Monday, December 20, 2021

Cancelling IAsyncEnumerable in C#

IAsyncEnumerable combines the power of IEnumerable (which lets us "foreach" through items) with the joys of async code (that we can "await"). Like many async methods, we can pass in a CancellationToken to short-circuit the process. But because of the way that we use IAsyncEnumerable, passing in a cancellation token is a bit different than we are used to.

Short Version:
The "WithCancellation" extension method on IAsyncEnumerable lets us pass in a cancellation token.
Let's take a quick look at how to use IAsyncEnumerable, and then we'll look at cancellation.

Using IAsyncEnumerable

I started using IAsyncEnumerable when I exploring channels in C#. Here's an example method that uses "await foreach" with an IAsyncEnumerable (taken from the Program.cs file of this GitHub repository: https://github.com/jeremybytes/csharp-channels-presentation):
    private static async Task ShowData(ChannelReader<Person> reader)
    {
        await foreach(Person person in reader.ReadAllAsync())
        {
            DisplayPerson(person);
        };
    }
The ChannelReader type has a "ReadAllAsync" method that returns an IAsyncEnumerable. What this means is that someone else can be writing to the channel while we read from it. The IAsyncEnumerable means that if there is not another item ready to read, we can wait for it. So, we can pull things off the channel as they are added, even if there are delays between each item getting added.

Waiting for items can cause a problem, and that's where the "async" part comes in. Since this is asynchronous, we can "await" each item. This gives us the advantages that we are used to with "await", meaning that we are not blocking processes and threads while we wait.

By using "await foreach" on an IAsyncEnumerable, we get this combined functionality: getting the next item in the enumeration and waiting asynchronously if the next item isn't ready yet.

For more information on channels, check the repository for list of articles and a recorded presentation: https://github.com/jeremybytes/csharp-channels-presentation.

The IAsyncEnumerable Interface

Things got a little more interesting when I was building my own classes that implement IAsyncEnumerable. The interface itself only has one method: GetAsyncEnumerator. An implementation could look something like this (we'll call this our "Processor" since it will process some custom data for us):
    public IAsyncEnumerator<int> GetAsyncEnumerator(
        CancellationToken cancellationToken = default)
    {
        while (...)
        {
            // interesting async stuff here
            yield return nextValue;
        }
    }
Like with IEnumerable, we can use "yield return" to return the next value from the enumeration. Unlike IEnumerable, we can also put asynchronous code in here, whether it's an asynchronous service call or waiting for a record to finish a complex process. (That goes in the "// interesting async stuff here" section; we won't look at that today.)

CancellationToken Parameter

An interesting thing about the GetAsyncEnumerable method is that it has a CancellationToken parameter. Notice that the the CancellationToken has a "default" value set. This means that the cancellation token is optional. If we do not pass in a token, the code will still work.

If we wanted to check the cancellation token in the code. This could look something like this:
    public async IAsyncEnumerator<int> GetAsyncEnumerator(
        CancellationToken cancellationToken = default)
    {
        while (...)
        {
            cancellationToken.ThrowIfCancellationRequested();
            // interesting stuff here
            yield return nextValue;
        }
    }
Each time through the "while" loop, the cancellation token will be checked. If cancellation is requested, then this will throw an OperationCanceledException.

Passing a Cancellation Token

The next part is where things get interesting. How do we actually pass a cancellation token to the IAsyncEnumerable? If we are using the "Processor" that we created above. That call could look something like this.
    await foreach (int currentItem in processor)
    {
        DisplayItem(currentItem);
    }
When we use "foreach", we do not call the "GetAsyncEnumerable" directly. That also means that we cannot pass in a cancellation token directly.

But there is an extension method available on IAsyncEnumerable that helps us out: WithCancellation.

Here's that same foreach loop with a cancellation token passed in:
    await foreach (int currentItem in processor.WithCancellation(tokenSource.Token))
    {
        DisplayItem(currentItem);
    }
This assumes that we have a CancellationTokenSource (called "tokenSource") elsewhere in our code.

For more information on Cancellation, you can refer to Task and Await: Basic Cancellation. The article was written a while back. Updated code samples (.NET 6) are available here: https://github.com/jeremybytes/using-task-dotnet6.

ConfigureAwait

As a side note, there is another extension method on IAsyncEnumerable: ConfigureAwait. This lets us use "ConfigureAwait(false)" in areas that we need it. ConfigureAwait isn't needed in the ASP.NET world anymore, but it can still be useful if we are doing desktop or other types of programming.

Wrap Up

IAsyncEnumerable gives us some pretty interesting abilities. I've been exploring it quite a bit lately. In some code comparisons, I was able to move async code into a library that made using it quite a bit easier. Once that sample code is ready, I'll be sharing it on GitHub.

Until then, keep exploring. Sometimes the answers are not obvious. It's okay if it takes some time to figure things out.

Happy Coding!

Monday, November 22, 2021

Running a .NET 6 Service on a Specific Port

I often have web services for test and demo purposes. For these services, I want to hard-code a localhost port so that I can minimize conflicts if I have multiple projects and services running at the same time. What I found is that the way I was doing this with .NET 5 does not work with .NET 6 services.

What I want is for my service to run at http://localhost:9874.

Short Version

Previously, I used "UseUrls" to set the location (this is the the Program.cs file of a project that uses a Startup class -- here's a link to a full sample on GitHub: Program.cs):
    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>()
                    .UseUrls("http://localhost:9874");
            });
The translation of this code into a top-level program version that we get with .NET 6 would look something like this (calling "UseUrls" on the WebHost of the WebApplication builder):
    var builder = WebApplication.CreateBuilder(args);
    builder.WebHost.UseUrls("http://localhost:9874");
    // Add services to the container.
    builder.Services.AddSingleton<IPeopleProvider, HardCodedPeopleProvider>();
However, THIS DOES NOT WORK.

