完成语义检查，整理项目

chusql-engine/src/ChuSQL/Model.hs

@@ -1,4 +1,11 @@
-module ChuSQL.Model where
+module ChuSQL.Model (Column (..), Value (..), Row, Table (..), Database, colNames, colType, lookupTable, allColumns) where
+
+-- 列的类型
+data Column
+    = TInt
+    | TStr
+    | TBool
+    deriving (Show, Eq)
 
 -- 列的值：整数、字符串、布尔
 data Value
@@ -13,10 +20,26 @@ type Row = [(String, Value)]
 -- 一张表
 data Table = Table
     { tableName :: String
-    , tableCols :: [String]
+    , tableCols :: [(String, Column)]
     , tableRows :: [Row]
     }
     deriving (Show)
 
 -- 数据库：多张表
 type Database = [(String, Table)]
+
+-- 获取列名
+colNames :: Table -> [String]
+colNames = map fst . tableCols
+
+-- | 查某一列的类型
+colType :: Table -> String -> Maybe Column
+colType t c = lookup c (tableCols t)
+
+-- | 按名字查表；表不存在就报错
+lookupTable :: Database -> String -> Either String Table
+lookupTable db t = maybe (Left ("unknown table: " ++ t)) Right (lookup t db)
+
+-- | 投影列清单里的哨兵值：它表示"全部列，不用挑"，不是列名
+allColumns :: String
+allColumns = "*"