如何构造grpc-proto-lens生成对象？解决类型构造器非法使用问题

我的proto定义如下：

message PortfolioRequest { string account_id = 1; }

使用http2-grpc-haskell（基于grpc-proto-lens）生成代码后，我尝试用let pr = PortfolioRequest (T.pack "0")创建对象，却报错Illegal term-level use of the type constructor。生成的Haskell代码如下：

{- | Fields : * 'Proto.Invest.Operations_Fields.accountId' @:: Lens' PortfolioRequest Data.Text.Text@ -} 
data PortfolioRequest = PortfolioRequest'_constructor 
  {_PortfolioRequest'accountId :: !Data.Text.Text, 
   _PortfolioRequest'_unknownFields :: !Data.ProtoLens.FieldSet} 
  deriving stock (Prelude.Eq, Prelude.Ord) 

instance Prelude.Show PortfolioRequest where 
  showsPrec _ __x __s = Prelude.showChar '{' (Prelude.showString (Data.ProtoLens.showMessageShort __x) (Prelude.showChar '}' __s)) 

instance Data.ProtoLens.Field.HasField PortfolioRequest "accountId" Data.Text.Text where 
  fieldOf _ = (Prelude..) (Lens.Family2.Unchecked.lens _PortfolioRequest'accountId (\ x__ y__ -> x__ {_PortfolioRequest'accountId = y__})) Prelude.id 

instance Data.ProtoLens.Message PortfolioRequest where 
  messageName _ = Data.Text.pack "tinkoff.public.invest.api.contract.v1.PortfolioRequest" 
  packedMessageDescriptor _ = "\n\DLEPortfolioRequest\DC2\GS\n\naccount_id\CAN\SOH \SOH(\tR\taccountId" 
  packedFileDescriptor _ = packedFileDescriptor 
  fieldsByTag = let accountId__field_descriptor = Data.ProtoLens.FieldDescriptor "account_id" (Data.ProtoLens.ScalarField Data.ProtoLens.StringField :: Data.ProtoLens.FieldTypeDescriptor Data.Text.Text) (Data.ProtoLens.PlainField Data.ProtoLens.Optional (Data.ProtoLens.Field.field @"accountId")) :: Data.ProtoLens.FieldDescriptor PortfolioRequest in Data.Map.fromList [(Data.ProtoLens.Tag 1, accountId__field_descriptor)] 
  unknownFields = Lens.Family2.Unchecked.lens _PortfolioRequest'_unknownFields (\ x__ y__ -> x__ {_PortfolioRequest'_unknownFields = y__}) 
  defMessage = PortfolioRequest'_constructor {_PortfolioRequest'accountId = Data.ProtoLens.fieldDefault, _PortfolioRequest'_unknownFields = []}

解决方法 #

从生成的代码可以看出，PortfolioRequest是类型构造器，不能直接在值层面调用。实际的数值构造器是PortfolioRequest'_constructor，不过更推荐使用proto-lens的标准方式构造消息对象：

方法1：默认消息+Lens修改（推荐） #

这是proto-lens官方推荐的方式，先获取默认消息实例，再通过生成的Lens设置目标字段：

import Data.ProtoLens (defMessage)
import Proto.Invest.Operations_Fields (accountId)  -- 导入生成的字段Lens
import qualified Data.Text as T

let pr = defMessage & accountId .~ T.pack "0"

方法2：直接调用生成的构造器 #

如果需要直接使用构造器，注意构造器名称是PortfolioRequest'_constructor，且必须同时指定_unknownFields字段（传空FieldSet即可）：

import qualified Data.Text as T
import Data.ProtoLens (FieldSet)

let pr = PortfolioRequest'_constructor (T.pack "0") []

错误原因说明 #

你写的PortfolioRequest (T.pack "0")是把类型构造器当作值构造器使用了。Haskell中类型和值属于不同的命名空间，类型构造器只能在类型层面使用，不能直接用来创建值，因此触发了Illegal term-level use of the type constructor错误。

内容的提问来源于stack exchange，提问作者Miezhiko