Dart supports referencing static functions, but is there a syntax to reference methods on objects as well, similar to Java/Kotlin?
[1, -3, 5].map((n) => n.abs()).forEach(print); // this works...
[1, -3, 5].map(int::abs).forEach(print); // ...but this won't compile
I said before that Dart can't do what you're trying to do, as in order to call an instance method you need an instance. That said, what you're doing is reminiscent of a Javascript coding style and isn't very idiomatic for Dart. I'd suggest the following instead:
final list = [1, -3, 5];
for (var n in list) {
print(n.abs());
}
or
final list = [1, -3, 5].map((n) => n.abs());
for (var n in list) {
print(n);
}
The forEach method isn't strictly recommended unless you have a pre-existing method you can pass in. Otherwise, it tends to make the code a bit denser and thus harder to read at a glance. (And this isn't Javascript, you don't need to worry about scope or closure issues when using a regular old for loop.)
EDIT:
There is a possible workaround, though I'm not sure I see much point in doing it this way unless it tickles a particular kind of fancy. You can "wrap" the call to the instance method within a local anonymous method, then pass the reference of that method to, in your case, the map method:
int abs(int i) => i.abs();
[1, -3, 5].map(abs).forEach(print);
In practice, this effectively just breaks the lambda method you created in your first example out into a method in its own right, so there's an argument to be made that this approach succeeds only in making the process more complicated than it needs to be.
Like it was mentioned earlier, you still need an instance to reference. If you're looking for means to shorten your code, you might want to use
[1, -3, 5].forEach((f) => print(f.abs()));
Related
Say I have a class ListContainer that contains and manages a list that must be accessible from outside. Since managing this list is complicated, I don't want to let anyone other than ListContainer modify it. In C++, I would create a function that returns const reference, but in Dart, const works completely differently. Just using getter will not prevent someone from modifying the list.
So how can I provide an access to the list values without allowing to modify the list?
I'm looking for something better than creating a getNth function because that would also require creating methods like length, map, and so on.
I think UnmodifiableListView is what you are looking for.
check out UnmodifiableListView Documentation
you can use it like this:
List<int> _myList = [1, 2, 3];
UnmodifiableListView<int> get myList => UnmodifiableListView(_myList);
I have seen a lot of tutorials using dot, while some use 2. What is the actual meaning of this?
Example,
Array().add()
Animation()..addListener(() {})
The .. operator is dart "cascade" operator. Useful for chaining operations when you don't care about the return value.
This is also dart solution to chainable functions that always return this
It is made so that the following
final foo = Foo()
..first()
..second();
Is strictly equals to this:
final foo = Foo();
foo.first();
foo.second();
Just to be a nitpicker, .. isn't actually an operator in Dart, just part of Dart's syntactic sugar.
In addition to the mentioned use of cascades for chaining calls to functions, you can also use it to access fields on the same object.
Consider this code, taken from the Dart documentation:
querySelector('#confirm') // Get an object.
..text = 'Confirm' // Use its members.
..classes.add('important')
..onClick.listen((e) => window.alert('Confirmed!'));
The first method call, querySelector(), returns a selector object. The code that follows the cascade notation operates on this selector object, ignoring any subsequent values that might be returned.
For more information about cascades, check out Dart's outstanding documentation!
I'm looking into blocks at the moment, and they have stumped me.
I used this as an example:
class ProcExample
attr_reader :proc_class
def initialize(&block)
#stored_proc = block
#proc_class = #stored_proc.class
end
def use_proc(arg)
#stored_proc.call(arg)
end
end
eg = ProcExample.new {|t| puts t}
p eg.proc_class
p eg.use_proc("Whoooooooo")
Now I kind of (not really( understand how the block is passed into #stored_proc. I used #proc_class because I was curious what class the block object was actually stored as.
But what if I wanted to store a block in a regular variable?
E.g.:
block_object = {|param| puts param**2}
But I found that this is treated as a Hash and not a block/proc. Naturally an error arises. I've tried assigning it with an ampersand in the variable name, and at the beginning of the block but that doesn't work.
Eventually I was wondering if it was possible to call a function and replace the block with a variable containing the block.
Like so:
(1..10).each block_object
Is this possible in Ruby?
You cannot assign blocks to a variable.
Blocks aren't really objects. They are special syntax for passing code to a higher-order method. If you want a piece of executable code that you can assign to a variable, pass around and manipulate, you need to use a Proc object.
