Writing a function that replaces a string using OpenText, ReadLine and WriteLine - f#

Question

I have to write a function, given a filename, needle and a replace, that swaps the two strings in a given text document. The function has to use the System.IO.File.OpenText, WriteLine and ReadLine syntax. I'm currently stuck here, where the function seems to override given text document instead of replacing the needle.
open System
let fileReplace (filename : string) (needle : string) (replace : string) : unit =
try // uses try-with to catch fail-cases
let lines = seq {
use file = IO.File.OpenText filename // uses OpenText
while not file.EndOfStream // runs through the file
do yield file.ReadLine().Replace(needle, replace)
file.Close()
}
use writer = IO.File.CreateText filename // creates the file
for line in lines
do writer.Write line
with
_ -> failwith "Something went wrong opening this file" // uses failwith exception
let filename = #"C:\Users\....\abc.txt"
let needle = "string" // given string already appearing in the text
let replace = "string" // Whatever string that needs to be replaced
fileReplace filename needle replace

Answer 1

The problem with your code is that you are using lazy sequence when reading lines. When you use seq { .. }, the body is not actually evaluated until it is needed. In your example, this is when iterating over lines in a for loop - but before the code gets there, you call CreateText and overwrite the file!
You can fix this by using a list, which is evaluated immediately. You also need to replace Write with WriteLine, but the rest works!
let fileReplace (filename : string) (needle : string) (replace : string) : unit =
try // uses try-with to catch fail-cases
let lines =
[ use file = IO.File.OpenText filename // uses OpenText
while not file.EndOfStream do // runs through the file
yield file.ReadLine().Replace(needle, replace)
]
use writer = IO.File.CreateText filename // creates the file
for line in lines do
writer.WriteLine line
with
_ -> failwith "Something went wrong opening this file" // uses failwith exception
I also removed the Close call, because use takes care of that for you.
EDIT: I put back the required do keywords - I was confused by your formatting. Most people would write them at the end of the previous line as in my updated version.