I'm having problem understanding the usefulness of Rust enums after reading The Rust Programming Language.
In section 17.3, Implementing an Object-Oriented Design Pattern, we have this paragraph:
If we were to create an alternative implementation that didn’t use the state pattern, we might instead use match expressions in the methods on Post or even in the main code that checks the state of the post and changes behavior in those places. That would mean we would have to look in several places to understand all the implications of a post being in the published state! This would only increase the more states we added: each of those match expressions would need another arm.
I agree completely. It would be very bad to use enums in this case because of the reasons outlined. Yet, using enums was my first thought of a more idiomatic implementation. Later in the same section, the book introduces the concept of encoding the state of the objects using types, via variable shadowing.
It's my understanding that Rust enums can contain complex data structures, and different variants of the same enum can contain different types.
What is a real life example of a design in which enums are the better option? I can only find fake or very simple examples in other sources.
I understand that Rust uses enums for things like Result and Option, but those are very simple uses. I was thinking of some functionality with a more complex behavior.
This turned out to be a somewhat open ended question, but I could not find a useful response after searching Google. I'm free to change this question to a more closed version if someone could be so kind as to help me rephrase it.
A fundamental trade-off between these choices in a broad sense has a name: "the expression problem". You should find plenty on Google under that name, both in general and in the context of Rust.
In the context of the question, the "problem" is to write the code in such a way that both adding a new state and adding a new operation on states does not involve modifying existing implementations.
When using a trait object, it is easy to add a state, but not an operation. To add a state, one defines a new type and implements the trait. To add an operation, naively, one adds a method to the trait but has to intrusively update the trait implementations for all states.
When using an enum for state, it is easy to add a new operation, but not a new state. To add an operation, one defines a new function. To add a new state, naively, one must intrusively modify all the existing operations to handle the new state.
If I explained this well enough, hopefully it should be clear that both will have a place. They are in a way dual to one another.
With this lens, an enum would be a better fit when the operations on the enum are expected to change more than the alternatives. For example, suppose you were trying to represent an abstract syntax tree for C++, which changes every three years. The set of types of AST nodes may not change frequently relative to the set of operations you may want to perform on AST nodes.
With that said, there are solutions to the more difficult options in both cases, but they remain somewhat more difficult. And what code must be modified may not be the primary concern.
I'm trying to get my head around mutable vs immutable objects. Using mutable objects gets a lot of bad press (e.g. returning an array of strings from a method) but I'm having trouble understanding what the negative impacts are of this. What are the best practices around using mutable objects? Should you avoid them whenever possible?
Well, there are a few aspects to this.
Mutable objects without reference-identity can cause bugs at odd times. For example, consider a Person bean with a value-based equals method:
Map<Person, String> map = ...
Person p = new Person();
map.put(p, "Hey, there!");
p.setName("Daniel");
map.get(p); // => null
The Person instance gets "lost" in the map when used as a key because its hashCode and equality were based upon mutable values. Those values changed outside the map and all of the hashing became obsolete. Theorists like to harp on this point, but in practice I haven't found it to be too much of an issue.
Another aspect is the logical "reasonability" of your code. This is a hard term to define, encompassing everything from readability to flow. Generically, you should be able to look at a piece of code and easily understand what it does. But more important than that, you should be able to convince yourself that it does what it does correctly. When objects can change independently across different code "domains", it sometimes becomes difficult to keep track of what is where and why ("spooky action at a distance"). This is a more difficult concept to exemplify, but it's something that is often faced in larger, more complex architectures.
Finally, mutable objects are killer in concurrent situations. Whenever you access a mutable object from separate threads, you have to deal with locking. This reduces throughput and makes your code dramatically more difficult to maintain. A sufficiently complicated system blows this problem so far out of proportion that it becomes nearly impossible to maintain (even for concurrency experts).
Immutable objects (and more particularly, immutable collections) avoid all of these problems. Once you get your mind around how they work, your code will develop into something which is easier to read, easier to maintain and less likely to fail in odd and unpredictable ways. Immutable objects are even easier to test, due not only to their easy mockability, but also the code patterns they tend to enforce. In short, they're good practice all around!
With that said, I'm hardly a zealot in this matter. Some problems just don't model nicely when everything is immutable. But I do think that you should try to push as much of your code in that direction as possible, assuming of course that you're using a language which makes this a tenable opinion (C/C++ makes this very difficult, as does Java). In short: the advantages depend somewhat on your problem, but I would tend to prefer immutability.
