evcxr_repl

evcxr_repl is A Rust REPL (Read-Eval-Print loop) built to provide evaluation context for Interactive programming.

Interactive programming, in which expressions are entered and evaluated in real-time, can be a powerful tool for exploring a language and problem solving. This capability is most often associated with dynamically evaluated languages such as JavaScript and Python. Compiled languages such as Java and C++ can also be used interactively, but tooling tends to lag by many years. Rust is a newer compiled language for which interactive programming has recently emerged. This article discusses interactive programming with Rust courtesy of the google/evcxr - https://github.com/google/evcxr crate.

testify

Go code (golang) set of packages that provide many tools for testifying that your code will behave as you intend.

Features include:

• Easy assertions

• Mocking

• Testing suite interfaces and functions

strconv

Package strconv implements conversions to and from string representations of basic data types.

10. Generic Types, Traits, and Lifetimes

Generics are abstract stand-ins for concrete types or other properties. When we’re writing code, we can express the behavior of generics or how they relate to other generics without knowing what will be in their place when compiling and running the code.

Then you’ll learn how to use traits to define behavior in a generic way. You can combine traits with generic types to constrain a generic type to only those types that have a particular behavior, as opposed to just any type.

Finally, we’ll discuss lifetimes, a variety of generics that give the compiler information about how references relate to each other. Lifetimes allow us to borrow values in many situations while still enabling the compiler to check that the references are valid.

15. Smart Pointers

15.3 Running Code on Cleanup with the Drop Trait

The second trait important to the smart pointer pattern is Drop, which lets you customize what happens when a value is about to go out of scope. You can provide an implementation for the Drop` trait on any type, and the code you specify can be used to release resources like files or network connections.

Specify the code to run when a value goes out of scope by implementing the Drop trait. The Drop trait requires you to implement one method named drop that takes a mutable reference to self. To see when Rust calls drop, let’s implement drop with println! statements for now.

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struct CustomSmartPointer {
    data: String,
}

impl Drop for CustomSmartPointer {
    fn drop(&mut self) {
        println!("Dropping CustomSmartPointer with data `{}`!", self.data);
    }
}

fn main() {
    let c = CustomSmartPointer {
        data: String::from("my stuff"),
    };
    let d = CustomSmartPointer {
        data: String::from("other stuff"),
    };
    println!("CustomSmartPointers created.");
}

When we run this program, we’ll see the following output:

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$ cargo run
CustomSmartPointers created.
Dropping CustomSmartPointer with data `other stuff`!
Dropping CustomSmartPointer with data `my stuff`!

Rust automatically called drop for us when our instances went out of scope, calling the code we specified. Variables are dropped in the reverse order of their creation, so d was dropped before c.

Dropping a Value Early with std::mem::drop

You might want to force the drop method that releases the lock so that other code in the same scope can acquire the lock. Rust doesn’t let you call the Drop trait’s drop method manually; instead you have to call the std::mem::drop function provided by the standard library if you want to force a value to be dropped before the end of its scope.

Rust doesn’t let us call drop of Drop trait explicitly because Rust would still automatically call drop on the value at the end of main. This would be a double free error because Rust would be trying to clean up the same value twice.

The std::mem::drop function is different from the drop method in the Drop trait. We call it by passing the value we want to force to be dropped early as an argument.

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struct CustomSmartPointer {
    data: String,
}

impl Drop for CustomSmartPointer {
    fn drop(&mut self) {
        println!("Dropping CustomSmartPointer with data `{}`!", self.data);
    }
}

fn main() {
    let c = CustomSmartPointer {
        data: String::from("some data"),
    };
    println!("CustomSmartPointer created.");
    drop(c);
    println!("CustomSmartPointer dropped before the end of main.");
}

Running this code will print the following:

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$ cargo run
CustomSmartPointer created.
Dropping CustomSmartPointer with data `some data`!
CustomSmartPointer dropped before the end of main.

References

[1] Smart Pointers - The Rust Programming Language - https://doc.rust-lang.org/book/ch15-00-smart-pointers.html

[2] The Rust Programming Language - The Rust Programming Language - https://doc.rust-lang.org/book/title-page.html

17. Object Oriented Programming Features of Rust

Many competing definitions describe what OOP is. In this chapter, we’ll explore certain characteristics that are commonly considered object oriented and how those characteristics translate to idiomatic Rust.

19. Advanced Features

19.3 Advanced Types

Using the Newtype Pattern for Type Safety and Abstraction

Creating Type Synonyms with Type Aliases

Rust provides the ability to declare a type alias to give an existing type another name. For this we use the type keyword.

The main use case for type Alias(synonyms) is to reduce repetition.

