A Simple Web App in Rust, Part 2b

My first attempt:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;

fn log_time(filename: &'static str) -> io::Result<()> {

    let local: DateTime<Local> = Local::now();
    let time_str = local.format("%Y").to_string();
    let mut f = try!(File::create(filename));
    try!(f.write_all(time_str));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
src/main.rs:13:22: 13:30 error: mismatched types:
 expected `&[u8]`,
    found `collections::string::String`
(expected &-ptr,
    found struct `collections::string::String`) [E0308]
src/main.rs:13     try!(f.write_all(time_str));
                                    ^~~~~~~~
<std macros>:1:1: 6:48 note: in expansion of try!
src/main.rs:13:5: 13:33 note: expansion site
error: aborting due to previous error
Could not compile `simple-log`.

To learn more, run the command again with --verbose.

Ugh. So, I know that there are many types of strings in Rust¹, and it looks like I need a different one, here. Thing is, I don't know how to do this, so I'll have to do some searching.

I remember seeing a section in the Rust book specifically about strings. Looking into it, it says that a can be converted from String to &str with an ampersand (&). I don't think this is quite right, because it looks like it's expecting a [u8] and not a &str ² Lemmie try that:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;

fn log_time(filename: &'static str) -> io::Result<()> {

    let local: DateTime<Local> = Local::now();
    let time_str = local.format("%Y").to_string();
    let mut f = try!(File::create(filename));
    try!(f.write_all(&time_str));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
src/main.rs:13:22: 13:31 error: mismatched types:
 expected `&[u8]`,
    found `&collections::string::String`
(expected slice,
    found struct `collections::string::String`) [E0308]
src/main.rs:13     try!(f.write_all(&time_str));
                                    ^~~~~~~~~
<std macros>:1:1: 6:48 note: in expansion of try!
src/main.rs:13:5: 13:34 note: expansion site
error: aborting due to previous error
Could not compile `simple-log`.

To learn more, run the command again with --verbose.

Well. Apparently, adding the ampersand just converted a String to an &String. That seems to directly contradict what the Rust book is saying, but I also probably don't know what is going on.

…And I just reached the end of the chapter on strings. Harumph. As far as I can tell, there isn't anything in here.

I walked away from this for a while (because, you know, life), and while I was gone it hit me. All this time, I have been reading u8 as a short form of UTF-8, but now that I think about it, it almost certainly actually means "unsigned 8-bit integer". And, I remember seeing an as_bytes method, so let me try that instead:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;

fn log_time(filename: &'static str) -> io::Result<()> {
    let local: DateTime<Local> = Local::now();
    let bytes = local.format("%Y").to_string().as_bytes();
    let mut f = try!(File::create(filename));
    try!(f.write_all(bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
main.rs:10:17: 10:47 error: borrowed value does not live long enough
main.rs:10     let bytes = local.format("%Y").to_string().as_bytes();
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.rs:10:59: 14:2 note: reference must be valid for the block suffix following statement 1 at 10:
58...
main.rs:10     let bytes = local.format("%Y").to_string().as_bytes();
main.rs:11     let mut f = try!(File::create(filename));
main.rs:12     try!(f.write_all(bytes));
main.rs:13     Ok(())
main.rs:14 }
main.rs:10:5: 10:59 note: ...but borrowed value is only valid for the statement at 10:4
main.rs:10     let bytes = local.format("%Y").to_string().as_bytes();
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.rs:10:5: 10:59 help: consider using a `let` binding to increase its lifetime
main.rs:10     let bytes = local.format("%Y").to_string().as_bytes();
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Could not compile `simple-log`.

To learn more, run the command again with --verbose.

Well, I hope this is progress. Does this error mean I fixed something, and there was something else wrong that was obscuring this problem? Did I introduce a whole new problem?

The strange thing about this error message is that it seems to be talking about error messages on the same line. I don't really understand most of it, but I'm thinking it is saying that I need to add a let in the middle of the sequence of method calls. Lets try:

fn log_time(filename: &'static str) -> io::Result<()> {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%Y").to_string();
    let bytes = formatted.as_bytes();
    let mut f = try!(File::create(filename));
    try!(f.write_all(bytes));
    Ok(())
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
     Running `target/debug/simple-log`
File created!
$ cat log.txt
2015$

Great! All the pieces are here. Before I move on, I want to reflect that I find this a little disappointing. It seems like Rust should be able to infer the correct behavior in the previous snippet without my guidance.

Testing the script:

$ ls
Cargo.lock      Cargo.toml      log.txt         src             target
$ rm log.txt
$ cargo run
     Running `target/debug/simple-log`
File created!
$ cat log.txt
2015$ cargo run
     Running `target/debug/simple-log`
File created!
$ cat log.txt
2015$

A few problems:

1. No newline. This is really gross.
2. The format needs some work.
3. It appears that the old value is being erased by the new value.

