Question

我正在尝试破译有关级别的错误消息。在Haskell中，我可以以一种简单的方式编写以下stream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，太好了。现在，当我尝试在Agda中定义类似函数时，我收到一条关于我不理解的级别的错误消息。具体来说，我收到此错误消息：

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

似乎在抱怨twist类型声明中类型变量a和b的级别，但是我不确定我是否理解问题所在。任何人都可以提供的任何指示或解释将不胜感激！

谢谢，比尔

以下是Agda代码，它可以使整个代码变得稀疏：

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')  }

Answer 1

欢迎堆栈溢出！这种错误表示未解决的元变量，这意味着Agda无法推断出隐式参数。错误消息指示该元变量的（自动生成的）名称及其类型。在这种情况下，问题可能与{a, b, c}中的twist类型有关：尚不清楚它们应位于哪个级别。要解决此问题，请指定级别：{a b c : Set}。

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

1 个答案: