class X extends Y {
X(int a, int b) : super(a,b);
}
Can someone give me an explanation about the syntax meaning of the colon :?
This feature in Dart is called "initializer list".
It allows you to initialize fields of your class, make assertions and call the super constructor.
This means that it is not the same as the constructor body. As I said, you can only initialize variables and only access static members. You cannot call any (non-static) methods.
The benefit is that you can also initialize final variables, which you cannot do in the constructor body. You also have access to all parameters that are passed to the constructor, which you do not have when initializing the parameters directly in the parentheses.
Additionally, you can use class fields on the left-hand of an assignment with the same name as a parameter on the right-hand side that refers to a parameter. Dart will automatically use the class field on the left-hand side.
Here is an example:
class X {
final int number;
X(number) : number = number ?? 0;
}
The code above assigns the parameter named number to the final field this.number if it is non-null and otherwise it assigns 0. This means that the left-hand number of the assignment actually refers to this.number. Now, you can even make an assertion that will never fail (and is redundant because of that, but I want to explain how everything works together):
class X {
final int number;
X(number): number = number ?? 0, assert(number != null);
}
Learn more.
It's ok to access non static member in initializer list.
class Point {
num x, y;
Point(this.x, this.y);
Point.origin(): this.x = 10, this.y = 10;
}
main() {
Point p = Point.origin();
print(p.x); // 10
}
Does Rascal support function pointers or something like this to do this like Java Interfaces?
Essentially I want to extract specific (changing) logic from a common logic block as separate functions. The to be used function is passed to the common block, which then call this function. In C we can do this with function pointers or with Interfaces in Java.
First I want to know how this general concept is called in the language design world.
I checked the Rascal Function Helppage, but this provide no clarification on this aspect.
So e.g. I have:
int getValue(str input) {
.... }
int getValue2(str input){
... }
Now I want to say:
WhatDatatype? func = getValue2; // how to do this?
Now I can pass this to an another function and then:
int val = invoke_function(func,"Hello"); // how to invoke?, and pass parameters and get ret value
Tx,
Jos
This page in the tutor has an example of using higher-order functions, which are the Rascal feature closest to function pointers:
http://tutor.rascal-mpl.org/Rascal/Rascal.html#/Rascal/Concepts/Functions/Functions.html
You can define anonymous (unnamed) functions, called closures in Java; assign them to variables; pass them as arguments to functions (higher-order functions); etc. Here is an example:
rascal>myfun = int(int x) { return x + 1; };
int (int): int (int);
rascal>myfun;
int (int): int (int);
rascal>myfun(3);
int: 4
rascal>int applyIntFun(int(int) f, int x) { return f(x); }
int (int (int), int): int applyIntFun(int (int), int);
rascal>applyIntFun(myfun,10);
int: 11
The first command defines an increment function, int(int x) { return x + 1; }, and assigns this to variable myfun. The rest of the code would work the same if instead this was
int myfun(int x) { return x + 1; }
The second command just shows the type, which is a function that takes and returns int. The third command calls the function with value 3, returning 4. The fourth command then shows a function which takes a function as a parameter. This function parameter, f, will then be called with argument x. The final command just shows an example of using it.
The following code got compilation error:
var a : Int = 0
var b : Int = 3
var sum : Int = 0
while (sum = a+b) < 2 {
}
The error message is:
Cannot invoke '<' with an argument list of type '((()),
IntegerLiteralConvertible)'
How to solve this problem? (Of course I can put sum assignment statement out side the while statement. But this is not convenient. Any other advice? Thanks
In many other languages, including C and Objective-C, sum = a+b would return the value of sum, so it could be compared.
In Swift, this doesn't work. This was done intentionally to avoid a common programmer error. From The Swift Programming Language:
Swift supports most standard C operators and improves several capabilities to eliminate common coding errors. The assignment operator (=) does not return a value, to prevent it from being mistakenly used when the equal to operator (==) is intended.
Since the assignment operator does not return a value, it can't be compared with another value.
It is not possible to overload the default assignment operator (=), but you could create a new operator or overload one of the compound operators to add this functionality. However, this would be unintuitive to future readers of your code, so you may want to simply move the assignment to a separate line.
In most languages, assignments propagate their value -- that is, when you call
sum = a + b
the new value of sum is available for another part of the expression:
doubleSum = (sum = a + b) * 2
Swift doesn't work that way -- the value of sum isn't available after the assignment, so it can't be compared in your while statement. From Apple's documentation:
This feature prevents the assignment operator (=) from being used by
accident when the equal to operator (==) is actually intended. By
making if x = y invalid, Swift helps you to avoid these kinds of
errors in your code.
The other answers explain why your code won't compile. Here is how you can clean it up without calculating sum in the while loop (I'm assuming you want to be able to reassign what sum's getter is, elsewhere.):
var a = 0, b = 3
var getSum = { a + b }
var sum: Int { return getSum() }
while sum < 2 {
...and if you're okay with invoking sum with parentheses:
var a = 0, b = 3
var sum = { a + b }
while sum() < 2 {
You can rewrite it as a for loop, although you'll have to repeat the assignment and addition:
for sum = a+b; sum < 2; sum = a+b {
}
Is there an easier (and cleaner) way to pass an optional parameter from one function to another than doing this:
void optional1({num x, num y}) {
if (?x && ?y) {
optional2(x: x, y: y);
} else if (?x) {
optional2(x: x);
} else if (?y) {
optional2(y: y);
} else {
optional2();
}
}
void optional2({num x : 1, num y : 1}) {
...
}
The one I actually want to call is:
void drawImage(canvas_OR_image_OR_video, num sx_OR_x, num sy_OR_y, [num sw_OR_width, num height_OR_sh, num dx, num dy, num dw, num dh])
At least I don't get a combinatorial explosion for positional optional parameters but I'd still like to have something simpler than lot's of if-else.
I have some code that uses the solution proposed in the first answer (propagating default values of named optional parameters, but I lose the ability to check whether or not the value was provided by the initial caller).
I've been burned by this corner of Dart several times. My current guidelines, which I encourage anyone to adopt are:
Do not use default values. Instead, use the implicit default of null and check for that in the body of the function. Document what value will be used if null is passed.
Do not use the ? argument test operator. Instead, just test for null.
This makes not passing an argument and explicitly passing null exactly equivalent, which means you can always forward by explicitly passing an argument.
So the above would become:
void optional1({num x, num y}) {
optional2(x: x, y: y);
}
/// [x] and [y] default to `1` if not passed.
void optional2({num x, num y}) {
if (x == null) x = 1;
if (y == null) y = 1;
...
}
I think this pattern is cleaner and easier to maintain that using default values and avoids the nasty combinatorial explosion when you need forward. It also avoids duplicating default values when you override a method or implement an interface with optional parameters.
However, there is one corner of the Dart world where this doesn't work: the DOM. The ? operator was designed specifically to address the fact that there are some JavaScript DOM methods where passing null is different from passing undefined (i.e. not passing anything).
If you're trying to forward to a DOM method in Dart that internally uses ? then you will have to deal with the combinatorial explosion. I personally hope we can just fix those APIs.
But if you're just writing your own Dart code, I really encourage you to avoid default values and ? entirely. Your users will thank you for it.
Would positional paremeters be an option four your case?
void optional([num x = 1, num y = 1]) {
}
Since you call optional2 with default parameters anyway, this seems like a good replacement without knowing, what the purpose of the function is
I wonder what this means in F#.
“a function taking an integer, which returns a function which takes an integer and returns an integer.”
But I don't understand this well.
Can anyone explain this so clear ?
[Update]:
> let f1 x y = x+y ;;
val f1 : int -> int -> int
What this mean ?
F# types
Let's begin from the beginning.
F# uses the colon (:) notation to indicate types of things. Let's say you define a value of type int:
let myNumber = 5
F# Interactive will understand that myNumber is an integer, and will tell you this by:
myNumber : int
which is read as
myNumber is of type int
F# functional types
So far so good. Let's introduce something else, functional types. A functional type is simply the type of a function. F# uses -> to denote a functional type. This arrow symbolizes that what is written on its left-hand side is transformed into what is written into its right-hand side.
Let's consider a simple function, that takes one argument and transforms it into one output. An example of such a function would be:
isEven : int -> bool
This introduces the name of the function (on the left of the :), and its type. This line can be read in English as:
isEven is of type function that transforms an int into a bool.
Note that to correctly interpret what is being said, you should make a short pause just after the part "is of type", and then read the rest of the sentence at once, without pausing.
