Find repeating values in List

问题

I want to return true if I find a repeating value in the list

let rec repeats L = 
   match L with
   | [] -> false
   | x::xs when x = xs.Head -> true
   | x::xs -> repeats xs;;


repeats [1;2;3;4;5]   

Should return false. But I get this error:

System.InvalidOperationException: The input list was empty.
   at Microsoft.FSharp.Collections.FSharpList`1.get_Head()
   at FSI_0003.repeats[a](FSharpList`1 L)
   at <StartupCode$FSI_0004>.$FSI_0004.main@()
   at main@dm()
Stopped due to error

What should I do to fix the error?

回答1:

The problem is that

x::xs = 5::[]

in the last case

You want to change it to

|x::xs::xss when x=xs -> true

来源：https://stackoverflow.com/questions/19107482/find-repeating-values-in-list