Fortunately, I found an option that does work:
    var builder = WebApplication.CreateBuilder(args);
    builder.WebHost.ConfigureKestrel(options => options.ListenLocalhost(9874));
    // Add services to the container.
    builder.Services.AddSingleton<IPeopleProvider, HardCodedPeopleProvider>();
If you are curious about why this might be, feel free to read on.

Note: If you want to look at the completed code, you can check the ".NET 6 Services (4 approaches)" repository on GitHub: https://github.com/jeremybytes/dotnet6-services-4-approaches. Specifically, you can look at the "Program.cs" file in any of the projects (here's one from the first project: Program.cs).

Different Ways of Setting URLs

There are several different ways to specify a URL for a service. This can be done with code, configuration, environment variables, or with a command-line argument. I'm not going to go through all of the options, just the ones that I tried. There is a pretty good article that shows these on the Microsoft Docs site: Configuring endpoints for the ASP.NET Core Kestrel web server.

At the time of writing for this post, there is a piece that is incorrect about .NET 6. The article mentions that the default endpoints are http://localhost:5000 and https://localhost:5001. This is true of .NET 5, however, the default endpoints where moved (randomized) for .NET 6. So the ports used by default will vary by project. (We'll see why this may be the cause of our problems a bit later.)

Configuration
You can put the settings into configuration (appsettings.json). Here's an example of how to set the URL:
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:9874"
      }
    }
  }
With this configuration, the default location will be overridden by our desired location: http://localhost:9874.

Command-Line Arguments
Another way to specify the location is to use the "--urls" option on the command line. Here's an example of running the service and overriding the URL from the command line:
PS D:\controller-api> dotnet run --urls "http://localhost:9874"
Building...
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:9874
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: D:\controller-api\
The output shows the service running at http://localhost:9874.

Code with "UseUrls"
Another options is to use code to set the location. This is the approach that I want to use for my scenario.

As mentioned above, previously, I used the "UseUrls" extension method in the Program class with .NET 5.
    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>()
                    .UseUrls("http://localhost:9874");
            });
This code still works when using either .NET 5 or .NET 6 with a Startup class.

But the default template for .NET 6 web api projects now uses the top-level programs feature, so I wanted to try to incorporate it into that template.

I found that the "WebHost" on the "WebApplication" builder object lets us use the "UseUrls" method. It even shows up in code completion in both Visual Studio Code and Visual Studio 2022. So, I tried it. Here are the first few lines of a "Program.cs" file that uses the .NET 6 template with the UseUrls added:
    var builder = WebApplication.CreateBuilder(args);
    builder.WebHost.UseUrls("http://localhost:9874");
    // Add services to the container.
    builder.Services.AddSingleton<IPeopleProvider, HardCodedPeopleProvider>();
This looks really promising, and everything builds without errors. But it doesn't work.

A Clue
Why doesn't this work? I was struggling with this, and I was looking to see if maybe I missed a setting or if there's something new that I don't know about.

The answer came when I was pairing with Lynn Langit (she's awesome, check out her stuff on GitHub: Lynn Langit). In the article mentioned above, she noticed that there was a section called "Limitations" (direct link: "Limitations"). The clue was in the following sentence:
When both the Listen and UseUrls approaches are used simultaneously, the Listen endpoints override the UseUrls endpoints.
This made me suspect that there was a call to "Listen" that was overriding my values.

Well, two can play at this game. So I started looking at "Listen" options. There are a lot of different methods that take various types of parameters. After a bit of struggling with methods that didn't quite do what I wanted, I stumbled across the exact one that I needed: ListenLocalHost.

Code with ListenLocalHost
The ListenLocalHost method is part of the KestrelServerOptions that can be set pretty easily in code (once you know it exists). Here are the first few lines of the Program.cs file with the options set:
    var builder = WebApplication.CreateBuilder(args);
    builder.WebHost.ConfigureKestrel(options => options.ListenLocalhost(9874));
    // Add services to the container.
    builder.Services.AddSingleton<IPeopleProvider, HardCodedPeopleProvider>();
This uses the "ConfigureKestrel" method on the same WebHost property that is part of our builder. The parameter is a delegate that takes a KestrelServerOptions. Inside the lambda expression, we can call "ListenLocalHost(9874)" on the options. This will configure our service to run at http://localhost:9874.

And this works!

I was very happy to find a solution. But why do we have this problem?

Some Speculation
I have some suspicions on why "UseUrls" doesn't work, but I have not been able to confirm them. What I guess is happening is that the new "random port" functionality that is part of .NET 6 uses one of the "Listen" methods (this would be in a part of the code that we don't see). If this internal code uses "Listen", then that means that "UseUrls" would not work as mentioned in the Limitations section of the article mentioned above.

I may be able to confirm this by digging through IL code looking at what is generated by the compiler, but I haven't taken the time to do that yet.

At this point, I'm happy to have a solution that works for me, and I'm moving forward with .NET 6 migrations.

Wrap Up

With new versions of frameworks, there is often a "who moved my cheese" problem where things are no longer where they used to be or things work differently that they did before. I've run into several of those things with .NET 6. The default templates and top-level programs can be really useful in a lot of scenarios. I use them mainly for prototyping or fake services that I use for testing or demos.

There is a lot of magic that happens with the default templates and top-level programs. If it works for you, it's pretty great. But if you need to step outside of the box a little, it can get pretty frustrating. The documentation is just not out there yet. And when searching for issues, you often get answers to the problem in previous iterations of the framework.

I will keep moving forward on .NET 6 and trying to figure out how to get things working like I want them to. Another example is that debugging in Visual Studio 2022 gets a bit interesting when you change the ports in code. But I'll save that for another article.

Happy Coding!

Thursday, September 30, 2021

Coding Practice: Learning Rust with Fibonacci Numbers

In my exploration of Rust, I built an application that calculates Fibonacci numbers (this was a suggestion from the end of Chapter 3 of The Rust Programming Language by Steve Klabnik and Carol Nichols).

