Interface Idris2：有没有在接口实现中使用隐式的方法

我正在使用Idris2和Idris跟踪TDD。在第6章中，我正在使用schema处理数据存储。首先，对于某些上下文：

infixr 5 .+.

data Schema = SString
            | SInt
            | (.+.) Schema Schema

SchemaType : Schema -> Type
SchemaType SString = String
SchemaType SInt = Int
SchemaType (x .+. y) = (SchemaType x, SchemaType y)

在某些时候，我们希望格式化类型为

SchemaType schema

的值以显示给用户。在这本书中，这个问题通过如下编写的

显示
函数得到解决：

display : SchemaType schema -> String
display {schema = SString} item = show item
display {schema = SInt} item = show item
display {schema = (x .+. y)} (iteml, itemr) = display iteml ++ ", " ++ display itemr

我想知道是否有可能使用
Show
界面来实现这一点，这样我就可以调用
Show item

我尝试了以下方法：

Show (SchemaType schema) where
  show {schema = SString} item = show item  
  show {schema = SInt} item = show item
  show {schema = (x .+. y)} (x, y) = "(" ++ show x ++ ", " ++ show y ++ ")" 

但它告诉我模式将被删除，因此无法使用

我曾试图让idris在运行时保存它，但我只是猜测语法和错误，我真的不知道如何解释

尝试1：
抛出：

Error: While processing left hand side of show. Can't match on ?postpone [no locals in scope] (Non linear pattern variable).

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:27:33--27:34
 23 |
 24 | {schema:_} -> Show (SchemaType schema) where
 25 |   show {schema = SString} item = show item
 26 |   show {schema = SInt} item = show item
 27 |   show {schema = (x .+. y)} (x, y) = "(" ++ show x ++ ", " ++ show y ++ ")"
                                      ^

Error: While processing left hand side of show. schema is not a valid argument in show ?item.

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:25:3--25:31
 24 | Show ({schema:_} -> SchemaType schema) where
 25 |   show {schema = SString} item = show item
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Error: While processing right hand side of show. Sorry, I can't find any elaboration which works. If Builtin.Pair: schema is not accessible in this context.

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:26:11--26:17
 24 | Show (SchemaType schema) where
 25 |   show item =
 26 |     case (schema, item) of
                ^^^^^^

尝试2：
抛出：

Error: While processing left hand side of show. Can't match on ?postpone [no locals in scope] (Non linear pattern variable).

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:27:33--27:34
 23 |
 24 | {schema:_} -> Show (SchemaType schema) where
 25 |   show {schema = SString} item = show item
 26 |   show {schema = SInt} item = show item
 27 |   show {schema = (x .+. y)} (x, y) = "(" ++ show x ++ ", " ++ show y ++ ")"
                                      ^

Error: While processing left hand side of show. schema is not a valid argument in show ?item.

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:25:3--25:31
 24 | Show ({schema:_} -> SchemaType schema) where
 25 |   show {schema = SString} item = show item
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Error: While processing right hand side of show. Sorry, I can't find any elaboration which works. If Builtin.Pair: schema is not accessible in this context.

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:26:11--26:17
 24 | Show (SchemaType schema) where
 25 |   show item =
 26 |     case (schema, item) of
                ^^^^^^

尝试3
抛出：

Error: While processing left hand side of show. Can't match on ?postpone [no locals in scope] (Non linear pattern variable).

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:27:33--27:34
 23 |
 24 | {schema:_} -> Show (SchemaType schema) where
 25 |   show {schema = SString} item = show item
 26 |   show {schema = SInt} item = show item
 27 |   show {schema = (x .+. y)} (x, y) = "(" ++ show x ++ ", " ++ show y ++ ")"
                                      ^

Error: While processing left hand side of show. schema is not a valid argument in show ?item.

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:25:3--25:31
 24 | Show ({schema:_} -> SchemaType schema) where
 25 |   show {schema = SString} item = show item
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Error: While processing right hand side of show. Sorry, I can't find any elaboration which works. If Builtin.Pair: schema is not accessible in this context.

/home/stefan/dev/tdd-idris/SchemaDataStore.idr:26:11--26:17
 24 | Show (SchemaType schema) where
 25 |   show item =
 26 |     case (schema, item) of
                ^^^^^^

有人能告诉我吗？我是不是在尝试一些不可能的事情，是不是语法有误？
schema
不是
show
的参数。我想这就是你想要的：

{schema:}->显示（SchemaType schema）其中
展示项目=
的案例（模式、项目）
（SString，str）=>show str
（SInt，int）=>显示int
（（x.+.y）、（左、右））=>“（“++显示x++”，“++显示y++”）

但是，这会产生另一个错误，因为类型类实际上只适用于
数据
，并且您正在将其与函数一起使用。也许不和谐的人（在标签描述中）知道如何让它工作。Idris堆栈溢出不是很活跃。
这怎么样

infixr 5 .+.

data Schema = SString
            | SInt
            | (.+.) Schema Schema

SchemaType : Schema -> Type
SchemaType SString = String
SchemaType SInt = Int
SchemaType (x .+. y) = (SchemaType x, SchemaType y)


namespace AdHoc
  public export
  show : {scm : _} -> SchemaType scm -> String
  show {scm = SString } str = show str
  show {scm = SInt    } num = show num
  show {scm = (a .+. b)} (x, y) = "(" ++ AdHoc.show x ++ ", " ++ AdHoc.show y ++ ")"

main : IO ()
main = putStrLn $ show {scm = (SString .+. SInt)} ("adhoc", 0)