Using filter on Swift Array gives "cannot invoke filter with an argument list of type (Object) throws -> Bool," but the argument doesn't throw

Question

I'm having trouble figuring out exactly what kind of argument list Swift is expecting here, because I am getting an error when I call "filter" on a swift array.

Here is my code:

func setDefaultBook()
{
    var book: Book

    let fetchRequest: NSFetchRequest<Book> = Book.fetchRequest()
    if let fetchResults = try? managedObjectContext.fetch(fetchRequest) {
        book = fetchResults.filter { $0.title == defaultBookTitle }
    }

    ...
}

A book is an NSManagedObject, and a book's title is a simple String property - an @NSManaged var. There is nothing special about the title here, and there's nothing to indicate that checking a title would throw. However, I get an error on the filter line that says:

"Cannot invoke 'filter' with an argument list of type '((Book) throws -> Bool)'

Clearly I'm missing something, because this comparison doesn't look to me like it would ever throw. I've tried a couple of other ways of doing this, and I keep getting the same error.

Building out the closure like so doesn't help:

book = fetchResults.filter { book in
    book.title == defaultBookTitle
}

And setting a predicate to do the same thing doesn't help either:

let predicate: (Book) -> Bool = { $0.title == self.defaultBookTitle }
book = fetchResults.filter(predicate)

I keep getting the same error as before.

Anyone have any idea why swift thinks that the argument here is (Book) throws -> Bool instead of just (Book) -> Bool?

Has anyone run into this error before? Any suggestions to fix it are appreciated.

vadian · Accepted Answer · 2016-12-07 17:49:50Z

6

The error message might be misleading. The result of filter is always an array

var books = [Book]()

let fetchRequest: NSFetchRequest<Book> = Book.fetchRequest()
if let fetchResults = try? managedObjectContext.fetch(fetchRequest) {
    books = fetchResults.filter { $0.title == defaultBookTitle }
}

But why not let Core Data filter the records:

var books = [Book]()

let fetchRequest: NSFetchRequest<Book> = Book.fetchRequest()
fetchRequest.predicate = NSPredicate(format:"title = %@", defaultBookTitle)
do {
   books = try managedObjectContext.fetch(fetchRequest)
} catch {
  print(error)
}

answered Dec 7, 2016 at 17:49

vadian

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2 Comments

rickster Over a year ago

Good answer on both counts. Some follow-ups: file bugs if you see compiler messages that need improving. And it's just about always better to let Core Data do filtering like this, because searching/filtering in the database is much cheaper than doing it on fetched/instantiated objects.

JAH-FH Over a year ago

You are correct in that it was a misleading error message; unfortunately I focused on that instead of what the actual return was. Thank you for your help.

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