Types mismatch when building list in F#

Question

I am trying to build a list from the names of some files. Unfortunately I am getting a type error that I do not know how to correct.

My code:-

open System

let buildStringList (list: string []) =

      let initial = []
           for i = 0 to list.Length do
              let foo = list.[i]
                 List.append initial foo.Substring(foo.IndexOf("."))

The type error:-

  error FS0001: This expression was expected to have type string -> 'a
  but here has type string

And this relates to the "foo.Substring(foo.IndexOf("."))" object. What is the best way to get the types correct here?

Many thanks.

Answer 1

Indentation of your function is totally off. At any rate, foo.Substring(foo.IndexOf(".")) is a string which isn't of type list as required by List.append .

What you want is to add an element to an accumulator list. A quick fix on your function is using a mutable value:

let buildStringList (arr: string []) =
    let mutable result = []
    for i = 0 to arr.Length do
        let foo = arr.[i]
        result <- foo.Substring(foo.IndexOf("."))::result
    List.rev result // Use List.rev if you would like to keep the original order

However, the recommended way is to use high-order functions. While using List.map as Mark's answer is a good approach, you can use Array.fold which is closer to your code:

let buildStringList (arr: string []) =
    arr
    |> Array.fold (fun acc foo -> foo.Substring(foo.IndexOf("."))::acc) []
    |> List.rev

To be complete, list comprehension is also helpful in certain situations:

let buildStringList (arr: string []) =
    [ for foo in arr -> foo.Substring(foo.IndexOf(".")) ]

Answer 2

I think this is a more functional way to do what you're trying:

open System

let buildStringList (list: string []) =
      list |> Array.toList |> List.map (fun x -> x.Substring(x.IndexOf(".")))

The reason for your particular error was that List.append takes two lists, not a list and a single item.