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let f: Box<dyn Fn() + Send + 'static> = Box::new(|| println!("hi"));

fn takes_long_type(f: Box<dyn Fn() + Send + 'static>) {
    // --snip--
}

fn returns_long_type() -> Box<dyn Fn() + Send + 'static> {
    // --snip--
}

A type alias makes this code more manageable by reducing the repetition.

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type Thunk = Box<dyn Fn() + Send + 'static>;

let f: Thunk = Box::new(|| println!("hi"));

fn takes_long_type(f: Thunk) {
    // --snip--
}

fn returns_long_type() -> Thunk {
    // --snip--
}

Type aliases are also commonly used with the Result<T, E> type for reducing repetition.

The Result<..., Error> is repeated a lot. As such, std::io has this type alias declaration:

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type Result<T> = std::result::Result<T, std::io::Error>;

Because this declaration is in the std::io module, we can use the fully qualified alias std::io::Result<T>—that is, a Result<T, E>with theEfilled in asstd::io::Error`. The Write trait function signatures end up looking like this:

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pub trait Write {
    fn write(&mut self, buf: &[u8]) -> Result<usize>;
    fn flush(&mut self) -> Result<()>;

    fn write_all(&mut self, buf: &[u8]) -> Result<()>;
    fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<()>;
}

! The Never Type that Never Returns

Rust has a special type named ! that’s known in type theory lingo as the empty type because it has no values. We prefer to call it the never type because it stands in the place of the return type when a function will never return. Here is an example:

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fn bar() -> ! {
    // --snip--
}

let guess: u32 = match guess.trim().parse() {
    Ok(num) => num,
    Err(_) => continue, // Because ! can never have a value, Rust decides that the type of guess is u32.
};

impl<T> Option<T> {
    pub fn unwrap(self) -> T {
        match self {
            Some(val) => val,
            None => panic!("called `Option::unwrap()` on a `None` value"),
        }
    }
}

print!("forever ");

loop {
    print!("and ever ");
    // break; 
}   // ! is the value of the expression.

// However, this wouldn’t be true if we included a break, because the loop would terminate when it got to the break.

Dynamically Sized Types and the Sized Trait

Rust needs to know how much memory to allocate for any value of a particular type, and all values of a type must use the same amount of memory.

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// let s1: str = "Hello there!";   // expected `str`, found `&str`
// let s2: str = "How's it going?";    // expected `str`, found `&str`

let s1: &str = "Hello there!";
let s2: &str = "How's it going?";

If Rust allowed us to write this code, these two str values would need to take up the same amount of space. But they have different lengths: s1 needs 12 bytes of storage and s2 needs 15. This is why it’s not possible to create a variable holding a dynamically sized type.

So although a &T is a single value that stores the memory address of where the T is located, a &str is two values: the address of the str and its length. As such, we can know the size of a &str value at compile time: it’s twice the length of a usize. That is, we always know the size of a &str, no matter how long the string it refers to is. In general, this is the way in which dynamically sized types are used in Rust: they have an extra bit of metadata that stores the size of the dynamic information. The golden rule of dynamically sized types is that we must always put values of dynamically sized types behind a pointer of some kind.

References

[1] The Rust Programming Language - The Rust Programming Language - https://doc.rust-lang.org/book/title-page.html

[2] Advanced Features - The Rust Programming Language - https://doc.rust-lang.org/book/ch19-00-advanced-features.html

Basic Example

This is a basic example that sets the value of a variable and exposes it for other contracts to access.

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// SPDX-License-Identifier: GPL-3.0 // the source code is licensed under the GPL version 3.0
pragma solidity >=0.4.16 <0.9.0; // that the source code is written for Solidity version 0.4.16, or a newer version of the language up to, but not including version 0.9.0.

contract SimpleStorage { // contract name
    // declares a state variable called storedData of type uint (unsigned integer of 256 bits)
    uint storedData;

    // set that can be used to modify the value of the variable
    function set(uint x) public {
        storedData = x;
    }

    // get that can be used to retrieve the value of the variable
    function get() public view returns (uint) {
        return storedData;
    }
}

The first line Machine-readable license specifiers are important in a setting where publishing the source code is the default.

pragma - are common instructions for compilers about how to treat the source code (e.g. pragma once - https://en.wikipedia.org/wiki/Pragma_once).

A contract in the sense of Solidity is a collection of code (its functions) and data (its state) that resides at a specific address on the Ethereum blockchain.

In this example, the contract declares a state variable called storedData of type uint (unsigned integer of 256 bits), and defines the functions set and get that can be used to modify or retrieve the value of the variable.

Anyone could call set again with a different value and overwrite your number, and the number is still stored in the history of the blockchain. Later, you will see how you can impose access restrictions so that only you can alter the number.

