YamlToJSON.swift

import SwiftUI
import Yams

extension String {
    var yamlToJSON: String {
        do {
            guard let parsed = try Yams.load(yaml: self) else { return "" }
            let data = try JSONSerialization.data(withJSONObject: parsed, options: [.sortedKeys, .prettyPrinted])
            return String(decoding: data, as: UTF8.self)
        } catch {
            return "\(error)"
        }
    }
}

@main
struct YamlToJSONApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

struct ContentView: View {
    @State var input = ""
    var body: some View {
        HSplitView {
            TextEditor(text: $input)
            TextEditor(text: .constant(input.yamlToJSON))
        }
    }
}

As a swift-sh script:

#!/usr/bin/swift sh

import Foundation
import Yams // @jpsim

guard CommandLine.arguments.count > 1 else {
    print("usage: tool file.yaml [file.json]")
    exit(42)
}

do {
    let input  = try String(contentsOfFile: CommandLine.arguments[1])
    let parsed = try Yams.load(yaml: input) ?? [:]
    let data   = try JSONSerialization
         .data(withJSONObject: parsed, options: [.sortedKeys, .prettyPrinted])
    
    if CommandLine.arguments.count > 2 {
        try data.write(to: URL(fileURLWithPath: CommandLine.arguments[2]))
    }
    else if let output = String(data: data, encoding: .utf8) {
        print(output)
    }
    else if !data.isEmpty {
        print("Could not convert data?!")
        exit(44)
    }
}
catch {
    print("Errors happen:", error)
    exit(43)
}