Haskell错误：无法匹配期望的类型'Integer'对比推断类型`Int' [英] Haskell Error: Couldn't match expected type `Integer' against inferred type `Int'

问题描述

我有一个haskell函数来计算有限Ints列表的大小。我需要输出类型为Integer，因为该值实际上会大于Int的最大范围（如果输出类型为Int，结果将为-1）

I have a haskell function that that calculates the size of the list of finite Ints. I need the output type to be an Integer because the value will actually be larger than the maximum bound of Int (the result will be -1 to be exact if the output type is an Int)

size :: a -> Integer
size a =  (maxBound::Int) - (minBound::Int)

我理解Ints（有界）和Integers（无界）之间的区别，但我想从Int构造一个Integer。我想知道是否有像fromInteger这样的函数，这将允许我将Int转换为Integer类型。

I understand the difference between Ints (bounded) and Integers (unbounded) but I'd like to make an Integer from an Int. I was wondering if there was a function like fromInteger, that will allow me to convert an Int to an Integer type.

推荐答案

您需要将值转换为整数，这可以通过fromIntegral函数完成（对Haskell进行数值转换）：

You'll need to convert the values to Integers, which can be done by the fromIntegral function (numeric casting for Haskell):

fromIntegral :: (Integral a, Num b) => a -> b

它将Integral类中的任何类型转换为（更大）Num类中的任何类型。例如

It converts any type in the Integral class to any type in the (larger) Num class. E.g.

fromIntegral (maxBound::Int) - fromIntegral (minBound::Int)

然而，我不太相信你正在采取的方法 - 它看起来非常脆弱。在存在允许环绕的类型的情况下的行为是相当可疑的。

However, I would not really trust the approach you're taking -- it seems very fragile. The behaviour in the presence of types that admit wraparound is pretty suspect.

你真正的意思是：有限Ints列表的大小。这种意义上的大小，如果不是列表的长度？

What do you really mean by: "the size of the list of finite Ints". What is the size in this sense, if it isn't the length of the list?

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