Let's verify #3 by fixing the format. If the time changes between runs, then we will know that's what is happening.

The format method of DateTime uses the standard strftime formatting conventions. Ideally, I would like times to be something like:

Sat, Jun 6 2015 05:32:00 PM
Sun, Jun 7 2015 08:35:00 AM

…etc. This should be readable enough for me to use. After reading the documentation for a while, I've come up with this:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;

fn log_time(filename: &'static str) -> io::Result<()> {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    let bytes = formatted.as_bytes();
    let mut f = try!(File::create(filename));
    try!(f.write_all(bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

Testing it:

$ rm log.txt
$ cargo run
     Running `target/debug/simple-log`
File created!
$ cat log.txt
Sun, Jun 07 2015 06:37:21 PM
$ sleep 5; cargo run
     Running `target/debug/simple-log`
File created!
$ cat log.txt
Sun, Jun 07 2015 06:37:41 PM

So, clearly the program is overwriting the log entries, which tbqh is what I expect, as I remember the documentation for File::create specifying that this is what would happen. So, I need to look at the documentation for manipulating files again.

I did some searching around, and basically finding the answer to this isn't trivial. After a while I found the documentation for std::path::Path, which has an exists method.

At this point, the interactions between types in my application is becoming increasingly hard to manage. I feel nervous, so I will commit before continuing.

I want to pull the time entry string generation out of the log_time function because it seems like the entry formatting/creation is distinct from the file manipulation code. So, trying this:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;


fn log_time_entry() -> String {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    formatted
}

fn log_time(filename: &'static str) -> io::Result<()> {
    let bytes = log_time_entry().as_bytes();
    let mut f = try!(File::create(filename));
    try!(f.write_all(bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
src/main.rs:16:17: 16:33 error: borrowed value does not live long enough
src/main.rs:16     let bytes = log_time_entry().as_bytes();
                               ^~~~~~~~~~~~~~~~
src/main.rs:16:45: 20:2 note: reference must be valid for the block suffix following statement 0 at
 16:44...
src/main.rs:16     let bytes = log_time_entry().as_bytes();
src/main.rs:17     let mut f = try!(File::create(filename));
src/main.rs:18     try!(f.write_all(bytes));
src/main.rs:19     Ok(())
src/main.rs:20 }
src/main.rs:16:5: 16:45 note: ...but borrowed value is only valid for the statement at 16:4
src/main.rs:16     let bytes = log_time_entry().as_bytes();
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:16:5: 16:45 help: consider using a `let` binding to increase its lifetime
src/main.rs:16     let bytes = log_time_entry().as_bytes();
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Could not compile `simple-log`.

To learn more, run the command again with --verbose.

So, this looks just like the problem I had earlier. Does borrowing/ownership require that a function have an explicit reference to resources? That seems a little strange. I will try to fix it again:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;

fn formatted_time_entry() -> String {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    formatted
}

fn log_time(filename: &'static str) -> io::Result<()> {
    let entry = formatted_time_entry();
    let bytes = entry.as_bytes();

    let mut f = try!(File::create(filename));
    try!(f.write_all(bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
     Running `target/debug/simple-log`
File created!

So, adding an explicit reference seems to be the solution. Whatever. It is an easy rule to learn and follow.

Next I want to extract the file manipulation code to its own function:

extern crate chrono;

use std::io::prelude::*;
use std::fs::File;
use std::io;
use chrono::*;

fn formatted_time_entry() -> String {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    formatted
}

fn record_entry_in_log(filename: &str, bytes: &[u8]) -> io::Result<()> {
    let mut f = try!(File::create(filename));
    try!(f.write_all(bytes));
    Ok(())
}

fn log_time(filename: &'static str) -> io::Result<()> {
    let entry = formatted_time_entry();
    let bytes = entry.as_bytes();

    try!(record_entry_in_log(filename, &bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

And this works. I made some initial errors, but they were quickly corrected and was all stuff that has been covered here before.

Looking into the documentation for std::fs::File, I notice a reference to std::fs::OpenOptions, which is exactly what I have been looking for. It would definitely be better than using std::path.

