Preamble: I'm teaching a course in object-functional programming using Scala and one of the things we do is to take sample problems and compare how they might be implemented using object-functional programming and using state-based, object-oriented programming, which is the background most of the students have.
So I want to implement a simple class in Scala that has a private var with a public accessor method (a very common idiom in state-based, object-oriented programming). Looking at Alvin Alexander's "Scala Cookbook" the recommended code to do this is pretty ghastly:
class Person(private var _age: Int):
def incrAge() = _age += 1
def age = _age
I say "ghastly" because I'm having to invent two names that essentially represent the age field, one used in the constructor and another used in the class interface. I'm curious if people more familiar with Scala would know of a simpler syntax that would avoid this?
EDIT: It seems clear to me now that Scala combines the val/var declaration with the given visibility (public/private), so for a var either both accessor&mutator are public or both are private. Depending on perspective, you might find this inflexible, or feel it rightly punishes you for using var 🙂.
Yes, a better way of doing it is not using var
class Person(val age: Int) {
def incrAge = new Person(age+1)
}
If you are going to write idiomatic scala code, you should start with pretending that certain parts of it simply do not exist: mostly vars, nulls and returns, but also mutable structures or collections, arrays, and certain methods like .get on a Try or an Option, or the Await object. Oh, and also isInstanceOf and asInstance.
You may ask "why do these things exist if they are not supposed to be used?". Well, because sometimes, in a very few very specific cases they are actually useful for achieving a very limited very specific purpose. But that would be probably fewer than 0.1% of the cases you will come across in your career, unless you are involved in some hard core low level library development (in which case, you would not be posting questions like this here).
So, until you acquire enough command of the language to be able to definitively distinguish those 0.1% of the cases from the other 99.9%, you are much better off simply ignoring those language features, and pretending they do not exist (if you can't figure out how to achieve a certain task without using one of those, post a question here, and people will gladly help you).
You said "Having to create two names to manage a single field is ugly." Indeed. But you know what's uglier? Using vars.
(Btw, the way you typically do this in java is getAge and setAge – still two names. The ugliness is rooted in allowing the value labeled with the given name to be different at different points of program execution, not in how specifically the semantics of mutation looks like).
I know that we can use
/// index variable
var i = 0
as a documentation comment for a single variable.
How can we do the same for a loop variable?
The following does not work:
var array = [0]
/// index variable
for i in array.indices {
// ...
}
or
var array = [0]
for /** index variable */ i in array.indices {
// ...
}
Background:
The reason why I don’t use "good" variable names is that I’m implementing a numerical algorithm which is derived using mathematical notation. It has in this case only single letter variable names. In order to better see the connection between the derivation and the implementation I use the same variable names.
Now I want to comment on the variables in code.
The use of /// is primarily intended for use of documenting the API of of a class, struct, etc. in Swift.
So if used before a class, func, a var/let in a class/struct, etc. you are attaching documentation to that code aspect that Xcode understands how to show inline. It doesn’t know how to pickup that information for things inside of function since at this time that is not the intention of /// (it may work for simple var/let but not likely fully on purpose).
Instead use a simple // code comment for the benefit of any those working in the code however avoid over documenting the code since good code is likely fairly self explaining to anyone versed in the language and adding unneeded documentations can get in the way of just reading the code.
This is a good reference for code documentation in Swift at this time Swift Documentation
I woud strongly push back on something like this if I saw it in a PR. i is a massively well adopted "term of art" for loop indices. Generally, if your variable declaration name needs to be commented, you need a better variable name. There are some exceptions, such as when it stores data with complicated uses/invariants that can't be captured in a better way in a type system.
I think commenting is one area that beginners get wrong, mainly from being misled by teachers or by not yet fully understanding the purpose of comments. Comments don't exist to create an english based, psuedo-programming language in which your entire app will be duplicated. Understanding the programming language is a minimal expectation out of contributors to a project. Absolutely no comments should be explaining programming language features. E.g. var x: Int = 0 // declares a new mutable variable called x, to the Int value 0, with the exception of tutorials for learning Swift.
