F＃和类型推断：“int list”不支持“+”

Question

为什么以下内容会产生错误？

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

第二送生成编译错误：

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

我不明白这一点。为什么它假设我正在输入一个列表？

全面上市：

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 

有一次，我在和函数的最后增加了一个0，它正确编译。

Answer 1

添加一个0到声明的末尾帮助编译器推断sum函数的返回类型：

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