In F# functions are values
In F#, functions are (almost) no more special than ordinary types. They are things that you can pass around to functions, return from functions, just like bools, ints or strings.
So if you have:
myNumber : int
isEven : int -> bool
You should consider int and int -> bool as two entities of the same kind: types. Here, myNumber is a value of type int, and isEven is a value of type int -> bool (this is what I'm trying to symbolize when I talk about the short pause above).
Function application
Values of types that contain -> happens to be also called functions, and have special powers: you can apply a function to a value. So, for example,
isEven myNumber
means that you are applying the function called isEven to the value myNumber. As you can expect by inspecting the type of isEven, it will return a boolean value. If you have correctly implemented isEven, it would obviously return false.
A function that returns a value of a functional type
Let's define a generic function to determine is an integer is multiple of some other integer. We can imagine that our function's type will be (the parenthesis are here to help you understand, they might or might not be present, they have a special meaning):
isMultipleOf : int -> (int -> bool)
As you can guess, this is read as:
isMultipleOf is of type (PAUSE) function that transforms an int into (PAUSE) function that transforms an int into a bool.
(here the (PAUSE) denote the pauses when reading out loud).
We will define this function later. Before that, let's see how we can use it:
let isEven = isMultipleOf 2
F# interactive would answer:
isEven : int -> bool
which is read as
isEven is of type int -> bool
Here, isEven has type int -> bool, since we have just given the value 2 (int) to isMultipleOf, which, as we have already seen, transforms an int into an int -> bool.
We can view this function isMultipleOf as a sort of function creator.
Definition of isMultipleOf
So now let's define this mystical function-creating function.
let isMultipleOf n x =
(x % n) = 0
Easy, huh?
If you type this into F# Interactive, it will answer:
isMultipleOf : int -> int -> bool
Where are the parenthesis?
Note that there are no parenthesis. This is not particularly important for you now. Just remember that the arrows are right associative. That is, if you have
a -> b -> c
you should interpret it as
a -> (b -> c)
The right in right associative means that you should interpret as if there were parenthesis around the rightmost operator. So:
a -> b -> c -> d
should be interpreted as
a -> (b -> (c -> d))
Usages of isMultipleOf
So, as you have seen, we can use isMultipleOf to create new functions:
let isEven = isMultipleOf 2
let isOdd = not << isEven
let isMultipleOfThree = isMultipleOf 3
let endsWithZero = isMultipleOf 10
F# Interactive would respond:
isEven : int -> bool
isOdd : int -> bool
isMultipleOfThree : int -> bool
endsWithZero : int -> bool
But you can use it differently. If you don't want to (or need to) create a new function, you can use it as follows:
isMultipleOf 10 150
This would return true, as 150 is multiple of 10. This is exactly the same as create the function endsWithZero and then applying it to the value 150.
Actually, function application is left associative, which means that the line above should be interpreted as:
(isMultipleOf 10) 150
That is, you put the parenthesis around the leftmost function application.
Now, if you can understand all this, your example (which is the canonical CreateAdder) should be trivial!
Sometime ago someone asked this question which deals with exactly the same concept, but in Javascript. In my answer I give two canonical examples (CreateAdder, CreateMultiplier) inf Javascript, that are somewhat more explicit about returning functions.
I hope this helps.
The canonical example of this is probably an "adder creator" - a function which, given a number (e.g. 3) returns another function which takes an integer and adds the first number to it.
So, for example, in pseudo-code
x = CreateAdder(3)
x(5) // returns 8
x(10) // returns 13
CreateAdder(20)(30) // returns 50
I'm not quite comfortable enough in F# to try to write it without checking it, but the C# would be something like:
public static Func<int, int> CreateAdder(int amountToAdd)
{
return x => x + amountToAdd;
}
Does that help?
EDIT: As Bruno noted, the example you've given in your question is exactly the example I've given C# code for, so the above pseudocode would become:
let x = f1 3
x 5 // Result: 8
x 10 // Result: 13
f1 20 30 // Result: 50
It's a function that takes an integer and returns a function that takes an integer and returns an integer.
This is functionally equivalent to a function that takes two integers and returns an integer. This way of treating functions that take multiple parameters is common in functional languages and makes it easy to partially apply a function on a value.