Commenting in this manner might seem like it's helpful, because you could argue it explains things for beginners. That may be the case, but it's suffocating for all other readers. Imagine if novel had to define all the English words they used.
Instead, the goal of documentation to explain the purpose and the use of things. To answer such questions as:
Why did you implement something this way, and not another way?
What purpose does this method serve?
When will this method of my delegate be called?
Case Study: Equatable
For a good example, take a look at the documentation of Equatable
Some things to notice:
It's written for an audience of Swift developers. It uses many things, which it does not explain such as, arrays, strings, constants, variable declaration, assignment, if statements, method calls (such as Array.contains(_:)), string interpolation, the print function.
It explains the general purpose of this protocol.
It explains how to use this protocol
It explains how you can adopt this protocol for your own use
It documents contractual requirements that cannot be enforced by the type system.
Since equality between instances of Equatable types is an equivalence relation, any of your custom types that conform to Equatable must satisfy three conditions, for any values a, b, and c:
a == a is always true (Reflexivity)
a == b implies b == a (Symmetry)
a == b and b == c implies a == c (Transitivity)
It explains possible misconceptions about the protocol ("Equality is Separate From Identity")
We are pretty familiar with implicits in Scala for now, but macros are pretty undiscovered area (at least for me) and, despite the presence of some great articles by Eugene Burmako, it is still not an easy material to just dive in.
In this particular question I'd like to find out if there is a possibility to achieve the analogous to the following code functionality using just macros:
implicit class Nonsense(val s: String) {
def ##(i:Int) = s.charAt(i)
}
So "asd" ## 0 will return 'a', for example. Can I implement macros that use infix notation? The reason to this is I'm writing a DSL for some already existing project and implicits allow making the API clear and concise, but whenever I write a new implicit class, I feel like introducing a new speed-reducing factor. And yes, I do know about value classes and stuff, I just think it would be really great if my DSL transformed into the underlying library API calls during compilation rather than in runtime.
TL;DR: can I replace implicits with macros while not changing the API? Can I write macros in infix form? Is there something even more suitable for this case? Is the trouble worth it?
UPD. To those advocating the value classes: in my case I have a little more than just a simple wrapper - they are often stacked. For example, I have an implicit class that takes some parameters, returns a lambda wrapping this parameters (i.e. partial function), and the second implicit class that is made specifically for wrapping this type of functions. I can achieve something like this:
a --> x ==> b
where first class wraps a and adds --> method, and the second one wraps the return type of a --> x and defines ==>(b). Plus it may really be the case when user creates considerable amount of objects in this fashion. I just don't know if this will be efficient, so if you could tell me that value classes cover this case - I'd be really glad to know that.
Back in the day (2.10.0-RC1) I had trouble using implicit classes for macros (sorry, I don't recollect why exactly) but the solution was to use:
an implicit def macro to convert to a class
define the infix operator as a def macro in that class
So something like the following might work for you:
implicit def toNonsense(s:String): Nonsense = macro ...
...
class Nonsense(...){
...
def ##(...):... = macro ...
...
}
That was pretty painful to implement. That being said, macro have become easier to implement since.
If you want to check what I did, because I'm not sure that applies to what you want to do, refer to this excerpt of my code (non-idiomatic style).
I won't address the relevance of that here, as it's been commented by others.
I found this code example in Programming in Scala, 2nd Ed. (Chapter 25, Listing 25.11):
object PrefixMap extends {
def empty[T] = ...
def apply[T](kvs: (String, T)*): PrefixMap[T] = ...
...
}
Why is the extends clause there without a superclass name? It looks like extending an anonymous class, but for what purpose? The accompanying text doesn't explain or even mention this construct anywhere. The code actually compiles and apparently works perfectly with or without it.
OTOH I found the exact same code on several web pages, including this (which looks like the original version of the chapter in the book). I doubt that a typo could have passed below the radars of so many readers up to now... so am I missing something?
I tried to google it, but struggled even to find proper search terms for it. So could someone explain whether this construct has a name and/or practical use in Scala?
