Haskell Netwire型误差

Question

我刚刚开始使用netwire，我在基本知识方面遇到了困难。

下面的代码对我来说很好：

main :: IO ()
main = testWire clockSession_ (for 3 . yeah)

yeah :: Monad m => Wire s () m a String
yeah = pure "yes"

但这并不意味着：

main :: IO ()
main = testWire clockSession_ forYeah

forYeah :: (Show b, Show e) => Wire s e Identity a b
forYeah = for 3 . yeah

失败时出错：

Could not deduce (b ~ [Char])
from the context (Show b, Show e)
bound by the type signature for
forYeah :: (Show b, Show e) => Wire s e Identity a b
  at /home/fiendfan1/workspace/Haskell/Haskell-OpenGL/src/Main.hs:12:12-54
  `b' is a rigid type variable bound by
      the type signature for
        forYeah :: (Show b, Show e) => Wire s e Identity a b
      at /home/fiendfan1/workspace/Haskell/Haskell-OpenGL/src/Main.hs:12:12
Expected type: Wire s e Identity a b
  Actual type: Wire s () Identity a String
In the second argument of `(.)', namely `yeah'
In the expression: for 3 . yeah
In an equation for `forYeah': forYeah = for 3 . yeah

所以我把它改成：

forYeah :: Show e => Wire s e Identity a String

这给了我一个错误：

Could not deduce (e ~ ())
from the context (Show e)
  bound by the type signature for
             forYeah :: Show e => Wire s e Identity a String
  at /home/fiendfan1/workspace/Haskell/Haskell-OpenGL/src/Main.hs:12:12-49
  `e' is a rigid type variable bound by
      the type signature for
        forYeah :: Show e => Wire s e Identity a String
      at /home/fiendfan1/workspace/Haskell/Haskell-OpenGL/src/Main.hs:12:12
Expected type: Wire s e Identity a String
  Actual type: Wire s () Identity a String
In the second argument of `(.)', namely `yeah'
In the expression: for 3 . yeah
In an equation for `forYeah': forYeah = for 3 . yeah

改为：

forYeah :: Wire s () Identity a String

给出以下错误：

No instance for (HasTime Integer s) arising from a use of `for'
Possible fix: add an instance declaration for (HasTime Integer s)
In the first argument of `(.)', namely `for 3'
In the expression: for 3 . yeah
In an equation for `forYeah': forYeah = for 3 . yeah

有人能解释为什么会发生这种情况，以及我如何修复我的第二个代码示例吗？

Answer 1

编辑：这里有一个完整的、编译的、运行的解决方案来解决这个问题：

module Main (
    main
) where

import Prelude hiding ((.), id)
import qualified Prelude as Prelude
import Control.Wire
import Control.Wire.Interval

main :: IO ()
main = testWire clockSession_ (withoutErrors forYeah)

yeah :: Monad m => Wire s e m a String
yeah = pure "yes"

forYeah :: (Num t, HasTime t s, Monoid e, Monad m) => Wire s e m a String
forYeah = for 3 . yeah

-- This just is an easy way to specify to use () as the type for errors in testWire
withoutErrors :: Wire s () m a b -> Wire s () m a b 
withoutErrors = Prelude.id

这是最初的答案，讨论了为什么我们应该更改yeah的类型，然后对forYeah的类型进行必要的更改

将yeah的类型更改为Monad m => Wire s e m a String。Monad m => (Wire s e m a)有一个Applicative实例，所以pure应该存在，而不指定yeah类型中Wire的第二个类型参数是()。

注意:我不使用netwire，也没有尝试编译这个。我只看过文档中的类型。

编辑：，您可能还需要更改forYeah的类型。

Wire还有一个Category实例：

Monad m => Category (Wire s e m)

Category的.操作符有以下类型：

(.) :: cat b c -> cat a b -> cat a c

因此，对于Wire来说，它是：

(.) :: Monad m => Wire s e m b c -> Wire s e m a b -> Wire s e m a c

for具有以下类型：

for :: (HasTime t s, Monoid e) => t -> Wire s e m a a

所以for 3会有一个类似于(HasTime Int s, Monoid e) => Wire s e m a a的类型。再加上‘s的Monad m => Wire s e m a String类型，for 3 . yeah的类型如下

(HasTime Int s, Monoid e, Monad m) => Wire s e m a String

因此，我们可能可以将forYeah的类型更改为：

forYeah :: (HasTime Int s, Monoid e, Monad m) => Wire s e m a String

编辑：更适合forYeah

由于整数(没有小数点)实际上等价于fromInteger的应用程序，其值为整数。和fromInteger :: (Num a) => Integer -> a，文字3实际上具有Num t => t类型。因此，我们可以选择的最佳类型可能是：

forYeah :: (Num t, HasTime t s, Monoid e, Monad m) => Wire s e m a String

Answer 2

当我将forYeah类型更改为

forYeah::Wire (Timed NominalDiffTime ()) () Identity a String

如果省略了forYeah的类型，它也会起作用。

Answer 3

我只是问GHCi这种类型是什么：

> :m Control.Wire
> :t for (3 :: Int) . pure "yes"
for 3 . pure "yes" :: (Monad m, HasTime Int s, Monoid e) => Wire s e m a [Char]
> :{
| let forYeah :: HasTime Int s => Wire s () Identity a String
|     forYeah = for 3 . pure "yes"
| :}
> :t forYeah
forYeah :: HasTime Int s => Wire s () Identity a String

这样就行了。但是，当询问testWire clockSession_ forYeah的类型时，我会看到一个错误，它不能将NominalDiffTime与Int相匹配，但是由于NominalDiffTime也是Num的一个实例，所以只需更改签名就相当容易了：

> :{
| let forYeah :: HasTime NominalDiffTime s => Wire s () Identity a String
|     forYeah = for 3 . pure "yes"
| :}
> :t testWire clockSession_ forYeah
testWire clockSession_ forYeah :: (Applicative m, MonadIO m) => m c

所以这看起来很管用。

而且，当我将yeah分别定义为

yeah :: Monad m => Wire s () m a String
yeah = pure "yes"

forYeah :: HasTime t s => Wire s () Identity a String
forYeah = for 3 . yeah

问题似乎在HasTime约束中。由于您忽略了它，编译器将文本3默认为Integer类型，但是没有任何s的HasTime Integer s实例。