It helped me learn a bit more about the language and environment.
  • for loops
  • Statements vs. expressions
  • Function returns (expressions)
  • checked_add to prevent overflow
  • Option enum (returned from checked_add)
  • Pattern matching on Option
  • Result enum (to return error rather than panic)
  • .expect with Result
  • Pattern matching on Result
So let's walk through this project.

The code is available on GitHub: https://github.com/jeremybytes/fibonacci-rust, and branches are set up for each step along the way. We will only be looking at the "main.rs" file in each branch, so all of the links will be directly to this file.

Fibonacci Numbers

The task is to calculate the nth Fibonacci number. The Fibonacci sequence is made by adding the 2 previous number in the sequence. So the sequence starts: 1, 1, 2, 3, 5, 8, 13, 21, 34. The 7th Fibonacci number (13) is the sum of the previous 2 numbers (5 and 8).

For our application, we will create a function to generate the nth Fibonacci number based on an input parameter. We'll call this function with multiple values and output the results.

Step 1: Creating the Project

Branch: 01-creation
The first step is to create the project. We can do this by typing the following in a terminal:
    cargo new fib
This will create a new folder called "fib" along with the Rust project file, a "src" folder to hold the code, and a "main.rs" file (in src) which is where we will be putting our code.

The "main.rs" file has placeholder code:
    fn main() {
        println!("Hello, world!");
    }
But we can use "cargo run" to make sure that everything is working in our environment.
    C:\rustlang\fib> cargo run
       Compiling fib v0.1.0 (C:\rustlang\fib)
        Finished dev [unoptimized + debuginfo] target(s) in 0.63s
         Running `target\debug\fib.exe`
    Hello, world!
Going forward, I'll just show the application output (without the compiling and running output).

Step 2: Basic Fibonacci

Branch: 02-basic
Now that we have the shell, let's create a function to return a Fibonacci number. Here's the completed function:
    fn fib(n: u8) -> u64 {
        let mut prev: u64 = 0;
        let mut curr: u64 = 1;
        for _ in 1..n {
            let next = prev + curr;
            prev = curr;
            curr = next;
        }
        curr
    }
There are several interesting bits here. Let's walk through them.

Declaring a Function
Let's start with the function declaration:
    fn fib(n: u8) -> u64 {

    }
The "fn" denotes that this is a function. "fib" is the function name. "n: u8" declares a parameter called "n" that is an unsigned 8-bit integer. And the "u64" after the arrow declares that this function returns an unsigned 64-bit integer.

When declaring parameters and return values for functions, the types are required. Rust does use type inference in some places (as we'll see), but function declarations need to have explicit types.

Declaring Variables
Next, we have some variables declared and assigned:
    let mut prev: u64 = 0;
    let mut curr: u64 = 1;
"let" declares a variable.

By default, variables are immutable. This means that once we assign a value, we cannot change it. For these variables, we use "mut" to denote that they are mutable. So we will be able to change the values later.

The variable names are "prev" and "curr". These will hold the "previous number" and the "current number" in the sequence.

The ": u64" declares these as unsigned 64-bit integer values. Fibonacci numbers tend to overflow very quickly, so I used a fairly large integer type.

Finally, we assign initial values of 0 and 1, respectively.

Looping with "for"
There are several ways to handle the loop required to calculate the Fibonacci number. I opted for a "for" loop:
    for _ in 1..n {

    }
"1..n" represents a range from 1 to the value of the incoming function argument. So if the argument is "3", this represents the range: 1, 2, 3.

The "for" statement will loop once for each value in the range. In this case the "_" denotes that we are not using the actual range value inside the loop. All we really need here is to run the loop 3 times. All of the calculation is done inside the loop itself.

Implicit Typing
Inside the "for" loop we do our calculations:
    let next = prev + curr;
    prev = curr;
    curr = next;
This creates a new variable called "next" inside the loop and assigns it the sum of "prev" and "curr". A couple of things to note. First, this variable is immutable (so we do not have the "mut" keyword). The value is assigned here and then it is not changed. Second, the "next" variable is implicitly typed. Instead of having a type declaration, it is set based on what is assigned to it. Since we are assigning the sum of two u64 values, "next" will also be a u64.

The next two lines update the "prev" and "curr" values. We needed to mark them as mutable when we declared them so that we could update them here.

This is a fairly naïve way of calculating Fibonacci numbers. If you'd like to see more details on how the calculation works, you can take a look at this article: TDDing into a Fibonacci Sequence with C#.

Statements vs. Expressions
The last line of the function is a bit interesting:
    curr
This returns the current value ("curr") from the function.

Rust does not use a "return" keyword to return a value. Instead, the last expression in a function is what is returned. (As a side note, this is similar to how F# works.)

What's the difference between a statement and an expression? A statement does some type of work; an expression returns a value.

In Rust, a statement ends with a semi-colon, and an expression does not end with a semi-colon. To make things more interesting, these are often combined.

Let's take a look back at a line of code:
    let next = prev + curr;
Overall, this is a statement: it declares and assigns a value to a variable called "next". And it ends with a semi-colon.
    prev + curr
"prev + curr" is an expression that returns the result of adding 2 values. So we really have a statement that includes an expression. (We can technically break this down further, but we won't do that here.)

So, let's get back to the return value of the function. The "fib" function returns a u64 value. The last expression in the function is:
    curr
It is important to note that this line does not end with a semi-colon. Because of this, the value of the "curr" variable (which is a u64) is returned for this function.

Because of my coding history, I'm used to putting semi-colons at the end of lines. So I'm sure that I'll mess this up many times before I get used to it. If you get an error that says a function is returning "()" instead of a particular type, it probably means that there's a semi-colon at the end of the expression you meant to return.