Immutable Objects vs. Immutable Collections
One of the finer points in the debate over mutable vs. immutable objects is the possibility of extending the concept of immutability to collections. An immutable object is an object that often represents a single logical structure of data (for example an immutable string). When you have a reference to an immutable object, the contents of the object will not change.
An immutable collection is a collection that never changes.
When I perform an operation on a mutable collection, then I change the collection in place, and all entities that have references to the collection will see the change.
When I perform an operation on an immutable collection, a reference is returned to a new collection reflecting the change. All entities that have references to previous versions of the collection will not see the change.
Clever implementations do not necessarily need to copy (clone) the entire collection in order to provide that immutability. The simplest example is the stack implemented as a singly linked list and the push/pop operations. You can reuse all of the nodes from the previous collection in the new collection, adding only a single node for the push, and cloning no nodes for the pop. The push_tail operation on a singly linked list, on the other hand, is not so simple or efficient.
Immutable vs. Mutable variables/references
Some functional languages take the concept of immutability to object references themselves, allowing only a single reference assignment.
In Erlang this is true for all "variables". I can only assign objects to a reference once. If I were to operate on a collection, I would not be able to reassign the new collection to the old reference (variable name).
Scala also builds this into the language with all references being declared with var or val, vals only being single assignment and promoting a functional style, but vars allowing a more C-like or Java-like program structure.
The var/val declaration is required, while many traditional languages use optional modifiers such as final in java and const in C.
Ease of Development vs. Performance
Almost always the reason to use an immutable object is to promote side effect free programming and simple reasoning about the code (especially in a highly concurrent/parallel environment). You don't have to worry about the underlying data being changed by another entity if the object is immutable.
The main drawback is performance. Here is a write-up on a simple test I did in Java comparing some immutable vs. mutable objects in a toy problem.
The performance issues are moot in many applications, but not all, which is why many large numerical packages, such as the Numpy Array class in Python, allow for In-Place updates of large arrays. This would be important for application areas that make use of large matrix and vector operations. This large data-parallel and computationally intensive problems achieve a great speed-up by operating in place.
Immutable objects are a very powerful concept. They take away a lot of the burden of trying to keep objects/variables consistent for all clients.
You can use them for low level, non-polymorphic objects - like a CPoint class - that are used mostly with value semantics.
Or you can use them for high level, polymorphic interfaces - like an IFunction representing a mathematical function - that is used exclusively with object semantics.
Greatest advantage: immutability + object semantics + smart pointers make object ownership a non-issue, all clients of the object have their own private copy by default. Implicitly this also means deterministic behavior in the presence of concurrency.
Disadvantage: when used with objects containing lots of data, memory consumption can become an issue. A solution to this could be to keep operations on an object symbolic and do a lazy evaluation. However, this can then lead to chains of symbolic calculations, that may negatively influence performance if the interface is not designed to accommodate symbolic operations. Something to definitely avoid in this case is returning huge chunks of memory from a method. In combination with chained symbolic operations, this could lead to massive memory consumption and performance degradation.
So immutable objects are definitely my primary way of thinking about object-oriented design, but they are not a dogma.
They solve a lot of problems for clients of objects, but also create many, especially for the implementers.
Check this blog post: http://www.yegor256.com/2014/06/09/objects-should-be-immutable.html. It explains why immutable objects are better than mutable. In short:
immutable objects are simpler to construct, test, and use
truly immutable objects are always thread-safe
they help to avoid temporal coupling
their usage is side-effect free (no defensive copies)
identity mutability problem is avoided
they always have failure atomicity
they are much easier to cache
You should specify what language you're talking about. For low-level languages like C or C++, I prefer to use mutable objects to conserve space and reduce memory churn. In higher-level languages, immutable objects make it easier to reason about the behavior of the code (especially multi-threaded code) because there's no "spooky action at a distance".
A mutable object is simply an object that can be modified after it's created/instantiated, vs an immutable object that cannot be modified (see the Wikipedia page on the subject). An example of this in a programming language is Pythons lists and tuples. Lists can be modified (e.g., new items can be added after it's created) whereas tuples cannot.
I don't really think there's a clearcut answer as to which one is better for all situations. They both have their places.