There are two kinds of Procs: lambdas and regular procs. They behave differently in two aspects: argument binding semantics and return semantics. lambdas bind arguments like methods and return returns from the lambda, just like return in a method returns from the method. Regular procs bind arguments like blocks and return returns from the enclosing method, not the proc, just like return in a block.
Regular procs can be created by passing a block to Proc.new or alternatively to Kernel#proc. Lambdas can be created by passing a block to Kernel#lambda or with the "stabby lambda" literal syntax:
lambda_object = ->param { puts param**2 }
In order to convert Procs to blocks and the other way around, Ruby has the unary prefix & modifier. This modifier is only valid in parameter lists and argument lists. When used in a parameter list, it means "wrap the block in a proc and bind it to this variable". When used in an argument list. it means "unwrap this proc into a block (and if it's not a proc already, call to_proc on it first) and pass it as a block argument".
(1..10).each(&lambda_object)
I'm surprised that you haven't already seen the unary prefix & modifier used in this way, it is actually fairly common, e.g. in something like ['1', '2'].map(&:to_s).
Another kind of object that also represents a piece of executable code is a Method object. It supports some of the same interface as Procs do, in particular #call, #to_proc, #arguments, #arity etc. There are two ways to get a Method object: either grab a method that is bound to a receiver from that receiver using the Object#method method or grab an UnboundMethod object from a class or module (e.g. using Module#instance_method) and bind it to a receiver using UnboundMethod#bind which will return a Method object.
Since Method implements to_proc, you can pass it to a method as a block using the unary prefix & modifier, e.g.:
# Warning: extremely silly example :-)
ary = []
(1..10).each(&ary.method(:<<))
ary
# => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
ary = []
(1..10).each(&Array.instance_method(:<<).bind(ary))
ary
# => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
You are looking for a proc object, I believe.
block = proc { ... }
You can use a proc or lambda. There are some subtle differences between them; and between Ruby versions. A good overview can been seen here: https://www.youtube.com/watch?v=VBC-G6hahWA given by Peter Cooper
I'm new to Dart, so maybe I'm missing something here:
This works:
In my main(), I have this:
var a = _someFunction;
var b = _someFunction;
print("${a == b}"); // true. correct!
Where _someFunction is another top-level function.
This does NOT work: (at least not how I'm expecting it to)
Given this class...
class Dummy {
void start() {
var a = _onEvent;
var b = _onEvent;
print(a == b); // false. ???????
}
void _onEvent() {
}
}
Instantiating it from main() and calling its start() method results in false. Apparently a new instance of some function or closure object is created and returned whenever my code obtains a reference to _onEvent.
Is this intentional behaviour?
I would expect that obtaining multiple references to the same method of the same instance returns the same object each time. Perhaps this is intended for some reason. If so; what reason? Or is this a bug/oversight/limitation of VM perhaps?
Thanks for any insights!
Currently, the behaviour seems to be intentional, but the following defect is open since May 2012: https://code.google.com/p/dart/issues/detail?id=144
If I were to guess, I'd say that setting "var a = _onEvent;" creates a bound method, which is some sort of object that contains both the function as well as this. You are asking for bound methods to be canonicalized. However, that would require the team to create a map of them, which could lead to worries about memory leaks.
I think they made "var a = _someFunction;" work early on because they needed static functions to be constants so that they could be assigned to consts. This was so that they could write things like:
const logger = someStaticLoggingFunction;
This was in the days before statics were lazily evaluated.
In any case, I would say that comparing closures for equality is a edge case for most languages. Take all of the above with a grain of salt. It's just my best guess based on my knowledge of the system. As far as I can tell, the language spec doesn't say anything about this.
Actually, now that I've read (https://code.google.com/p/dart/issues/detail?id=144), the discussion is actually pretty good. What I wrote above roughly matches it.
I've come across conversions of the form Array(value), String(value), and Integer(value) on occasion. It appears to me that these are just syntactic sugar for a call to the corresponding value.to_a, value.to_s, or value.to_i methods.
So I'm wondering:
Where/how are these are defined? I can't find them in Object, Module, Class, etc
Are there any common scenarios for which it's preferable to use these rather than the corresponding/underlying to_X method?
Could these be used in type-generic coercion? That is, can I do something along the lines of
[Integer, String, Array].each {|klass| klass.do_generic_coercion(foo) }
? (...and no, I don't really want to do that; I know the type I want out, but I'm looking to avoid the case statement.)
