Question

How could I write this C# code in F# or Haskel, or a similar functional language?

var lines = File.ReadAllLines(@"\\ad1\\Users\aanodide\Desktop\APIUserGuide.txt");

// XSDs are lines 375-471
var slice = lines.Skip(374).Take(471-375+1);

var kvp = new List<KeyValuePair<string, List<string>>>(); 
slice.Aggregate(kvp, (seed, line) => 
{
    if(line.StartsWith("https"))
        kvp.Last().Value.Add(line);
    else
        kvp.Add(
            new KeyValuePair<string,List<string>>(
                line, new List<string>()
            )
        );
    }
    return kvp;
});

Answer 1

So, if I read your code correctly, your input looks something like this:

[...]
Foo
https://example1.com
https://example2.com
Bar
https://example3.com
Baz
Xyzzy
https://example4.com
[...]

From this, you want the headers grouped with the URLs below them. Here’s a Haskell program that does this:

import Data.List (isPrefixOf)

groupUrls :: [String] -> [(String, [String])]
groupUrls [] = []
groupUrls (header:others) = (header, urls) : groupUrls remaining
  where (urls, remaining) = span (isPrefixOf "https") others

main = do
    input <- readFile "\\\\ad1\\\\Users\\aanodide\\Desktop\\APIUserGuide.txt"
    let slice = take (471 - 375 + 1) $ drop 374 $ lines input
    let kvp = groupUrls slice
    print kvp

Output:

[("Foo",["https://example1.com","https://example2.com"]),("Bar", ["https://example3.com"]),("Baz",[]),("Xyzzy",["https://example4.com"])]

The key function of interest here is span, which is used here to take the consecutive lines starting with "https" and return them together with the remaining lines, which are then dealt with recursively.