ++ col), db)
+                    else
+                        Right
+                            ( Right ()
+                            , replaceTable
+                                name
+                                tbl
+                                    { tableCols = filter ((/= col) . fst) (tableCols tbl)
+                                    , tableRows = map (filter ((/= col) . fst)) (tableRows tbl)
+                                    }
+                                db
+                            )
+
     -- \| 原样返回当前库
     snapshot = MemoryStorage $ \db -> Right (db, db)
 
--- * 工具
-
 -- | 用给定表替换同名表
 replaceTable :: String -> Table -> Database -> Database
 replaceTable name table =