For example, assume there's an add function that takes two integers and adds them together:
let add x y = x + y
You have a list and you want to add 10 to each item. You'd partially apply add function to the value 10. It would bind one of the parameters to 10 and leaves the other argument unbound.
let list = [1;2;3;4]
let listPlusTen = List.map (add 10)
This trick makes composing functions very easy and makes them very reusable. As you can see, you don't need to write another function that adds 10 to the list items to pass it to map. You have just reused the add function.
You usually interpret this as a function that takes two integers and returns an integer.
You should read about currying.
a function taking an integer, which returns a function which takes an integer and returns an integer
The last part of that:
a function which takes an integer and returns an integer
should be rather simple, C# example:
public int Test(int takesAnInteger) { return 0; }
So we're left with
a function taking an integer, which returns (a function like the one above)
C# again:
public int Test(int takesAnInteger) { return 0; }
public int Test2(int takesAnInteger) { return 1; }
public Func<int,int> Test(int takesAnInteger) {
if(takesAnInteger == 0) {
return Test;
} else {
return Test2;
}
}
You may want to read
F# function types: fun with tuples and currying
In F# (and many other functional languages), there's a concept called curried functions. This is what you're seeing. Essentially, every function takes one argument and returns one value.
This seems a bit confusing at first, because you can write let add x y = x + y and it appears to add two arguments. But actually, the original add function only takes the argument x. When you apply it, it returns a function that takes one argument (y) and has the x value already filled in. When you then apply that function, it returns the desired integer.
This is shown in the type signature. Think of the arrow in a type signature as meaning "takes the thing on my left side and returns the thing on my right side". In the type int -> int -> int, this means that it takes an argument of type int — an integer — and returns a function of type int -> int — a function that takes an integer and returns an integer. You'll notice that this precisely matches the description of how curried functions work above.
Example:
let f b a = pown a b //f a b = a^b
is a function that takes an int (the exponent) and returns a function that raises its argument to that exponent, like
let sqr = f 2
or
let tothepowerofthree = f 3
so
sqr 5 = 25
tothepowerofthree 3 = 27
The concept is called Higher Order Function and quite common to functional programming.
Functions themselves are just another type of data. Hence you can write functions that return other functions. Of course you can still have a function that takes an int as parameter and returns something else. Combine the two and consider the following example (in python):
def mult_by(a):
def _mult_by(x):
return x*a
return mult_by
mult_by_3 = mult_by(3)
print mylt_by_3(3)
9
(sorry for using python, but i don't know f#)
There are already lots of answers here, but I'd like to offer another take. Sometimes explaining the same thing in lots of different ways helps you to 'grok' it.
I like to think of functions as "you give me something, and I'll give you something else back"
So a Func<int, string> says "you give me an int, and I'll give you a string".
I also find it easier to think in terms of 'later' : "When you give me an int, I'll give you a string". This is especially important when you see things like myfunc = x => y => x + y ("When you give curried an x, you get back something which when you give it a y will return x + y").
(By the way, I'm assuming you're familiar with C# here)
So we could express your int -> int -> int example as Func<int, Func<int, int>>.
Another way that I look at int -> int -> int is that you peel away each element from the left by providing an argument of the appropriate type. And when you have no more ->'s, you're out of 'laters' and you get a value.
(Just for fun), you can transform a function which takes all it's arguments in one go into one which takes them 'progressively' (the official term for applying them progressively is 'partial application'), this is called 'currying':
static void Main()
{
//define a simple add function
Func<int, int, int> add = (a, b) => a + b;
//curry so we can apply one parameter at a time
var curried = Curry(add);
//'build' an incrementer out of our add function
var inc = curried(1); // (var inc = Curry(add)(1) works here too)
Console.WriteLine(inc(5)); // returns 6
Console.ReadKey();
}
static Func<T, Func<T, T>> Curry<T>(Func<T, T, T> f)
{
return a => b => f(a, b);
}
Here is my 2 c. By default F# functions enable partial application or currying. This means when you define this:
let adder a b = a + b;;
You are defining a function that takes and integer and returns a function that takes an integer and returns an integer or int -> int -> int. Currying then allows you partiallly apply a function to create another function:
let twoadder = adder 2;;
//val it: int -> int
The above code predifined a to 2, so that whenever you call twoadder 3 it will simply add two to the argument.
The syntax where the function parameters are separated by space is equivalent to this lambda syntax:
let adder = fun a -> fun b -> a + b;;
Which is a more readable way to figure out that the two functions are actually chained.