f# - 使用 "fst"从元组中提取第一个元素抛出错误 "type mismatch"

Question

这是我采用的元组

 let person = ("Prathap Reddy SV", "Male", 16)
 let name = fst person   

 or 

 let person = ("Prathap", "Male", 16)
 let name = fst person  

当我编译它时，它向我展示了下面的输出

 > let person = ("Prathap Reddy SV", "Male", 16)
   let name = fst person

   let name = fst person
   ---------------^^^^^^

   stdin(152,16): error FS0001: Type mismatch. Expecting a
   string * string    
 but given a
   string * string * int    
 The tuples have differing lengths of 2 and 3   

但是当我给 tuple 提供两个字符串值时，它工作正常。

Answer 1

签名fst是 ('a * 'b -> 'a) ，这就是您收到给定错误的原因。

函数fst期望一个包含任何类型的两个元素的元组，但您提供了一个包含三个元素的元组。对于三个字符串的元组，错误是相同的:fst ("a","b","c")会屈服

stdin(1,6): error FS0001: Type mismatch. Expecting a
    'a * 'b    
but given a
    'a * 'b * 'c    
The tuples have differing lengths of 2 and 3