Apply function to each element to generate sequence with yield in F#

Question

I am relative new with F# and one of the more complicate things to me is how to use yield properly. I am trying to generate a sequence by reading a text file and extracting for each line the values with a and add them into a sequence(the result)

regexp function from http://www.fssnip.net/29/title/Regular-expression-active-pattern

let (|Regex|_|) pattern input =
    let m = Regex.Match(input, pattern)
    if m.Success then Some(List.tail [ for g in m.Groups -> g.Value ])
    else None

let extractCoordinates input =
    match input with
    | Regex @"\(([0-9]{3})\)[-. ]?([0-9]{3})[-. ]?([0-9]{4})" [ area; prefix; suffix ] ->
        [ area; prefix; suffix ]
    | _ -> []

read file and generate sequence

open System.IO

let filepath = __SOURCE_DIRECTORY__ + @"../../test_input_01.txt"

let values =
        File.ReadAllLines 
        |> Seq.map (fun l -> extractCoordinates l)

but it is no working

error FS0001: The type 'string -> string []' is not compatible with the type 'seq<'a>'

Could anyone tell me how to apply a function to each element of a list and store the output into a sequence result? Using yield or not...

nilekirk nilekirk · Accepted Answer · 2019-12-14T15:11:47

Your approach with Seq.map is correct. The error you get is from not providing the argument to File.ReadAllLines:

let values =
    File.ReadAllLines filepath
    |> Seq.map (fun l -> extractCoordinates l)

Apply function to each element to generate sequence with yield in F#

1 Answers