为什么GHCi在无法编译的提示符下接受某些内容? [英] Why is GHCi accepting something at the prompt that won't compile?
问题描述
我正在尝试使用Haskell类型，创建一个采用类型构造函数和具体类型的数据(受
I'm trying to play around with Haskell types, creating a data taking a type constructor and a concrete type (inspired by this).
这是我的kung.hs文件:
Here is my kung.hs file:
data Kung t a = Kung { field :: t a } deriving (Show, Eq)
val1 = Kung { field = [1,5] }
val2 = Kung { field = Just 3 }
--val3 = Kung { field = 3 }
它可以正常编译并可以加载:
It compiles fine and loads ok:
*Main> :load C:\Test\Haskell\kung.hs
[1 of 1] Compiling Main ( C:\Test\Haskell\kung.hs, interpreted )
Ok, one module loaded.
*Main> val1
Kung {field = [1,5]}
*Main> val2
Kung {field = Just 3}
*Main>
现在是相同版本，但取消注释val3:
Now the same version, but uncommenting val3:
data Kung t a = Kung { field :: t a } deriving (Show, Eq)
val1 = Kung { field = [1,5] }
val2 = Kung { field = Just 3 }
val3 = Kung { field = 3 }
这不能编译:
*Main> :load C:\Test\Haskell\kung.hs
[1 of 1] Compiling Main ( C:\Test\Haskell\kung.hs, interpreted )
C:\Test\Haskell\kung.hs:7:24: error:
* No instance for (Num (t0 a0)) arising from the literal `3'
* In the `field' field of a record
In the expression: Kung {field = 3}
In an equation for `val3': val3 = Kung {field = 3}
|
7 | val3 = Kung { field = 3 }
| ^
Failed, no modules loaded.
这似乎很好.无法通过某种类型构造函数和某些具体类型来分解"/构造"(也许不是此处使用的正确术语)类型为Num的值3.
which seems fine. There is no way to "decompose" / "construct" (maybe not the right terminology used here) the value 3 of type Num from some type constructor and some concrete type.
返回到GHCi解释器，加载文件的第一个版本，不带val3的注释，然后:
Going back to the GHCi interpreter, load the first version of the file without the val3 commented and then:
Prelude> :load C:\Test\Haskell\kung.hs
[1 of 1] Compiling Main ( C:\Test\Haskell\kung.hs, interpreted )
Ok, one module loaded.
*Main> val3 = Kung { field = 3 }
*Main> :t val3
val3 :: Num (t a) => Kung t a
我应该如何理解?为什么GHCi人为地管理"分解3? (不提供实型)
How should I understand that? Why did GHCi artificially "manage" to decompose 3? (without giving a real type)
然后，这个val3似乎并不可行:
Then this val3 does not really seem viable:
*Main> val3
<interactive>:50:1: error:
* Ambiguous type variables `t0', `a0' arising from a use of `print'
prevents the constraint `(Show (t0 a0))' from being solved.
Probable fix: use a type annotation to specify what `t0', `a0' should be.
These potential instances exist:
instance (Show b, Show a) => Show (Either a b)
-- Defined in `Data.Either'
instance [safe] Show (t a) => Show (Kung t a)
-- Defined at C:\Test\Haskell\kung.hs:1:49
instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
...plus 15 others
...plus one instance involving out-of-scope types
(use -fprint-potential-instances to see them all)
* In a stmt of an interactive GHCi command: print it
*Main>
这里发生的细微变化是什么?
What is the subtlety happening here?
推荐答案
这是工作中的可怕的单态限制.以下可以正常编译:
This is the Dreaded Monomorphism Restriction at work. The following compiles fine:
data Kung t a = Kung { field :: t a } deriving (Show, Eq)
val3 :: Num (t a) => Kung t a
val3 = Kung { field = 3 }
但是，同构限制阻止了GHC自己推断此签名.而是尝试查找单态类型.为此，它仅具有Haskell 默认规则.通常，这意味着受Num约束的类型变量被单态化为Integer ...，但整数的形式不为t a，因此将失败.
however, the monomorphism restriction prevents GHC from inferring this signature itself. Instead, it tries to find a monomorphic type. For this it only has the Haskell defaulting rules available. Normally, these imply that a Num-constrained type variable is monomorphised to Integer... but integer is not of the form t a, so this fails.
正确修复程序的确可以自己编写类型签名，但是您也可以关闭单态性限制:
The correct fix is to, indeed, write the type signature yourself, but you can also turn off the monomorphism restriction:
{-# LANGUAGE NoMonomorphismRestriction #-}
data Kung t a = Kung { field :: t a } deriving (Show, Eq)
val3 = Kung { field = 3 }
在GHCi中，由于我相信是GHC-7.8，因此默认情况下，单态性限制已关闭.这就是为什么问题不会在那里出现的原因.
In GHCi, the monomorphism restriction is turned off by default since, GHC-7.8 I believe it was. That's why the problem doesn't arise there.
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