Here's the full function:
    fn fib(n: u8) -> u64 {
        let mut prev: u64 = 0;
        let mut curr: u64 = 1;
        for _ in 1..n {
            let next = prev + curr;
            prev = curr;
            curr = next;
        }
        curr
    }
Using the "fib" Function
Now that we have a function that returns a Fibonacci number, it's time to update the "main" function to use it.
    fn main() {
        println!("Fibonacci 1st = {}", fib(1));
        println!("Fibonacci 2nd = {}", fib(2));
        println!("Fibonacci 3rd = {}", fib(3));
        println!("Fibonacci 4th = {}", fib(4));
        println!("Fibonacci 5th = {}", fib(5));
    }
This uses the "println!" macro to output a string to the standard output. On each line, the set of curly braces represents a placeholder in the string. So in the first statement, the curly braces will be replaced by the value that comes back from calling "fib(1)".

So let's run and check the output:
    Fibonacci 1st = 1
    Fibonacci 2nd = 1
    Fibonacci 3rd = 2
    Fibonacci 4th = 3
    Fibonacci 5th = 5
It works!

Well, it mostly works. We'll see a shortcoming in a bit.

Step 3: Testing More Values

Branch: 03-mainloop
Before looking at where we have a problem in the "fib" function, let's make it easier to test for different values. For this, we'll add an array of numbers to test, and then loop through them.

Here's an updated "main" function:
    fn main() {
        let nths = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

        for nth in nths {
            println!("Fibonacci {} = {}", nth, fib(nth));
        }
    }
Creating an Array
The first line sets up an array called "nths" and initializes it with the values of 1 through 10. I use this name because our original task was to calculate the "nth" Fibonacci number. This is a collection of all the ones we want to calculate.

We're using type inference to let the compiler pick the type for "nths". In this case, it determines that it is an array with 10 elements of type u8. It decides on u8 because the values are used as arguments for the "fib" function, and that takes a u8.

As an interesting note, if you comment out the "println!" statement, the "nths" variable is an array with 10 elements of type i32 (a signed 32-bit integer). This is the default integer type.

Type inference works as long as it can be determined at compile time. If it cannot be determined at compile time, then an explicit type needs to be added.

Another "for" Loop
We use a "for" loop to go through the array. Instead of discarding the value from the "for" loop (like we did above), we capture it in the "nth" variable.

Inside the loop, we have a "println!" with 2 placeholders, one for the loop value and one for the result of the "fib" function.

Here's what that output looks like:
    Fibonacci 1 = 1
    Fibonacci 2 = 1
    Fibonacci 3 = 2
    Fibonacci 4 = 3
    Fibonacci 5 = 5
    Fibonacci 6 = 8
    Fibonacci 7 = 13
    Fibonacci 8 = 21
    Fibonacci 9 = 34
    Fibonacci 10 = 55
And now we can more easily test values by adding to the array.

Overflow!
As I noted at the beginning, Fibonacci sequences tend to overflow pretty quickly (they increase the value by half for each item). We can see this by adding a "100" to our array.
    let nths = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100];
Here's the output when we run with these values:
    Fibonacci 1 = 1
    Fibonacci 2 = 1
    Fibonacci 3 = 2
    Fibonacci 4 = 3
    Fibonacci 5 = 5
    Fibonacci 6 = 8
    Fibonacci 7 = 13
    Fibonacci 8 = 21
    Fibonacci 9 = 34
    Fibonacci 10 = 55
    thread 'main' panicked at 'attempt to add with overflow', src\main.rs:13:20
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    error: process didn't exit successfully: `target\debug\fib.exe` (exit code: 101)
This creates a "panic" in our application, and it exits with an error.

With a little experimentation, we will find that the 93rd Fibonacci number is fine, but the 94th will overflow the u64 value.

Checking for Overflow

Branch: 04-checkedadd
Now we can work on fixing the overflow. Our problem is with this line:
    let next = prev + curr;
If "prev" and "curr" are near the upper limits of the u64 range, then adding them together will go past that upper limit.

Some other languages will "wrap" the value (starting over again at 0). Rust will generate an error instead (in the form of a panic). If you do want to wrap the value, Rust does offer a "wrapped_add" function that does just that.

But we do not want to wrap, we would like to catch the error and give our users a better experience.

checked_add
Instead of using the default "+" operator, we can use the "checked_add" function. Here is that code:
    let result = prev.checked_add(curr);
"checked_add" does not panic if the value overflows. This is because it uses the Option enum.

Option Enum
The Option enum lets us return either a valid value or no value. "Some<T>" is used if there is a valid value, otherwise "None" is used.

For example, let's say that "prev" is 1 and "curr" is 2. The "result" would be "Some(3)".

If "prev" and "curr" are big enough to cause an overflow when added together, then "result" would be "None".

Pattern Matching
The great thing about having an Option as a return type is that we can use pattern matching with it.

Here is the inside of the updated "for" loop:
    let result = prev.checked_add(curr);
    match result {
        Some(next) => {
            prev = curr;
            curr = next;
        }
        None => {
            curr = 0;
            break;
        }
    }
The "match" keyword sets up the pattern matching for us.

The first "arm" has "Some(next)" as the pattern. The "next" part lets us assign a name to the value that we can use inside the block. In this case, "next" will hold the same value that it did in the earlier version ("prev" + "curr"), so inside the block, we can assign the "prev" and "curr" values like we did before.

The second "arm" has "None" as the pattern. This will be used if there is an overflow. If there is an overflow, then we set the "curr" variable to 0 and then break out of the "for" loop.

Here is the updated "fib" function:
    fn fib(n: u8) -> u64 {
        let mut prev: u64 = 0;
       let mut curr: u64 = 1;
        for _ in 1..n {
            let result = prev.checked_add(curr);
            match result {
                Some(next) => {
                    prev = curr;
                    curr = next;
                }
                None => {
                    curr = 0;
                    break;
                }
            }
        }
        curr
    }
Here's an updated array to test valid values and overflow values:
    let nths = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 90, 91, 92, 93, 94, 95, 96];
And here's the output:
    Fibonacci 1 = 1
    Fibonacci 2 = 1
    Fibonacci 3 = 2
    Fibonacci 4 = 3
    Fibonacci 5 = 5
    Fibonacci 6 = 8
    Fibonacci 7 = 13
    Fibonacci 8 = 21
    Fibonacci 9 = 34
    Fibonacci 10 = 55
    Fibonacci 90 = 2880067194370816120
    Fibonacci 91 = 4660046610375530309
    Fibonacci 92 = 7540113804746346429
    Fibonacci 93 = 12200160415121876738
    Fibonacci 94 = 0
    Fibonacci 95 = 0
    Fibonacci 96 = 0
Setting "curr" to 0 is not a great way to handle our error state. For now, it is fine because it gets rid of the panic, and our application keeps running.