This is a good and difficult question. Let's answer the three parts.
First part
To find the definition, it is important to realize that the name of the method is "Array", etc., which can be quite counterintuitive, since methods are usually lowercase...
irb> method(:Array)
=> #<Method: Object(Kernel)#Array>
This tells you these are defined in Kernel, and thus available everywhere without requiring an explicit prefix.
Second part
Array(), String(),... are conversion methods. Calling obj.to_a will return an array, but will raise an NoMethodError if obj doesn't respond_to? :to_a. So the typical case when you'd prefer using Array(), String(), instead of to_a or to_s is when you are not positive an object responds to a given conversion method.
String(obj) will return nil if obj doesn't respond_to? :to_s. String(obj) will also check that the result of to_s is actually a string; it should be, but maybe an overly creative programmer decided to return something else?
Most other conversion methods act the same way, but Array(obj) is different. It will return [obj] if obj doesn't respond_to? :to_a. It will actually call to_ary (which is the implicit conversion operation, while to_a is the explicit one).
There is another important way to convert objects in 1.9 (and upcoming 1.8.8): Array.try_convert(obj). This returns nil if the obj does not respond_to? :to_ary. It will not call the to_a. Although they are longer to type, you might prefer using them when writing very general code that might accept different types of objects and want to avoid converting a hash to an array by mistake, for example (since Hash has a to_a method but not to_ary). When your method requires an array-like object and you are willing to do an explicit conversion, then obj.to_a is fine. The typical use of Array(obj) would be in a method that accepts either a single obj to act on, or a list of objects (although typically this is written as [*obj]).
Last part
Hopefully, the answers to the first two parts give you your final answer...
You can use:
[Integer, String, Array].each {|klass| klass.try_convert(foo) }
or
[:Integer, :String, :Array].each{|method| send(method, obj)}
Good question! Let's see if we can figure it out.
Ross-Harveys-MacBook-Pro:ruby-1.9.1-p376 ross$ irb
irb(main):001:0> Object.ancestors
=> [Object, Kernel]
irb(main):002:0> Kernel.ancestors
=> [Kernel]
irb(main):003:0> Kernel.class
=> Module
irb(main):004:0> Kernel.public_methods.include? "Array"
=> true
So, it looks like these are methods in the Kernel module that are mixed in to Object, so they are available without specifying a receiver. We might also want to peek at the C implementation, in object.c:
VALUE
rb_Array(VALUE val)
{
VALUE tmp = rb_check_array_type(val);
if (NIL_P(tmp)) {
tmp = rb_check_convert_type(val, T_ARRAY, "Array", "to_a");
if (NIL_P(tmp)) {
return rb_ary_new3(1, val);
}
}
return tmp;
}
One thing seems easy to conclude, the default .to_a is deprecated, so it does seem like Array(x) is the canonical way to do the conversion. It apparently does nothing if given an Array, calls .to_a if that's present, and if not it just wraps its argument in an Array.
Regarding whether to_a is deprecated...well, I said "the default":
Ross-Harveys-MacBook-Pro:puppet_sd ross$ irb
irb(main):001:0> class X; X; end.new.to_a
(irb):1: warning: default `to_a' will be obsolete
They are Defined in Ruby Kernel Module, like:
Array(), Complex(), Float(), Integer(), Rational(), Stirng(), etc.
I found those method references in Dave Thomas's Pickaxe book "Progamming Ruby 1.9", page 555.
For example: Array(arg) will convert arg as an Array, following are copied from the book:
"Returns arg as an Array. First tries to call rg.to_ary, then arg.to_a. If both fail, creates a single element array containing arg( or an empty array if arg is nil)."
ex. Array(1..5) # => [1, 2, 3, 4, 5]
From what I understand, the simple version is like this:
object.to_a tries to convert 'object' to an Array using a class member function.
Array(object) tries to make a new Array using 'object'.
I can re-define what .to_a means for a given class (it is just another member after all). The Array(...) call is defined in Kernel so it behaves the same for any class. I typically use type conversions of the style Array(...) when I don't know ahead of time what type of object will be passed in. It's better at handling cases where an object doesn't know how to convert itself to an array or can't be converted to an array. If the object to be converted is the result of a long or complex expression, using the Array(...) style is often clearer. I save the .to_a form for instances when I know the class of the object and exactly what to expect from the output of .to_a (mostly instances when I have written or modified the .to_a member function myself).