小编vpo*_*hok的帖子

我试着通过一些订单从集合中获取数据:

db.data.aggregate([
    {$limit: 1000},
    {$group: {
        _id: "$service",
        count: {$sum: 1},
        data: {$push: '$$ROOT'}
    }}
]);

但得到下一个错误:

Error("Printing Stack Trace")@:0
()@src/mongo/shell/utils.js:37
([object Array])@src/mongo/shell/collection.js:866
@(shell):6

uncaught exception: aggregate failed: {
    "errmsg" : "exception: FieldPath field names may not start with '$'.",
    "code" : 16410,
    "ok" : 0
}

我哪里做错了？

我是Haskell的初学者,在下一个街区遇到麻烦:

up_heap :: Num a => [a] -> [a] -> [a]
up_heap heap (h:t) =
let ppos = quot (length heap) 2 in
        case ((ppos > 0) && ((heap !! ppos) > h)) of
            True ->
                let (upper, (p:lower)) = splitAt ppos heap in
                    up_heap upper (h:lower) ++ (p:t)
            False ->
                heap ++ h:t

下一个错误:无法推断(Ord a)因使用">"而产生

怎么改进这个？