My first attempt:

extern crate chrono;

use std::io::prelude::*;
use std::fs::{File,OpenOptions};
use std::io;
use chrono::{DateTime,Local};


fn formatted_time_entry() -> String {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    formatted
}

fn record_entry_in_log(filename: &str, bytes: &[u8]) -> io::Result<()> {
    let mut file = try!(OpenOptions::new().
                        append(true).
                        create(true).
                        open(filename));
    try!(file.write_all(bytes));
    Ok(())
}

fn log_time(filename: &'static str) -> io::Result<()> {
    let entry = formatted_time_entry();
    let bytes = entry.as_bytes();

    try!(record_entry_in_log(filename, &bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(..) => println!("Error: could not create file.")
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
src/main.rs:4:15: 4:19 warning: unused import, #[warn(unused_imports)] on by default
src/main.rs:4 use std::fs::{File,OpenOptions};
                            ^~~~
     Running `target/debug/simple-log`
Error: could not create file.

Interesting. I see that it is actually creating the file, after which I notice this is the message I've hard-coded into main. Ugh; I think this will work:

extern crate chrono;

use std::io::prelude::*;
use std::fs::{File,OpenOptions};
use std::io;
use chrono::{DateTime,Local};


fn formatted_time_entry() -> String {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    formatted
}

fn record_entry_in_log(filename: &str, bytes: &[u8]) -> io::Result<()> {
    let mut file = try!(OpenOptions::new().
                        append(true).
                        create(true).
                        open(filename));
    try!(file.write_all(bytes));
    Ok(())
}

fn log_time(filename: &'static str) -> io::Result<()> {
    let entry = formatted_time_entry();
    let bytes = entry.as_bytes();

    try!(record_entry_in_log(filename, &bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(e) => println!("Error: {}", e)
    }
}

=>

$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
src/main.rs:4:15: 4:19 warning: unused import, #[warn(unused_imports)] on by default
src/main.rs:4 use std::fs::{File,OpenOptions};
                            ^~~~
     Running `target/debug/simple-log`
Error: Bad file descriptor (os error 9)

Weird. Searching for this "bad file descriptor" error message seems to indicate that this happens when a file descriptor is used has been closed. what happens if I comment out the file.write_all call?

$ rm log.txt
$ cargo run
   Compiling simple-log v0.1.0 (file:///Users/joel/Projects/simple-log)
src/main.rs:3:5: 3:25 warning: unused import, #[warn(unused_imports)] on by default
src/main.rs:3 use std::io::prelude::*;
                  ^~~~~~~~~~~~~~~~~~~~
src/main.rs:4:15: 4:19 warning: unused import, #[warn(unused_imports)] on by default
src/main.rs:4 use std::fs::{File,OpenOptions};
                            ^~~~
src/main.rs:15:40: 15:45 warning: unused variable: `bytes`, #[warn(unused_variables)] on by default
src/main.rs:15 fn record_entry_in_log(filename: &str, bytes: &[u8]) -> io::Result<()> {
                                                      ^~~~~
src/main.rs:16:9: 16:17 warning: unused variable: `file`, #[warn(unused_variables)] on by default
src/main.rs:16     let mut file = try!(OpenOptions::new().
                       ^~~~~~~~
src/main.rs:16:9: 16:17 warning: variable does not need to be mutable, #[warn(unused_mut)] on by de
fault
src/main.rs:16     let mut file = try!(OpenOptions::new().
                       ^~~~~~~~
     Running `target/debug/simple-log`
File created!
$ ls
Cargo.lock      Cargo.toml      log.txt         src             target

Unsurprisingly, there are a bunch unused messages, but aside from that the file is indeed created.

It seems a little silly, but I tried adding .write(true) to the chain of functions, and it worked. It seems like .append(true) should imply .write(true), but I guess it doesn't.

And with that, its working! The final version:

extern crate chrono;

use std::io::prelude::*;
use std::fs::{File,OpenOptions};
use std::io;
use chrono::{DateTime,Local};


fn formatted_time_entry() -> String {
    let local: DateTime<Local> = Local::now();
    let formatted = local.format("%a, %b %d %Y %I:%M:%S %p\n").to_string();
    formatted
}

fn record_entry_in_log(filename: &str, bytes: &[u8]) -> io::Result<()> {
    let mut file = try!(OpenOptions::new().
                        append(true).
                        write(true).
                        create(true).
                        open(filename));
    try!(file.write_all(bytes));
    Ok(())
}

fn log_time(filename: &'static str) -> io::Result<()> {
    let entry = formatted_time_entry();
    let bytes = entry.as_bytes();

    try!(record_entry_in_log(filename, &bytes));
    Ok(())
}

fn main() {
    match log_time("log.txt") {
        Ok(..) => println!("File created!"),
        Err(e) => println!("Error: {}", e)
    }
}

=>

$ ls
Cargo.lock      Cargo.toml      src             target
$ cargo run
     Running `target/debug/simple-log`
File created!
$ cargo run
     Running `target/debug/simple-log`
File created!
$ cat log.txt
Sun, Jun 07 2015 10:40:01 PM
Sun, Jun 07 2015 10:40:05 PM