F#和类型推断:"int list"不支持"+" [英] F# and type inference: "int list" does not support "+"

问题描述

为什么以下内容会产生错误?

Why does the following generate an error?

let mult a b = a * b
let sum a b = a + b

第二个let产生编译错误:

The second let generates a compile error:

为类型推断变量应用默认类型"int列表"时，类型约束不匹配.类型"int列表"不支持任何名为"+"的运算符，请考虑添加其他类型约束

Type constraint mismatch when applying the default type 'int list' for a type inference variable. The type 'int list' does not support any operators named '+' Consider adding further type constraints

我不明白.为什么假定我正在输入列表?

I don't get it. Why does it assume I'm inputting a list?

完整列表:

let sampleTimeSeries = 
  dict [
  "20110131", 1.5; 
  "20110228", 1.5;
  "20110331", 1.5; 
  "20110431", 1.5; 
  "20110531", 1.5; 
]
//  Recursive reduce function
//  func  : The function to apply
//  terminatingvalue : The terminating value
//  sequence : The list to apply the value on.
let rec reduce func sequence terminatingValue:int =
  match sequence with 
  | [] -> terminatingValue 
  | h::t -> func h (reduce func t terminatingValue)

let mult a b = a * b
let sum a b = a + b + 0

一旦我在sum函数的末尾添加了0，它就可以正确编译.

Once I added a 0 at the end of the sum function, it compiled correctly.

推荐答案

在声明的末尾添加0有助于编译器推断sum函数的返回类型:

Adding a 0 to the end of the declaration helped the compiler infer the return type of the sum function:

let mult a b = a * b 
let sum a b = a + b + 0 

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