F# - Sanity Checks and Options - f#

Question

I'm pretty new to F# so it's hard for me to change my mindset after many years of C#/Java OOP.
I have an event handler MyForm.SelectFile(filePath:String) that opens a dialog and let you select the file to read. Once the file is selected, Parser.LoadFile(filePath:String) is called:
static member LoadFile(filePath:String) =
if not <| ZipFile.IsZipFile(filePath) then
failwith "invalid file specified."
use zipFile = new ZipFile(filePath)
if zipFile.Count <> 2 || zipFile |> Seq.exists(fun x -> x.FileName <> "alpha" && x.FileName <> "beta") then
failwith "invalid file specified."
zipFile |> fun x -> Parser.Parse(x.OpenReader())
I'm always expecting the selected file to be a valid zip archive containing 2 files without extension: "alpha" and "beta".
First, is there a better way to sanitize my input?
My if statements are pretty long and I'm sure F# can provide better solutions, but I really can't figure out.
Second, using failwith is forcing me to handle exceptions in my MyForm.SelectFile(filePath:String) method and I think Options could be a better solution.
I can't figure out how to use them if I need to perform two different and consecutive checks (ZipFile.IsZipFile and content) because in between I have to instantiate a ZipFile.
In C# I would just return null whenever a check fails and then checking the return value against null would let me know whether I need to prompt an error or continue.
Current code:
type Parser with
static member isValidZipFile (zipFile:ZipFile) =
(zipFile.Count = 2) && (zipFile |> Seq.forall(fun x -> (x.FileName = "alpha") || (x.FileName = "beta")))
static member LoadFile(filePath:String) =
if not <| ZipFile.IsZipFile(filePath) then
None
else
use zipFile = new ZipFile(filePath)
if not <| Parser.isValidZipFile(zipFile) then
None
else
Some(seq { for zipEntry in zipFile do yield Parser.Parse(zipEntry.OpenReader()) } |> Seq.toArray)

Answer 1

First, the last line of your function could be a bit more elegant if it was written like:
zipFile.OpenReader() |> Parser.Parse
Second, you're on the right track as far as your thinking about using Option. It's really pretty simple in this case:
static member LoadFile(filePath:String) =
if not <| ZipFile.IsZipFile(filePath) then None else
use zipFile = new ZipFile(filePath)
if zipFile.Count <> 2 || zipFile |> Seq.exists(fun x -> x.FileName <> "alpha" && x.FileName <> "beta") then None else
Some (zipFile.OpenReader() |> Parser.Parse)
That last line could also be written as:
zipFile.OpenReader() |> Parser.Parse |> Some
Now, you mentioned that you don't like the long if statement. Let's turn it into a function! And I usually prefer functions with "positive" names, i.e. an isValidInput function is usually more helpful than an isInvalidInput. So let's make a function that checks if a zipfile is actually valid:
let isValid (z:ZipFile) =
z.Count = 2 && z |> Seq.forAll(fun x -> x.FileName = "alpha" || x.FileName = "beta")
Now your LoadFile function can become:
static member LoadFile(filePath:String) =
if not <| ZipFile.IsZipFile(filePath) then None else
use zipFile = new ZipFile(filePath)
if not <| isValid zipFile then None else
zipFile.OpenReader() |> Parser.Parse |> Some
And that looks pretty easy to read, so we can stop refactoring for now.

Answer 2

This piece of code looks weird. Using Sequence expressions for such a simple piece of code is overkill.
Some(seq { for zipEntry in zipFile do yield Parser.Parse(zipEntry.OpenReader()) } |> Seq.toArray)
You could write it better like this
zipFile |> Seq.map (fun ze -> ze.OpenReader () |> Parser.parse) |> Some
Or if you insist in doing it in an array (why?)
zipFile |> Seq.map (fun ze -> ze.OpenReader () |> Parser.parse) |> Seq.toArray |> Some
You'll end up with type signature option<seq<value>>. I am not sure if this is a good idea, but it is not possible to tell without looking at the rest of your code.