为什么是“（2 + 2.0）”？双.hs文件，但“分数a =>一个"在GHCi？ [英] Why is "(2+2.0)" Double in a .hs file but "Fractional a => a" in GHCi?

问题描述

从GHCi中的 .hs 文件加载 a = 2 + 2.0 并执行 ：ta 显示 a :: Double 。

另一方面， GHCi中的code> let b = 2 + 2.0 和：tb 显示 b ::小数a => ;一个。

你怎么能从这两个文件中推断出这个？

• 4.3.4重载数字操作的模糊类型和默认值 https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-750004.3


• 2.4.8在GHCi中输入违约类型： https://downloads.haskell.org/~ghc/ 7.8.3 / docs / html / users_guide / interactive-evaluation.html

我发现这个官方文档几乎不可理解。

解决方案

本文档是您想要的，我想。基本上，GHC在GHCi中默认实现与模块中不同的语言;特别是，可怕的单态限制在模块中是默认启用的（按照语言），而在GHCi中它是禁用的。单态的约束是什么导致GHC为你的 a 选择一个单形类型，而在关闭时，GHC可以自由地概括 b ，产生你看到的多态类型。

Loading a = 2+2.0 from a .hs file in GHCi and doing :t a shows a :: Double.

On the other hand, doing let b = 2+2.0 and :t b in GHCi shows b :: Fractional a => a.

How are you able to deduce this from these two documents?

• 4.3.4 Ambiguous Types, and Defaults for Overloaded Numeric Operations https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-750004.3
• 2.4.8 Type defaulting in GHCi: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/interactive-evaluation.html

I find this official documentation almost incomprehensible.

解决方案

This documentation is what you want, I think. Basically, GHC implements a different language by default in GHCi than in a module; in particular, the dreaded monomorphism restriction is enabled by default in a module (as per the language) whereas in GHCi it's disabled. The monomorphism restriction is what causes GHC to pick a monomorphic type for your a, whereas, with it turned off, GHC is free to generalize the type of b, yielding the polymorphic type you see.

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