如何在 F# 中将文件读入一系列行

Question

This is C# version:

public static IEnumerable<string> ReadLinesEnumerable(string path) {
  using ( var reader = new StreamReader(path) ) {
    var line = reader.ReadLine();
    while ( line != null ) {
      yield return line;
      line = reader.ReadLine();
    }
  }
}

But directly translating needs a mutable variable.

Answer 1

If you're using .NET 4.0, you can just use File.ReadLines .

> let readLines filePath = System.IO.File.ReadLines(filePath);;

val readLines : string -> seq<string>

Answer 2

open System.IO

let readLines (filePath:string) = seq {
    use sr = new StreamReader (filePath)
    while not sr.EndOfStream do
        yield sr.ReadLine ()
}

Answer 3

To answer the question whether there is a library function for encapsulating this pattern - there isn't a function exactly for this, but there is a function that allows you to generate sequence from some state called Seq.unfold . You can use it to implement the functionality above like this:

new StreamReader(filePath) |> Seq.unfold (fun sr -> 
  match sr.ReadLine() with
  | null -> sr.Dispose(); None 
  | str -> Some(str, sr))

The sr value represents the stream reader and is passed as the state. As long as it gives you non-null values, you can return Some containing an element to generate and the state (which could change if you wanted). When it reads null , we dispose it and return None to end the sequence. This isn't a direct equivalent, because it doesn't properly dispose StreamReader when an exception is thrown.

In this case, I would definitely use sequence expression (which is more elegant and more readable in most of the cases), but it's useful to know that it could be also written using a higher-order function.

Answer 4

    let lines = File.ReadLines(path)                

    // To check
    lines |> Seq.iter(fun x -> printfn  "%s" x) 

Answer 5

On .NET 2/3 you can do:

let readLines filePath = File.ReadAllLines(filePath) |> Seq.cast<string>

and on .NET 4:

let readLines filePath = File.ReadLines(filePath);;

Answer 6

In order to avoid the "System.ObjectDisposedException: Cannot read from a closed TextReader." exception, use:

let lines = seq { yield! System.IO.File.ReadLines "/path/to/file.txt" }