All identifiers (contract names, function names and variable names) are restricted to the ASCII character set. It is possible to store UTF-8 encoded data in string variables.

Subcurrency Example

The contract allows only its creator to create new coins (different issuance schemes are possible). Anyone can send coins to each other

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// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;

contract Coin {
    // The keyword "public" makes variables
    // accessible from other contracts
    address public minter;
    mapping (address => uint) public balances;

    // Events allow clients to react to specific
    // contract changes you declare
    event Sent(address from, address to, uint amount);

    // Constructor code is only run when the contract
    // is created
    constructor() {
        minter = msg.sender;
    }

    // Sends an amount of newly created coins to an address
    // Can only be called by the contract creator
    function mint(address receiver, uint amount) public {
        require(msg.sender == minter);
        require(amount < 1e60);
        balances[receiver] += amount;
    }

    // Sends an amount of existing coins
    // from any caller to an address
    function send(address receiver, uint amount) public {
        require(amount <= balances[msg.sender], "Insufficient balance.");
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        emit Sent(msg.sender, receiver, amount);
    }
}

The line address public minter; declares a state variable of type address - https://docs.soliditylang.org/en/v0.8.3/types.html#address. The address type is a 160-bit value that does not allow any arithmetic operations. It is suitable for storing addresses of contracts, or a hash of the public half of a keypair belonging to external accounts - https://docs.soliditylang.org/en/v0.8.3/introduction-to-smart-contracts.html#accounts.

The keyword public automatically generates a function that allows you to access the current value of the state variable from outside of the contract. Without this keyword, other contracts have no way to access the variable. The code of the function generated by the compiler is equivalent to the following

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function minter() external view returns (address) { return minter; }

Arrays

An array holds a specific number of elements, and it cannot grow or shrink. Different data types can be handled as elements in arrays such as Int, String, Boolean, and others.

An array is a fixed-length sequence of zero or more elements of a particular type. Because of their fixed length, arrays are rarely used directly in Go. Slices, which can grow and shrink, are much more versatile.

Maps

Go provides another important data type named map which maps unique keys to values. A key is an object that you use to retrieve a value at a later date. Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key.

Create a map

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var m map[string]int                // nil map of string-int pairs

m1 := make(map[string]float64)      // Empty map of string-float64 pairs
m2 := make(map[string]float64, 100) // Preallocate room for 100 entries

m3 := map[string]float64{           // Map literal
    "e":  2.71828,
    "pi": 3.1416,
}
fmt.Println(len(m3))                // Size of map: 2

• A map (or dictionary) is an unordered collection of key-value pairs, where each key is unique.

• You create a new map with a make statement or a map literal.

• The default zero value of a map is nil. A nil map is equivalent to an empty map except that elements can’t be added.

• The len function returns the size of a map, which is the number of key-value pairs.

Warning: If you try to add an element to an uninitialized map you get the mysterious run-time error Assignment to entry in nil map.

Set, Get, Delete, Length keys/values,

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m := make(map[string]float64)

m["pi"] = 3.14             // Set a new key-value pair
m["pi"] = 3.1416           // Update value
fmt.Println(m)             // Print map: "map[pi:3.1416]"

v := m["pi"]               // Get value: v == 3.1416
v = m["pie"]               // Not found: v == 0 (zero value)

_, found := m["pi"]        // found == true
_, found = m["pie"]        // found == false

if x, found := m["pi"]; found {
    fmt.Println(x)
}                           // Prints "3.1416"

delete(m, "pi")             // Delete a key-value pair
fmt.Println(m)              // Print map: "map[]"

When you index a map you get two return values; the second one (which is optional) is a boolean that indicates if the key exists.

If the key doesn’t exist, the first value will be the default zero value.

For-each range loop

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m := map[string]float64{
    "pi": 3.1416,
    "e":  2.71828,
}
fmt.Println(m) // "map[e:2.71828 pi:3.1416]"

for key, value := range m { // Order not specified 
    fmt.Println(key, value)
}

Iteration order is not specified and may vary from iteration to iteration.

• If an entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced.

• If an entry is created during iteration, that entry may or may not be produced during the iteration.

Starting with Go 1.12, the fmt package prints maps in key-sorted order to ease testing.

Performance and implementation

Maps are backed by hash tables.

Set, get and delete operations run in constant expected time. The time complexity for the add operation is amortized.

The comparison operators == and != must be defined for the key type.

References

Maps explained: create, add, get, delete · YourBasic Go - https://yourbasic.org/golang/maps-explained/

Go - Maps - Tutorialspoint - https://www.tutorialspoint.com/go/go_maps.htm

Go map - working with maps in Golang - https://zetcode.com/golang/map/