Next up, we'll work on getting a real error that we can handle.

Using the Result Enum

Branch: 05-result
In the last article, I wrote a bit about my first impressions of error handling in Rust: Initial Impressions of Rust. A big part of that involves the Result enum.

Similar to the Option enum, the Result enum represents exclusive states. For Result, the options are "Ok" and "Err". Each of these can have their own type.

To update our "fib" function to return a Result, we'll need to make 2 updates.

Updating the Function Declaration
First, we'll need to update the signature of the function to return a Result. Here's the new signature:
    fn fib(n: u8) -> Result<u64, &'static str> {

    }
The "Result" enum has 2 generic type parameters. The first represents the type for the "Ok" value; the second represents the type for the "Err".

In this case, the "Ok" will be a u64.

The "Err" is a bit more confusing. We want to return a string, but if we try to use just "str", we get an error that the compiler cannot determine the size at compile time. And as we've seen, Rust needs to be able to determine things at compile time.

Instead of using "str", we can use "&str" to use the address of a string (Rust does use pointers; we won't talk too much about them today). The address is a fixed size, so that gets rid of the previous error. But we get a new error that there is a "missing lifetime specifier".

UPDATE Technical Note: '&str' is a string slice. This allows the Err to borrow the value of the string without taking ownership of it. (I've learned more about ownership since I wrote this article. It's pretty interesting.)

The good news is that the error also gives you a hint to consider using the "static" lifetime with an example.

I'm using Visual Studio Code with the Rust extension, so I get these errors and hints in the editor. But these same messages show up if you build using "cargo build".

Returning a Result
Now that we've updated the function signature, we need to actually return a Result. We can do this with some pattern matching.

Replace the previous expression at the end of the function:
    curr
with a "match":
    match curr == 0 {
        false => Ok(curr),
        true => Err("Calculation overflow")
    }
This looks at the value of the "curr" variable and compares it to 0. (Again, this isn't the best way to handle this, but we'll fix it a bit later).

If "curr" is not 0 (meaning there is a valid value), then we hit the "false" arm and return an "Ok" with the value.

If "curr" is 0 (meaning there was an overflow), then we hit the "true" arm and return an "Err" with an appropriate message.

Here's the updated "fib" function:
    fn fib(n: u8) -> Result<u64, &'static str> {
        let mut prev: u64 = 0;
        let mut curr: u64 = 1;
        for _ in 1..n {
            let result = prev.checked_add(curr);
            match result {
                Some(next) => {
                    prev = curr;
                    curr = next;
                }
                None => {
                    curr = 0;
                    break;
                }
            }
        }
        match curr == 0 {
            false => Ok(curr),
            true => Err("Calculation overflow")
        }
    }
Side Note: In thinking about this later, I could have done the pattern matching more elegantly. I started with an if/else block (which is why I'm matching on a boolean value). But we could also write the pattern matching to use the "curr" value directly:
    match curr {
        0 => Err("Calculation overflow"),
        _ => Ok(curr),
    }
This is more direct (but not really less confusing since 0 is a magic number here). Now the match is on the "curr" value itself. If the value is "0", then we return the Err. For the second arm, the underscore represents a catch-all. So if the value is anything other than "0", we return "Ok". Notice that I did have to reverse the order of the arms. The first match wins with pattern matching, so the default case needs to be at the end.

Both of these matches produce the same results. We won't worry about them too much because we'll be replacing this entirely in just a bit.

But since we changed the return type, our calling code needs to be updated.

Using ".expect"
One way that we can deal with the Result enum is to use the "expect()" function.

Here is the updated code from the "main" function:
    println!("Fibonacci {} = {}", nth, fib(nth).expect("Fibonacci calculation failed"));
After the call to "fib(nth)", we add an ".expect()" call and pass in a message.

"expect()" works on a Result enum. If the Result is "Ok", then it pulls out the value and returns it. So if there is no overflow, then the expected Fibonacci number is used for the placeholder.

But if Result is "Err", then "expect" will panic. That's not exactly what we want here, but this gets us one step closer.

With the "expect" in place, here is our output:
    Fibonacci 1 = 1
    Fibonacci 2 = 1
    Fibonacci 3 = 2
    Fibonacci 4 = 3
    Fibonacci 5 = 5
    Fibonacci 6 = 8
    Fibonacci 7 = 13
    Fibonacci 8 = 21
    Fibonacci 9 = 34
    Fibonacci 10 = 55
    Fibonacci 90 = 2880067194370816120
    Fibonacci 91 = 4660046610375530309
    Fibonacci 92 = 7540113804746346429
    Fibonacci 93 = 12200160415121876738
    thread 'main' panicked at 'Fibonacci calculation failed: "Calculation overflow"', src    \main.rs:5:53
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    error: process didn't exit successfully: `target\debug\fib.exe` (exit code: 101)
Because we get a panic when trying to fetch #94, the application halts and does not process the rest of the values (95 and 96).

If we look at the output, we see both of the messages that we added. "Fibonacci calculation failed" is what we put into the "expect", and "Calculation overflow" is what we put into the "Err" Result.

But I'd like to get rid of the panic. And we can do that with more pattern matching.

Matching on Result

Branch: 06-matchresult
Just like we used pattern matching with Option in the "fib" function, we can use pattern matching with Result in the "main" function.

Pattern Matching
Here's the updated "for" loop from the "main" function:
    for nth in nths {
        match fib(nth) {
            Ok(result) => println!("Fibonacci {} = {}", nth, result),
            Err(e) => println!("Error at Fibonacci {}: {}", nth, e),
        }
    }
Inside the "for" loop, we match on the result of "fib(nth)".

