我在Haskell代码中看到了这一点：
data QueryParam (sym :: Symbol) a
这是什么意思？构造函数在哪里？这是没有任何扩展名的有效代码吗？
答案 0 :(得分：3)
是的，这是一个没有构造函数的数据声明 - 这样的声明表明// DocumentDB query
// POINT TO PONDER: create the formatted query, so that after creating the dynamic query we'll replace it with dynamically created "SQL Parameter/s"
var queryText = @"SELECT
us.id,
us.email,
us.status,
us.role
FROM user us
WHERE us.status = @userStatus AND us.email IN ({0})";
// contain's list of emails
IList<string> emailIds = new List<string>();
emailIds.Add("a@gmail.com");
emailIds.Add("b@gmail.com");
#region Prepare the query
// simple parameter: e.g. check the user status
var userStatus = "active";
var sqlParameterCollection = new SqlParameterCollection { new SqlParameter("@userStatus", userStatus) };
// IN clause: with list of parameters:
// first: use a list (or array) of string, to keep the names of parameter
// second: loop through the list of input parameters ()
var namedParameters = new List<string>();
var loopIndex = 0;
foreach (var email in emailIds)
{
var paramName = "@namedParam_" + loopIndex;
namedParameters.Add(paramName);
var newSqlParamter = new SqlParameter(paramName, email);
sqlParameterCollection.Add(newSqlParamter);
loopIndex++;
}
// now format the query, pass the list of parameter into that
if (namedParameters.Count > 0)
queryText = string.Format(queryText, string.Join(" , ", namedParameters));
// after this step your query is something like this
// SELECT
// us.id,
// us.email,
// us.status,
// us.role
// FROM user us
// WHERE us.status = @userStatus AND us.email IN (@namedParam_0, @namedParam_1, @namedParam_2)
#endregion //Prepare the query
// now inject the parameter collection object & query
var users = Client.CreateDocumentQuery<Users>(CollectionUri, new SqlQuerySpec
{
QueryText = queryText,
Parameters = sqlParameterCollection
}).ToList();
类型的种类型为QueryParam
它与在类型级别有效的信息一起使用 - 它与Symbol相当，但在类型级别而不是值级别。