Shortly:
Mutable instance is passed by reference.
Immutable instance is passed by value.
Abstract example. Lets suppose that there exists a file named txtfile on my HDD. Now, when you are asking me to give you the txtfile file, I can do it in the following two modes:
I can create a shortcut to the txtfile and pass shortcut to you, or
I can do a full copy of the txtfile file and pass copied file to you.
In the first mode, the returned file represents a mutable file, because any change into the shortcut file will be reflected into the original one as well, and vice versa.
In the second mode, the returned file represents an immutable file, because any change into the copied file will not be reflected into the original one, and vice versa.
If a class type is mutable, a variable of that class type can have a number of different meanings. For example, suppose an object foo has a field int[] arr, and it holds a reference to a int[3] holding the numbers {5, 7, 9}. Even though the type of the field is known, there are at least four different things it can represent:
A potentially-shared reference, all of whose holders care only that it encapsulates the values 5, 7, and 9. If foo wants arr to encapsulate different values, it must replace it with a different array that contains the desired values. If one wants to make a copy of foo, one may give the copy either a reference to arr or a new array holding the values {1,2,3}, whichever is more convenient.
The only reference, anywhere in the universe, to an array which encapsulates the values 5, 7, and 9. set of three storage locations which at the moment hold the values 5, 7, and 9; if foo wants it to encapsulate the values 5, 8, and 9, it may either change the second item in that array or create a new array holding the values 5, 8, and 9 and abandon the old one. Note that if one wanted to make a copy of foo, one must in the copy replace arr with a reference to a new array in order for foo.arr to remain as the only reference to that array anywhere in the universe.
A reference to an array which is owned by some other object that has exposed it to foo for some reason (e.g. perhaps it wants foo to store some data there). In this scenario, arr doesn't encapsulate the contents of the array, but rather its identity. Because replacing arr with a reference to a new array would totally change its meaning, a copy of foo should hold a reference to the same array.
A reference to an array of which foo is the sole owner, but to which references are held by other object for some reason (e.g. it wants to have the other object to store data there--the flipside of the previous case). In this scenario, arr encapsulates both the identity of the array and its contents. Replacing arr with a reference to a new array would totally change its meaning, but having a clone's arr refer to foo.arr would violate the assumption that foo is the sole owner. There is thus no way to copy foo.
In theory, int[] should be a nice simple well-defined type, but it has four very different meanings. By contrast, a reference to an immutable object (e.g. String) generally only has one meaning. Much of the "power" of immutable objects stems from that fact.
Mutable collections are in general faster than their immutable counterparts when used for in-place
operations.
However, mutability comes at a cost: you need to be much more careful sharing them between
different parts of your program.
It is easy to create bugs where a shared mutable collection is updated
unexpectedly, forcing you to hunt down which line in a large codebase is performing the unwanted update.
A common approach is to use mutable collections locally within a function or private to a class where there
is a performance bottleneck, but to use immutable collections elsewhere where speed is less of a concern.
That gives you the high performance of mutable collections where it matters most, while not sacrificing
the safety that immutable collections give you throughout the bulk of your application logic.
If you return references of an array or string, then outside world can modify the content in that object, and hence make it as mutable (modifiable) object.
Immutable means can't be changed, and mutable means you can change.
Objects are different than primitives in Java. Primitives are built in types (boolean, int, etc) and objects (classes) are user created types.
Primitives and objects can be mutable or immutable when defined as member variables within the implementation of a class.
A lot of people people think primitives and object variables having a final modifier infront of them are immutable, however, this isn't exactly true. So final almost doesn't mean immutable for variables. See example here
http://www.siteconsortium.com/h/D0000F.php.
General Mutable vs Immutable
Unmodifiable - is a wrapper around modifiable. It guarantees that it can not be changed directly(but it is possibly using backing object)
Immutable - state of which can not be changed after creation. Object is immutable when all its fields are immutable. It is a next step of Unmodifiable object
Thread safe
The main advantage of Immutable object is that it is a naturally for concurrent environment. The biggest problem in concurrency is shared resource which can be changed any of thread. But if an object is immutable it is read-only which is thread safe operation. Any modification of an original immutable object return a copy
source of truth, side-effects free
As a developer you are completely sure that immutable object's state can not be changed from any place(on purpose or not). For example if a consumer uses immutable object he is able to use an original immutable object
compile optimisation
Improve performance
Disadvantage:
Copying of object is more heavy operation than changing a mutable object, that is why it has some performance footprint
To create an immutable object you should use:
1. Language level
Each language contains tools to help you with it. For example:
Java has final and primitives
Swift has let and struct[About].
