16_realworld.nula

// 16_realworld.nula — Real-world CLI: JSON Fetcher & Reporter
//
// Fetches JSON from a URL, parses and inspects it, logs a timestamped
// summary, and writes the result to disk. A realistic workflow exercising
// HTTP, JSON, FS, datetime, args, and env together.
//
// Demonstrates:
//   System.arg   — read CLI arguments
//   Http.get     — fetch data from the web
//   json.parse / json.stringify / json.get_string — process JSON
//   datetime.now — capture a timestamp
//   FS.write     — persist output to a file
//   Env.get      — read configuration from the environment
//   IO.print     — user-facing progress messages
//
// Usage:
//   nulang examples/16_realworld.nula <url>
//
// Environment:
//   NU_OUTPUT_DIR — output directory (default: ./output/)
//
// Example:
//   NU_OUTPUT_DIR=/tmp nulang examples/16_realworld.nula \
//     https://jsonplaceholder.typicode.com/todos/1

import stdlib::json
import stdlib::datetime


// ── Helpers ───────────────────────────────────────────────────────────────

// Format a DateTime record as "YYYY-MM-DD HH:MM:SS".
let format_datetime = fn(dt) {
    let pad = fn(n) {
        if n < 10 then "0" + perform Int.to_string(n)
        else perform Int.to_string(n)
    };
    perform Int.to_string(dt.year) + "-" +
    pad(dt.month) + "-" +
    pad(dt.day) + " " +
    pad(dt.hour) + ":" +
    pad(dt.minute) + ":" +
    pad(dt.second)
}

// Produce a human-readable description of any JsonValue.
let describe = fn(v) {
    match v {
        JsonNull       => "null",
        JsonBool(b)    => if b then "true" else "false",
        JsonNumber(n)  => "number",
        JsonString(s)  => "string (" + perform Int.to_string(perform String.length(s)) + " chars)",
        JsonArray(a)   => "array[" + perform Int.to_string(perform Array.length(a)) + " items]",
        JsonObject(f)  => "object{" + perform Int.to_string(perform Array.length(f)) + " keys}"
    }
}

// Check whether a value is non-nil.
let is_some = fn(v) {
    v != nil
}


// ── Main program ──────────────────────────────────────────────────────────

// 1. Read the target URL from the command line.
//    System.arg(0) = program name, System.arg(1) = script path,
//    System.arg(2) = first user argument.
let url = perform System.arg(2)

// 2. Check that a URL was provided.
let got_url = is_some(url)

if got_url then {
    // 3. Read output directory from environment, falling back to ./output/.
    //    Env.get returns nil when the variable is not set.
    let env_dir = perform Env.get("NU_OUTPUT_DIR")
    let has_env = is_some(env_dir)
    let out_dir = if has_env then env_dir else "./output/"

    perform IO.print("=== JSON Fetcher ===")
    perform IO.print("URL:        " + url)
    perform IO.print("Output dir: " + out_dir)

    // 4. Fetch JSON from the URL.
    perform IO.print("Fetching...")
    let response = perform Http.get(url)

    // 5. Check that the response is valid.
    let ok = is_some(response)
    if ok then {
        let resp_len = perform String.length(response)
        perform IO.print("Received " + perform Int.to_string(resp_len) + " bytes")

        // 6. Parse the JSON response.
        let parsed = parse(response)
        let summary = describe(parsed)
        perform IO.print("Parsed: " + summary)

        // 7. Capture the current timestamp.
        let now = now()
        let ts = format_datetime(now)
        perform IO.print("Timestamp: " + ts)

        // 8. Build the output report (plain-text for readability).
        let report =
            "JSON Fetch Report\n" +
            "=================\n" +
            "Fetched at:   " + ts + "\n" +
            "Source URL:   " + url + "\n" +
            "Response:     " + perform Int.to_string(resp_len) + " bytes\n" +
            "Data summary: " + summary + "\n" +
            "\n" +
            "── Original JSON ──\n" +
            stringify(parsed) + "\n"

        // 9. Write the report to disk.
        let out_path = out_dir + "fetch_report.txt"
        let write_ok = perform FS.write(out_path, report)

        // FS.write returns Unit on success, nil on failure.
        // Check whether the write succeeded by reading back the file.
        let verify = perform FS.read(out_path)
        let wrote = is_some(verify)
        if wrote then {
            perform IO.print("Wrote output to " + out_path)
            perform IO.print("=== Done ===")
            unit
        } else {
            perform IO.print("Error: could not write to " + out_path)
            perform IO.print("(Does the output directory exist?)")
        }
    } else {
        perform IO.print("Error: failed to fetch from " + url)
        perform IO.print("(Check the URL and network connection)")
    }
} else {
    perform IO.print("Usage: nulang examples/16_realworld.nula <url>")
    perform IO.print("")
    perform IO.print("  Fetches JSON from the given URL, parses it, prints a")
    perform IO.print("  timestamped summary, and writes a report to disk.")
    perform IO.print("")
    perform IO.print("Environment:")
    perform IO.print("  NU_OUTPUT_DIR   output directory (default: ./output/)")
    perform IO.print("")
    perform IO.print("Example:")
    perform IO.print("  nulang examples/16_realworld.nula https://jsonplaceholder.typicode.com/todos/1")
}