If the Result is "Ok", then we use "println!" with the same string that we had before.

If the Result is "Err", then we output an error message.

Adding an Overflow Flag
The last thing I want to do is get rid of the "curr = 0" that denotes an overflow. Even though this works, it's a bit unclear. (And it can cause problems since some implementations of Fibonacci consider "0" to be a valid value.)

For this, we'll add a new variable called "overflow" to the "fib" function. Here's the completed function with "overflow" in place:
    fn fib(n: u8) -> Result<u64, &'static str> {
        let mut prev: u64 = 0;
        let mut curr: u64 = 1;
        let mut overflow = false;
        for _ in 1..n {
            let result = prev.checked_add(curr);
            match result {
                Some(next) => {
                    prev = curr;
                    curr = next;
                }
                None => {
                    overflow = true;
                    break;
                }
            }
        }
        match overflow {
            false => Ok(curr),
            true => Err("Calculation overflow")
        }
  }
A new mutable "overflow" variable is created and set to "false". Then if there is an overflow, it is set to "true". Finally, "overflow" is used in the final pattern matching to determine whether to return "Ok" or "Err".

Final Output
With these changes in place, here is our final output:
    Fibonacci 1 = 1
    Fibonacci 2 = 1
    Fibonacci 3 = 2
    Fibonacci 4 = 3
    Fibonacci 5 = 5
    Fibonacci 6 = 8
    Fibonacci 7 = 13
    Fibonacci 8 = 21
    Fibonacci 9 = 34
    Fibonacci 10 = 55
    Fibonacci 90 = 2880067194370816120
    Fibonacci 91 = 4660046610375530309
    Fibonacci 92 = 7540113804746346429
    Fibonacci 93 = 12200160415121876738
    Error at Fibonacci 94: Calculation overflow
    Error at Fibonacci 95: Calculation overflow
    Error at Fibonacci 96: Calculation overflow
This version no longer panics if there is an overflow. If we do have an overflow, it gives us an error message. And all of the values will get calculated, even if an overflow occurs in the middle.

Wrap Up

Calculating Fibonacci numbers is not a very complex task. But in this walkthrough, we got to use several features of Rust and understand a bit more about how the language works.
  • for loops
  • Statements vs. expressions
  • Function returns (expressions)
  • checked_add to prevent overflow
  • Option enum (returned from checked_add)
  • Pattern matching on Option
  • Result enum (to return error rather than panic)
  • .expect with Result
  • Pattern matching on Result
Quite honestly, I didn't expect to get this much out of this exercise. I've calculate Fibonacci sequences lots of times before. I was surprised about what I learned.

It's okay to do "simple" exercises. And it's okay to be surprised when they don't go quite as you expected.

Happy Coding!

Sunday, September 26, 2021

Initial Impressions of Rust

I experimented a little with Rust this past week. I haven't gone very deep at this point, but there are a few things I found interesting. To point some of these out, I'm using a number guessing game (details on the sample and where I got it are at the end of the article). The code can be viewed here: https://github.com/jeremybytes/guessing-game-rust.

Disclaimer: these are pretty raw impressions. My opinions are subject to change as I dig in further.

The code is for a "guess the number" game. Here is a completed game (including both input and output):

    Guess the number!
    Please input your guess.
    23
    You guessed: 23
    Too small!
    Please input your guess.
    78
    You guessed: 78
    Too big!
    Please input your guess.
    45
    You guessed: 45
    Too small!
    Please input your guess.
    66
    You guessed: 66
    Too small!
    Please input your guess.
    71
    You guessed: 71
    You win!
So let's get on to some of the language and environment features that I find interesting.

Automatic Git

Cargo is Rust's build system and package manager. The following command will create a new project:

cargo new guessing-game

Part of the project creation is a new Git repository (and a .gitignore file). So there are no excuses about not having source control.

Pattern Matching

Here's a pattern matching sample (from the main.rs file in the repository mentioned above):

    match guess.cmp(&secret_number) {
        Ordering::Less => println!("Too small!"),
        Ordering::Greater => println!("Too big!"),
        Ordering::Equal => {
            println!("You win!");
            break;
        },
    }

In this block "guess.cmp(&secret_number)" compares the number the user guessed to the actual number. "cmp" returns an Ordering enumeration. So "Ordering::Less" denotes the "less than" value of the enumeration.

Each "arm" of the match expression has the desired functionality: either printing "Too small!", "Too big!", or "You win!".

As a couple of side notes: the "println!" (with an exclamation point) is a macro that prints to the standard output. I haven't looked into macros yet, so that will be interesting to see how this expands. Also the "break" in the last arm breaks out of the game loop. We won't look at looping in this article.

Error Handling

I like looking at different ways of approaching error handling. Earlier this year, I wrote about the approach that Go takes: Go (golang) Error Handling - A Different Philosophy. Go differs quite a bit from C#. While C# uses exceptions and try/catch blocks, Go uses strings - it's up to the programmer to specifically check for those errors.

Rust takes an approach that is somewhere in between by using a Result enumeration. It is common to return a Result which will provide an "Ok" with the value or an "Err".

Let's look at 2 approaches. For this, we'll look at the part of the program that converts the input value (a string) into a number.

Using "expect"
Let's look at the following code (also from the main.rs file):

    let guess: u32 = guess.trim().parse()
        .expect("invalid string (not a number)");
This code parses the "guess" string and assigns it to the "guess" number (an unsigned 32-bit integer). We'll talk about why there are two things called "guess" in a bit.

The incoming "guess" string is trimmed and then parsed to a number. This is done by stringing functions together. But after the parse, there is another function: expect.

The "parse" function returns a Result enumeration. If we try to assign this directly to the "guess" integer, we will get a type mismatch. The "expect" function does 2 things for us. (1) If "parse" is successful (meaning Result is "Ok"), then it returns the value that we can assign to the variable. (2) If "parse" fails (meaning Result is "Err"), then our message is added to the resulting error.