Language defines a type of variable. For example:
Java has primitive and reference type,
Swift has value and reference type[About].
For immutable object more convenient is primitives and value type which make a copy by default. As for reference type it is more difficult(because you are able to change object's state out of it) but possible. For example you can use clone pattern on a developer level to make a deep(instead of shallow) copy.
2. Developer level
As a developer you should not provide an interface for changing state
[Swift] and [Java] immutable collection
I would like to make it a compiler error to allow a type to be dropped, instead it must be forgotten. My use case is for a type the represents a handle of sorts that must be returned to its source for cleanup. This way a user of the API cannot accidentally leak the handle. They would be required to either return the handle to its source or explicitly forget it. In the source, the associated resources would be cleaned up and the handle explicitly forgotten.
The article The Pain Of Real Linear Types in Rust mentions this. Relevant quote:
One extreme option that I've seen is to implement drop() as
abort("this value must be used"). All "proper" consumers then
mem::forget the value, preventing this "destructor bomb" from going
off. This provides a dynamic version of strict must-use values.
Although it's still vulnerable to the few ways destructors can leak,
this isn't a significant concern in practice. Mostly it just stinks
because it's dynamic and Rust users Want Static Verification.
Ultimately, Rust lacks "proper" support for this kind of type.
So, assuming you want static checks, the answer is no.
You could require the user to pass a function object that returns the handle (FnOnce(Handle) -> Handle), as long as there aren't any other ways to create a handle.
From the Rust book's chapter on ownership, non-copyable values can be passed to functions by either transferring ownership or by using a mutable or immutable reference. When you transfer ownership of a value, it can't be used in the original function anymore: you must return it back if you want to. When you pass a reference, you borrow the value and can still use it.
I come from languages where values are immutable by default (Haskell, Idris and the like). As such, I'd probably never think about using references at all. Having the same value in two places looks dangerous (or, at least, awkward) to me. Since references are a feature, there must be a reason to use them.
Are there situations I should force myself to use references? What are those situations and why are they beneficial? Or are they just for convenience and defaulting to passing ownership is fine?
Mutable references in particular look very dangerous.
They are not dangerous, because the Rust compiler will not let you do anything dangerous. If you have a &mut reference to a value then you cannot simultaneously have any other references to it.
In general you should pass references around. This saves copying memory and should be the default thing you do, unless you have a good reason to do otherwise.
Some good reasons to transfer ownership instead:
When the value's type is small in size, such as bool, u32, etc. It's often better performance to move/copy these values to avoid a level of indirection. Usually these values implement Copy, and actually the compiler may make this optimisation for you automatically. Something it's free to do because of a strong type system and immutability by default!
When the value's current owner is going to go out of scope, you may want to move the value somewhere else to keep it alive.
Entities and value objects are both domain objects. What's the use of knowing the distinction between the two in DDD? Eg does thinking about domain objects as being either an entity or value object foster a cleaner domain model?
Yes, it is very helpful to be able to tell the difference, particularly when you are designing and implementing your types.
One of the main differences is when it comes to dealing with equality, since Entities should have quite different behavior than Value Objects. Knowing whether your object is an Entity or a Value Object tells you how you should implement equality for the type. This is helpful in itself, but it doesn't stop there.
Entities are mutable types (at least by concept). The whole idea behind an Entity is that it represents a Domain concept with a known lifetime progression (i.e. it is created, it undergoes several transformations, it is archived and perhaps eventually deleted). It represents the same particular 'thing' even if months or years pass by, and it changes state along the way.
Value Objects, on the other hand, simply represent values without any inherent identity. Although you don't have to do this, they lend themselves tremendously well to be implemented as immutable types. This is very interesting because any immutable type is by definition thread-safe. As we are moving into the multi-core age, knowing when to implement an object as an immutable type is very valuable.
It also helps a lot in unit testing when the equality semantics are well-known. In both cases, equality is well-defined. I don't know what language you use, but in many languages (C#, Java, VB.NET) equality is determined by reference by default, which in many cases isn't particularly useful.