A simple way to generate random sentences using NSSpellChecker.

RandomSentenceGenerator.swift

import AppKit

let filteredWords = Set<String>(["app", "apps"])

let questionPrefixes = Set<String>([
    "Which", "What", "Who", "Whom", "Whose", "Where", "When", "How", "Why", "Can",
    "May", "Will", "Is", "Do", "Shall", "Has", "Must", "Would", "Could", "Should"
    ])

extension String {
    func addingRandomCompletion(maximumIndex: Int? = 4) -> String {
        let completions = NSSpellChecker.shared().completions(
            forPartialWordRange: NSRange(location: (self as NSString).length, length: 0),
            in: self, language: "en", inSpellDocumentWithTag: 0)!
            .filter { filteredWords.contains($0) == false }
        var upperIndexBound = completions.count
        if let mi = maximumIndex { upperIndexBound = min(mi+1, completions.count) }
        return self + completions[Int(arc4random_uniform(UInt32(upperIndexBound)))]
    }
    func lexicalClassTags() -> [String] {
        return linguisticTags(in: startIndex ..< endIndex, scheme: "LexicalClass",
                              options: [.joinNames, .omitWhitespace])
    }
}

func randomSentence() -> String {
    var s = ""
    while true {
        s = s.addingRandomCompletion(maximumIndex: s.isEmpty ? nil : 3)
        let ltags = s.lexicalClassTags()
        if ltags.count > 3 && ltags.last == "Noun" && ltags.contains("Verb") && arc4random() & 7 > 0 {
            return s + (questionPrefixes.contains(where: { s.hasPrefix($0) }) ? "?" : ".")
        } else {
            s += " "
        }
    }
}

for _ in 0 ..< 10 {
    print(randomSentence() + "\n")
}