Looks like a print error to me. It will work all the same, though, which probably helped hide it all this time.
Anyway, that object is extending a structural type, though it could also be an early initialization, if you had with XXX at the end. MMmmm. It looks more like an early initialization without any class or trait to be initialized later, actually... structure types do not contain code, I think.
I keep seeing the phrase "duck typing" bandied about, and even ran across a code example or two. I am way too lazy busy to do my own research, can someone tell me, briefly:
the difference between a 'duck type' and an old-skool 'variant type', and
provide an example of where I might prefer duck typing over variant typing, and
provide an example of something that i would have to use duck typing to accomplish?
I don't mean to seem fowl by doubting the power of this 'new' construct, and I'm not ducking the issue by refusing to do the research, but I am quacking up at all the flocking hype i've been seeing about it lately. It looks like no typing (aka dynamic typing) to me, so I'm not seeing the advantages right away.
ADDENDUM: Thanks for the examples so far. It seems to me that using something like 'O->can(Blah)' is equivalent to doing a reflection lookup (which is probably not cheap), and/or is about the same as saying (O is IBlah) which the compiler might be able to check for you, but the latter has the advantage of distinguishing my IBlah interface from your IBlah interface while the other two do not. Granted, having a lot of tiny interfaces floating around for every method would get messy, but then again so can checking for a lot of individual methods...
...so again i'm just not getting it. Is it a fantastic time-saver, or the same old thing in a brand new sack? Where is the example that requires duck typing?
In some of the answers here, I've seen some incorrect use of terminology, which has lead people to provide wrong answers.
So, before I give my answer, I'm going to provide a few definitions:
Strongly typed
A language is strongly typed if it enforces the type safety of a program. That means that it guarantees two things: something called progress and something else called preservation. Progress basically means that all "validly typed" programs can in fact be run by the computer, They may crash, or throw an exception, or run for an infinite loop, but they can actually be run. Preservation means that if a program is "validly typed" that it will always be "Validly typed", and that no variable (or memory location) will contain a value that does not conform to its assigned type.
Most languages have the "progress" property. There are many, however, that don't satisfy the "preservation" property. A good example, is C++ (and C too). For example, it is possible in C++ to coerce any memory address to behave as if it was any type. This basically allows programmers to violate the type system any time they want. Here is a simple example:
struct foo
{
int x;
iny y;
int z;
}
char * x = new char[100];
foo * pFoo = (foo *)x;
foo aRealFoo;
*pFoo = aRealFoo;
This code allows someone to take an array of characters and write a "foo" instance to it. If C++ was strongly typed this would not be possible. Type safe languages, like C#, Java, VB, lisp, ruby, python, and many others, would throw an exception if you tried to cast an array of characters to a "foo" instance.
Weakly typed
Something is weakly typed if it is not strongly typed.
Statically typed
A language is statically typed if its type system is verified at compile time. A statically typed language can be either "weakly typed" like C or strongly typed like C#.
Dynamically typed
A dynamically typed language is a language where types are verified at runtime. Many languages have a mixture, of some sort, between static and dynamic typing. C#, for example, will verify many casts dynamically at runtime because it's not possible to check them at compile time. Other examples are languages like Java, VB, and Objective-C.
There are also some languages that are "completely" or "mostly" dynamically typed, like "lisp", "ruby", and "small talk"
Duck typing
Duck typing is something that is completely orthogonal to static, dynamic, weak, or strong typing. It is the practice of writing code that will work with an object regardless of its underlying type identity. For example, the following VB.NET code:
function Foo(x as object) as object
return x.Quack()
end function
Will work, regardless of what the type of the object is that is passed into "Foo", provided that is defines a method called "Quack". That is, if the object looks like a duck, walks like a duck, and talks like a duck, then it's a duck. Duck typing comes in many forms. It's possible to have static duck typing, dynamic duck typing, strong duck typing, and weak duck typing. C++ template functions are a good example of "weak static duck typing". The example show in "JaredPar's" post shows an example of "strong static duck typing". Late binding in VB (or code in Ruby or Python) enables "strong dynamic duck typing".