Here's a sample output:

    Guess the number!
    Please input your guess.
    bad number
    thread 'main' panicked at 'invalid string (not a number): ParseIntError { kind: InvalidDigit }', src\main.rs:19:14
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    error: process didn't exit successfully: `target\debug\guessing_game.exe` (exit code: 101)
I typed "bad number", and the parsing failed. This tells us that "'main' panicked", which means that we have an error that caused the application to exit. And the message has our string along with the actual parsing error.

Pattern Matching
Another way of dealing with errors is to look at the Result directly. And for this, we can use pattern matching.

Here is the same code as above, but we're handing the error instead:

    let guess: u32 = match guess.trim().parse() {
        Ok(num) => num,
        Err(msg) => {
            println!("{}", msg);
            continue;
        },
    };
Notice that we have a "match" here. This sets up a match expression to use the Result that comes back from the "parse" function. This works similarly to the match expression that we saw above.

If Result is "Ok", then it returns the value from the function ("num") which gets assigned to the "guess" integer.

If Result is "Err", then it prints out the error message and then continues. Here "continue" tells the containing loop to go to its next iteration.

Side note: The "println!" macro uses placeholders. The curly braces within the string are replaced with the value of "msg" when this is printed.

Here is some sample output:

    Guess the number!
    Please input your guess.
    bad number
    invalid digit found in string
    Please input your guess.
    23
    You guessed: 23
    Too small!
This time, when I type "bad number" it prints out the error message and then goes to the next iteration of the loop (which asks for another guess).

Overall, this is an interesting approach to error handling. It is more structured than Go and its error strings but also a lot lighter than C# and its exceptions. I'm looking forward to learning more about this and seeing what works well and where it might be lacking.

Immutability

Another feature of Rust is that variables are immutable by default. Variables must be made explicitly mutable. 

Here is the variable that is used to get the input from the console (from the same main.rs file):

    let mut guess = String::new();

    io::stdin().read_line(&mut guess)
        .expect("failed to read line");
The first line creates a mutable string called "guess". The next line reads from the standard input (the console in this case) and assigns it to the "guess" variable.

A couple more notes: You'll notice the "&" in the "read_line" argument. Rust does have pointers. Also the double colon "::" denotes a static. So "new" is a static function on the "String" type.

Immutable by default is interesting since it forces us into a different mindset where we assume that variable cannot be changed. If we want to be able to change them, we need to be explicit about it.

Variable Shadowing

The last feature that we'll look at today is how we can "shadow" a variable. In the code that we've seen already, there are two "guess" variables:

    let mut guess = String::new();
This is a string, and it is used to hold the value typed in on the console.

    let guess: u32 = guess.trim().parse()
This is a 32-bit unsigned integer. It is used to compare against the actual number that we are trying to guess.

These can have the same name because the second "guess" (the number) "shadows" the first "guess" (the string). This means that after the second "guess" is created, all references to "guess" will refer to the number (not the string).

At first, this seems like it could be confusing. But the explanation that goes along with this code helps it make sense (from The Rust Programming Language, Chapter 2):
We create a variable named guess. But wait, doesn’t the program already have a variable named guess? It does, but Rust allows us to shadow the previous value of guess with a new one. This feature is often used in situations in which you want to convert a value from one type to another type. Shadowing lets us reuse the guess variable name rather than forcing us to create two unique variables, such as guess_str and guess for example. 
I often use intermediate variables in my code -- often to help make debugging easier. Instead of having to come up with unique names for variable that represent the same thing but with different types, I can use the same name.

I'm still a bit on the fence about this. I'm sure that it can be misused (like most language features), but it seems to make a lot of sense in this particular example.

Rust Installation

One other thing I wanted to mention was the installation process on Windows. Rust needs a C++ compiler, so it recommends that you install the "Visual Studio C++ Build Tools". This process was not as straight forward as I would have liked. Microsoft does have a stand-alone installer if you do not have Visual Studio, but it starts up the Visual Studio Installer. I ended up just going to my regular Visual Studio Installer and checking the "C++" workload. I'm sure that this installed way more than I needed (5 GBs worth), but I got it working on both of my day-to-day machines.

Other than the C++ build tools, the rest of the installation was pretty uneventful.

I'm assuming that installation is a bit easier on Unix-y OSes (like Linux and macOS) since a C++ compiler (such as gcc) is usually already installed.

There is also a way to run Rust in the browser: https://play.rust-lang.org/. I haven't played much with this, so I'm not sure what the capabilities and limitations are.

For local editing, I've been using Visual Studio Code with the Rust extension.

Documentation and Resources

The sample code is taken from The Rust Programming Language by Steve Klabnik and Carol Nichols. it is available online (for free) or in printed form.

I have only gone as far as Chapter 2 at this point. I really liked Chapter 2 because it walks through building this guessing game project. With each piece of code, various features were showcased, and I found it to be a really good way to get a quick feel for how the language works and some of the basic paradigms.


Wrap Up

I'm not sure how far I'll go with Rust. There are definitely some interesting concepts (that's why I wrote this article). Recently, I converted one of my applications to Go to get a better feel for the language and stretch my understanding a bit (https://github.com/jeremybytes/digit-display-golang). I may end up doing the same thing with Rust.

Exploring other languages can help us expand our ways of thinking and how we approach different programming tasks. And this is useful whether or not we end up using the language in our day-to-day coding. Keep expanding, and keep learning.

Happy Coding!

Friday, July 9, 2021

A Collection of 2020 Recorded Presentations

2020 was "interesting". One good thing that came out of it is that I had the chance to speak remotely for some user groups and conferences that I would not normally get to attend in person. Many of those presentations were recorded. Here's a list for anyone who is interested

Abstract Art: Getting Abstraction "Just Right" (Apr 2020)

Modern Devs Charlotte

Abstraction is awesome. And abstraction is awful. Too little, and our applications are difficult to extend and maintain. Too much, and our applications are difficult to extend and maintain. Finding the balance is the key to success. The first step is to identify your natural tendency as an under-abstractor or an over-abstractor. Once we know that, we can work on real-world techniques to dial in the level of abstraction that is "just right" for our applications.


