Types Agda级别错误消息的含义：…：。Agda.Primitive.Level

我正试图破译一条关于等级的错误信息。在Haskell中，我可以直接编写以下流函数twist：

data Stream a = a :> Stream a
twist :: (a -> (b , (Either a c))) -> (c -> (b , (Either a c))) -> (Either a c) -> Stream b
twist lt rt (Left a) = b :> twist lt rt ac
   where
     (b , ac) = lt a
twist lt rt (Right c) = b :> twist lt rt ac
   where
     (b , ac) = rt c

到目前为止，一切顺利。现在，当我试图在Agda中定义类似函数时，我得到了一条关于我不理解的级别的错误消息。具体而言，我收到以下错误消息：

_a_41 : .Agda.Primitive.Level  [ at ...snip.../MinimalStream.agda:20,34-35 ]
_b_42 : .Agda.Primitive.Level  [ at ...snip.../MinimalStream.agda:20,34-35 ]

它似乎在抱怨twist类型声明中类型变量a和b的级别，但我不确定我是否理解问题所在。任何人如能提供任何建议或解释，将不胜感激

谢谢， 账单

---

以下是Agda代码，该代码生成了整个精简版本：

module MinimalStream where

open import Data.Product using (_×_; _,_; proj₁)
open import Data.Sum -- using (_⊎_)

case_of_ : ∀ {a b} {A : Set a} {B : Set b} → A → (A → B) → B
case x of f = f x

record Stream A : Set where
  coinductive
  field headStr : A
        tailStr : Stream A
open Stream; S = Stream

-- standard kinds of stream functions work as expected.
unzip₁ : ∀ {a b : Set} → Stream (a × b) → Stream a
headStr (unzip₁ sab) = proj₁ (headStr sab)
tailStr (unzip₁ sab) = unzip₁ (tailStr sab)

twist : ∀ {a b c} → (a → (b × (a ⊎ c))) → (c → (b × (a ⊎ c))) → (a ⊎ c) → Stream b
headStr (twist lt rt (inj₁ a)) = case lt a of
                                    λ { (b , (inj₁ _)) → b ;
                                        (b , (inj₂ _)) → b }
headStr (twist lt rt (inj₂ c)) = case rt c of
                                    λ { (b , (inj₁ _)) → b ;
                                        (b , (inj₂ _)) → b }
tailStr (twist lt rt (inj₁ a)) = case lt a of
                                    λ { (_ , (inj₁ a')) → twist lt rt (inj₁ a') ;
                                        (_ , (inj₂ c))  → twist lt rt (inj₂ c)  }
tailStr (twist lt rt (inj₂ c)) = case rt c of
                                    λ { (_ , (inj₁ a))  → twist lt rt (inj₁ a) ;
                                        (_ , (inj₂ c')) → twist lt rt (inj₂ c')  }

---

欢迎来到堆栈溢出！这类错误表示未解决的元变量，这意味着Agda无法推断隐式参数。错误消息指示元变量的（自动生成的）名称及其类型。在这种情况下，问题可能与

twist

中的

{a，b，c}

类型有关：不清楚这些类型应处于哪个级别。要解决此问题，请指定级别：

{abc:Set}

@BillHarrison很高兴听到这个消息。请随时接受答案，这样问题就不会显示为“未回答”。