Variant
A variant is a dynamically typed data structure that can hold a range of predefined data types, including strings, integer types, dates, and com objects. It then defines a bunch of operations for assigning, converting, and manipulating data stored in variants. Whether or not a variant is strongly typed depends on the language in which it is used. For example, a variant in a VB 6 program is strongly typed. The VB runtime ensures that operations written in VB code will conform to the typing rules for variants. Tying to add a string to an IUnknown via the variant type in VB will result in a runtime error. In C++, however, variants are weakly typed because all C++ types are weakly typed.
OK.... now that I have gotten the definitions out of the way, I can now answer your question:
A variant, in VB 6, enables one form of doing duck typing. There are better ways of doing duck typing (Jared Par's example is one of the best), than variants, but you can do duck typing with variants. That is, you can write one piece of code that will operate on an object regardless of its underlying type identity.
However, doing it with variants doesn't really give a lot of validation. A statically typed duck type mechanism, like the one JaredPar describes gives the benefits of duck typing, plus some extra validation from the compiler. That can be really helpful.
The simple answer is variant is weakly typed while duck typing is strongly typed.
Duck typing can be summed up nicely as "if it walks like a duck, looks like a duck, acts like a duck, then it's a duck." It computer science terms consider duck to be the following interface.
interface IDuck {
void Quack();
}
Now let's examine Daffy
class Daffy {
void Quack() {
Console.WriteLine("Thatsssss dispicable!!!!");
}
}
Daffy is not actually an IDuck in this case. Yet it acts just like a Duck. Why make Daffy implement IDuck when it's quite obvious that Daffy is in fact a duck.
This is where Duck typing comes in. It allows a type safe conversion between any type that has all of the behaviors of a IDuck and an IDuck reference.
IDuck d = new Daffy();
d.Quack();
The Quack method can now be called on "d" with complete type safety. There is no chance of a runtime type error in this assignment or method call.
Duck typing is just another term for dynamic typing or late-binding. A variant object that parses/compiles with any member access (e.g., obj.Anything) that may or not actually be defined during runtime is duck typing.
Probably nothing requires duck-typing, but it can be convenient in certain situations.
Say you have a method that takes and uses an object of the sealed class Duck from some 3rd party library. And you want to make the method testable. And Duck has an awfully big API (kind of like ServletRequest) of which you only need to care about a small subset. How do you test it?
One way is to make the method take something that quacks. Then you can simply create a quacking mock object.
Try reading the very first paragraph of the Wikipedia article on duck typing.
Duck typing on Wikipedia
I can have an interface (IRunnable) that defines the method Run().
If I have another class with a method like this:
public void RunSomeRunnable(IRunnable rn) { ... }
In a duck type friendly language I could pass in any class that had a Run() method into the RunSomeRunnable() method.
In a statically typed language the class being passed into RunSomeRunnable needs to explicitly implement the IRunnable interface.
"If it Run() like a duck"
variant is more like object in .NET at least.
#Kent Fredric
Your example can most certainly be done without duck typing by using explicit interfaces...uglier yes, but it's not impossible.
And personally, I find having well defined contracts in interfaces much better for enforcing quality code, than relying on duck typing...but that's just my opinion and take it with a grain of salt.
public interface ICreature { }
public interface IFly { fly();}
public interface IWalk { walk(); }
public interface IQuack { quack(); }
// ETC
// Animal Class
public class Duck : ICreature, IWalk, IFly, IQuack
{
fly() {};
walk() {};
quack() {};
}
public class Rhino: ICreature, IWalk
{
walk();
}
// In the method
List<ICreature> creatures = new List<ICreature>();
creatures.Add(new Duck());
creatures.Add(new Rhino());
foreach (ICreature creature in creatures)
{
if (creature is IFly)
(creature as IFly).fly();
if (creature is IWalk)
(creature as IWalk).walk();
}
// Etc
In regards to your request for an example of something you'd need to use duck typing to accomplish, I don't think such a thing exists. I think of it like I think about whether to use recursion or whether to use iteration. Sometimes one just works better than the other.