I'll Get Back to You: Task, Await, and Asynchronous Methods in C# (Apr 2020)

Enterprise Developers Guild

There's a lot of confusion about async/await, Task/TPL, and asynchronous and parallel programming in general. So let's start with the basics and look at how we can consume asynchronous methods using Task and then see how the "await" operator can makes things easier for us. Along the way, we’ll look at continuations, cancellation, and exception handling.


A Banjo Made Me a Better Developer (May 2020)

Dev Around the Sun

What does a banjo have to do with software development? They both require learning. And picking up a banjo later in life showed me 3 things that I've brought into my developer life. (1) You can learn; a growth mindset removes blockages. (2) You don't have to be afraid to ask for help; experienced banjoists/developers can point you in the right direction. (3) You don't have to be perfect before you share what you've learned; it's okay to show what you have "in progress". In combination, these have made me a better banjo player and a better developer.


I'll Get Back to You: Task, Await, and Asynchronous Methods in C# (Jun 2020)

Houston .NET User Group

There's a lot of confusion about async/await, Task/TPL, and asynchronous and parallel programming in general. So let's start with the basics and look at how we can consume asynchronous methods using Task and then see how the "await" operator can makes things easier for us. Along the way, we’ll look at continuations, cancellation, and exception handling.


What's New in C# 8 Interfaces (and how to use them effectively) (Jun 2020)

Southeast Valley .NET User Group
North West Valley .NET User Group

C# 8 brings new features to interfaces, including default implementation, access modifiers, and static members. We'll look at these new features and see where they are useful and where they should be avoided. With some practical tips, "gotchas", and plenty of examples, we'll see how to use these features effectively in our code.


What's New in C# 8 Interfaces (and how to use them effectively) (Jul 2020)

Tulsa Developers Association

C# 8 brings new features to interfaces, including default implementation, access modifiers, and static members. We'll look at these new features and see where they are useful and where they should be avoided. With some practical tips, "gotchas", and plenty of examples, we'll see how to use these features effectively in our code.


I'll Get Back to You: Task, Await, and Asynchronous Methods in C# (Aug 2020)

Code PaLOUsa

There's a lot of confusion about async/await, Task/TPL, and asynchronous and parallel programming in general. So let's start with the basics and look at how we can consume asynchronous methods using Task and then see how the "await" operator can makes things easier for us. Along the way, we’ll look at continuations, cancellation, and exception handling.


Get Func-y: Understanding Delegates in C# (Oct 2020)

St Pete .NET

Delegates are the gateway to functional programming. So let's understand delegates and how we can change the way we program by using functions as parameters, return types, variables, and properties. In addition, we'll see how the built in delegate types, Func and Action, are waiting to make our lives easier. By the time we're done, we'll see how delegates can add elegance, extensibility, and safety to our programming. And as a bonus, you'll have a few functional programming concepts to take with you.


More to Come!

I have a few more recordings from 2021 presentations, and I'll post those soon. As always, if you'd like me to visit your user group, drop me a line. I'll see what I can do about being there online or in person (once in-person stuff is a bit more back to normal).

Happy Coding!

Thursday, June 24, 2021

New and Updated: "C# Interfaces" Course on Pluralsight

An updated version of my "C# Interfaces" course is now available on Pluralsight.


You can click on the link and watch a short promo for the course. If you are not a Pluralsight subscriber, you can sign up for a free trial and watch the entire course.

"C# Interfaces" title slide with a photo of the author, Jeremy Clark

Description

Code that is easy to maintain, extend, and test is key to applications that can move quickly to meet your users’ needs. In this course, C# Interfaces, you’ll learn to use interfaces to add flexibility to your code. First, you’ll explore the mechanics (“What are interfaces?”) and why you want to use them. Next, you’ll discover how to create your own interfaces to make it easy to change or swap out functionality. Finally, you’ll learn about default member implementation and how to avoid some common stumbling blocks. When you’re finished with this course, you’ll have the skills and knowledge of C# interfaces needed to write application code that is easy to maintain, extend, and test.

What's Different

For those who have seen a previous version of this course, here are a few key things that have been added or updated:
  • New section on default implementation
  • A new comparison between interfaces and abstract classes
  • New demo on using configuration and compile-time binding
  • Updated content for dynamic loading of assemblies
  • All code samples are cross-platform friendly (Windows, Linux, macOS)
For a full list of changes, you can check out the change log: "C# Interfaces" Change Log.

Additional Resources

I have added an "Additional Resources" repository on GitHub that includes:
  • The latest code samples (to be updated when .NET 6 drops later this year)
  • Instructions on running the samples with Visual Studio 2019 and Visual Studio Code
  • A list of relevant files for each code sample
  • Articles and links for further study

There are also a couple of samples that I was not able to fit into the course. So stay tuned to this blog; I'll be posting those in the near future.

Happy Coding!

Sunday, February 28, 2021

Recorded Presentation: ASP.NET MVC for Absolute Beginners - Your Secret Decoder Ring

ASP.NET MVC has a lot of conventions around routing, controllers, actions, views, and parameters. Coding-by-convention has both good and bad parts. A good part is that it can be fairly light in configuration. A bad part is that it is easy to get lost if you don't know the conventions.

In January, I had the opportunity to share with the Central Ohio .NET Developer's Group (Meetup link).

Video link:  

In this presentation, we create a new MVC project and look at the default routes, controllers, and views to get an initial understanding of some of the conventions. With that information, we can take the next steps of adding our own code -- collecting parameters and generate mazes.

The parameter collection lets us pick a maze size, the color, and a maze-generation algorithm to use.


Clicking the "Generate Maze" buttons gets us a result:


If this sounds interesting, take a look at the GitHub repo with the code and the video on YouTube:



Happy Coding!