OverloadedStrings不太合适

此代码给出以下错误.我该如何解决？

230     let pipeline = [ ["$match" =: matchSelect],
231                         ["$group" =: ["_id" =: empty, "sum" =:
232                             ["$sum" =: "$score"]]] ]

Add.hs:232:40:
    No instance for (Data.String.IsString v0)
      arising from the literal `"$score"'
    The type variable `v0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Data.String.IsString Data.Text.Internal.Text
        -- Defined in `Data.Text'
      instance Data.String.IsString [Char] -- Defined in `Data.String'
    In the second argument of `(=:)', namely `"$score"'
    In the expression: "$sum" =: "$score"
    In the second argument of `(=:)', namely `["$sum" =: "$score"]'
Failed, modules loaded: Utils, Delete, Get, Migrate, Review, Validate.

GS - Apologi.. 8

(=:)具有多态类型,其中在结果类型中未提及参数中提到的类型变量,因此编译器没有足够的数量来决定表达式中第二个参数的类型"$sum" =: "$score".

字符串文字"$score"给出约束IsString v0,并(=:)给出约束Val v0,但实际上并没有说"它必须是这种类型",并且由于最终结果是Field没有提及的类型,v因此最终结果的使用方式无关紧要v.

最简单的方法是给出"$score"一个类型签名,例如("$score" :: String)你真正想要的其他类型.