In my experience, duck typing makes code more readable and easier to grasp (both for the programmer and the reader). But I find that more traditional static typing eliminates a lot of needless typing errors. There's simply no way to objectively say one is better than another or even to say what situations one is more effective than the other.
I say that if you're comfortable using static typing, then use it. But you should at least try duck typing out (and use it in a nontrivial project if possible).
To answer you more directly:
...so again i'm just not getting it. Is it a fantastic time-saver, or the same old thing in a brand new sack?
It's both. You're still attacking the same problems. You're just doing it a different way. Sometimes that's really all you need to do to save time (even if for no other reason to force yourself to think about doing something a different way).
Is it a panacea that will save all of mankind from extinction? No. And anyone who tells you otherwise is a zealot.
A variant (at least as I've used them in VB6) holds a variable of a single, well-defined, usually static type. E.g., it might hold an int, or a float, or a string, but variant ints are used as ints, variant floats are used as floats, and variant strings are used as strings.
Duck typing instead uses dynamic typing. Under duck typing, a variable might be usable as an int, or a float, or a string, if it happens to support the particular methods that an int or float or string supports in a particular context.
Example of variants versus duck typing:
For a web application, suppose I want my user information to come from LDAP instead of from a database, but I still want my user information to be useable by the rest of the web framework, which is based around a database and an ORM.
Using variants: No luck. I can create a variant that can contain a UserFromDbRecord object or a UserFromLdap object, but UserFromLdap objects won't be usable by routines that expect objects from the FromDbRecord hierarchy.
Using duck typing: I can take my UserFromLdap class and add a couple of methods that make it act like a UserFromDbRecord class. I don't need to replicate the entire FromDbRecord interface, just enough for the routines that I need to use. If I do this right, it's an extremely powerful and flexible technique. If I do it wrong, it produces very confusing and brittle code (subject to breakage if either the DB library or the LDAP library changes).
I think the core point of duck typing is how it is used. One uses method detection and introspection of the entity in order to know what to do with it, instead of declaring in advance what it will be ( where you know what to do with it ).
It's probably more practical in OO languages, where primitives are not primitives, and are instead objects.
I think the best way to sum it up, in variant type, an entity is/can be anything, and what it is is uncertain, as opposed to an entity only looks like anything, but you can work out what it is by asking it.
Here's something I don't believe is plausible without ducktyping.
sub dance {
my $creature = shift;
if( $creature->can("walk") ){
$creature->walk("left",1);
$creature->walk("right",1);
$creature->walk("forward",1);
$creature->walk("back",1);
}
if( $creature->can("fly") ){
$creature->fly("up");
$creature->fly("right",1);
$creature->fly("forward",1);
$creature->fly("left", 1 );
$creature->fly("back", 1 );
$creature->fly("down");
} else if ( $creature->can("walk") ) {
$creature->walk("left",1);
$creature->walk("right",1);
$creature->walk("forward",1);
$creature->walk("back",1);
} else if ( $creature->can("splash") ) {
$creature->splash( "up" ) for ( 0 .. 4 );
}
if( $creature->can("quack") ) {
$creature->quack();
}
}
my #x = ();
push #x, new Rhinoceros ;
push #x, new Flamingo;
push #x, new Hyena;
push #x, new Dolphin;
push #x, new Duck;
for my $creature (#x){
new Thread(sub{
dance( $creature );
});
}
Any other way would require you to put type restrictions on for functions, which would cut out different species, needing you to create different functions for different species, making the code really hellish to maintain.
And that really sucks in terms of just trying to perform good choreography.
Everything you can do with duck-typing you can also do with interfaces. Duck-typing is fast and comfortable, but some argue it can lead to errors (if two distinct methods/properties are named alike). Interfaces are safe and explicit, but people might say "why state the obvious?". Rest is a flame. Everyone chooses what suits him and no one is "right".