F#: Breaking out of a loop - for-loop

Question

I am new to programming and F# is my first language.
I have a list of URLs that, when first accessed, either returned HTTP error 404 or experienced gateway timeout. For these URLs, I would like to try accessing them another 3 times. At the end of these 3 attempts, if a WebException error is still thrown, I will assume that the URL doesn't exist, and I will add it to a text file containing all of the invalid URLs.
Here is my code:
let tryAccessingAgain (url: string) (numAttempts: int) =
async {
for attempt = 1 to numAttempts do
try
let! html = fetchHtmlAsync url
let name = getNameFromPage html
let id = getIdFromUrl url
let newTextFile = File.Create(htmlDirectory + "\\" + id.ToString("00000") + " " + name.TrimEnd([|' '|]) + ".html")
use file = new StreamWriter(newTextFile)
file.Write(html)
file.Close()
with
:? System.Net.WebException -> File.AppendAllText("G:\User\Invalid URLs.txt", url + "\n")
}
I have tested fetchHtmlAsync, getNameFromPage and getIdFromUrl in F# Interactive. All of them work fine.
If I succeed in downloading the HTML contents of a URL without using all 3 attempts, obviously I want to break out of the for-loop immediately. My question is: How may I do so?

Answer 1

use recursion instead of the loop:
let rec tryAccessingAgain (url: string) (numAttempts: int) =
async {
if numAttempts > 0 then
try
let! html = fetchHtmlAsync url
let name = getNameFromPage html
let id = getIdFromUrl url
let newTextFile = File.Create(htmlDirectory + "\\" + id.ToString("00000") + " " + name.TrimEnd([|' '|]) + ".html")
use file = new StreamWriter(newTextFile)
file.Write(html)
file.Close()
with
| :? System.Net.WebException ->
File.AppendAllText("G:\User\Invalid URLs.txt", url + "\n")
return! tryAccessingAgain url (numAttempts-1)
}
please note that I could not test it and there might be some syntax errors - sorry if
as we are at it - you might want to rewrite the logging of the invalid url like this:
let rec tryAccessingAgain (url: string) (numAttempts: int) =
async {
if numAttempts <= 0 then
File.AppendAllText("G:\User\Invalid URLs.txt", url + "\n")
else
try
let! html = fetchHtmlAsync url
let name = getNameFromPage html
let id = getIdFromUrl url
let newTextFile = File.Create(htmlDirectory + "\\" + id.ToString("00000") + " " + name.TrimEnd([|' '|]) + ".html")
use file = new StreamWriter(newTextFile)
file.Write(html)
file.Close()
with
| :? System.Net.WebException ->
return! tryAccessingAgain url (numAttempts-1)
}
this way it will only be logged once the attempts where all made