I am new to Dart and I can see Dart has num which is the superclass of int and double (and only those two, since it's a compile time error to subclass num to anything else).
So far I can't see any benefits of using num instead of int or double, are there any cases where num is better or even recommended? Or is it just to avoid thinking about the type of the number so the compiler will decide if the number is an int or a double for us?
One benefit for example
before Dart 2.1 :
suppose you need to define a double var like,
double x ;
if you define your x to be a double, when you assign it to its value, you have to specify it say for example 9.876.
x = 9.876;
so far so good.
Now you need to assign it a value like say 9
you can't code it like this
x = 9; //this will be error before dart 2.1
so you need to code it like
x = 9.0;
but if you define x as num
you can use
x = 9.0;
and
x = 9;
so it is a convenient way to avoid these type mismatch errors between integer and double types in dart.
both will be valid.
this was a situation before Dart 2.1 but still can help explain the concept
check this may be related
Not sure if this is useful to anyone, but I just ran into a case where I needed num in a way.
I defined a utility function like this:
T maximumByOrNull<T, K extends Comparable<K>>(Iterable<T> it, K key(T),) {
return it.isEmpty
? null
: it.reduce((a, b) => key(a).compareTo(key(b)) > 0 ? a : b);
}
And invoking it like this…
eldest = maximumByOrNull(students, (s) => s.age);
… caused trouble when age is an int, because int itself does not implement Comparable<int>.
So Dart cannot infer the type K in the invocation to maximumByOrNull.
However, num does implement Comparable<num>. And if I specified:
eldest = maximumByOrNull(students, (s) => s.age as num); // this
eldest = maximumByOrNull<Student, num>(students, (s) => s.age); // or this
the complaint went away.
Bottom line: it seems num implements Comparable when int and double do not, themselves, and sometimes this causes trouble for generic functions.
A good use case of num are extensions that work with int and double.
As an example I include the extension MinMax on List<num> that provides the getters min and max.
extension MinMax on List<num>{
/// Returns the maximum value or `null` if the list is empty.
num get max {
return (isNotEmpty)
? fold<num>(0, (prev, current) => (prev > current) ? prev : current)
: null;
}
/// Returns the minimum value or `null` if the list is empty.
num get min {
return (isNotEmpty)
? fold<num>(0, (prev, current) => (prev < current) ? prev : current)
: null;
}
}
Using the extension above one can access the min/max values without a need to create specific implementations for the classes int and double.
void main() {
final a = <int>[1,3,5];
final b = <double>[ 0.5, 0.8, -5.0];
print(a.min);
print(b.max);
}
I just ran into a use case when it is useful.
My app stores weight, which was originally defined as a double.
When using with a local database (sqlite) it works fine, since sqlite handles integer and real types.
However, when I converted my app to use Firestore database, I ran into issues with all my double fields. If a decimal value is stored everything works fine. However, when the weight happens to be a whole number, Firestore returns it as an int and suddenly type errors - int is not a subtype of type double - start to appear.
In the above scenario changing the double fields and variables to num was a quite simple solution.
I was trying to create a Lex program that does the following:
INPUT
float hey1(int a, int b);
OUTPUT
Function: hey1
it returns: float
Signature:
a - float
b - float
With the current knowledge I have of LEX, I decided to input the test file and output the 1st part to another file, which would become the input for the next part. Kindly guide me forward, as I have come to a dead end, and can't proceed forward.
My attempt :-
%{
FILE *out;
%}
letter [A-Za-z0-9]
%%
("float"|"void"|"int")" "{letter}*"(" fprintf(out,"Function: %s", yytext) ;
%%
int main()
{
yyin = fopen("test.c","r") ;
out=fopen("out.c","w");
yylex() ;
fclose(yyin) ;
}
Thanks
To keep structures clear is it possible to name them. So essentially I asking for a 'struct' in Rascal. So eg:
list[tupple[map[str,int],int]]
to:
treeLabel :: str
occurences :: int
treeData :: map[treeLabel,int]
treeNode :: tupple[treeData,int]
tree :: list[treeNode]
tree x=[];
Tx
Jos
How about using Abstract Data Types?
See Rascal Tutor. The above could then look like this:
data MyStruct = ms(str treeLabel,
int occurrence,
map[treeLabel, int] treeData,
tuple[TreeData td, int n] treeNode,
list[TreeNode] tree);
given some variable m with a myStruct value you can access elements with the usual dot notation:
m.treeLabel;
m.treeLabel = "xyz";
etc.
I am writing D2 bindings for Lua. This is in one of the Lua header files.
typedef int (*lua_CFunction) (lua_State *L);
I assume the equivalent D2 statement would be:
extern(C) alias int function( lua_State* L ) lua_CFunction;
Lua also provides an api function:
void lua_pushcfunction( lua_State* L, string name, lua_CFunction func );
If I want to push a D2 function does it have to be extern(C) or can I just use the function?
int dfunc( lua_State* L )
{
std.stdio.writeln("dfunc");
}
extern(C) int cfunc( lua_State* L )
{
std.stdio.writeln("cfunc");
}
lua_State* L = lua_newstate();
lua_pushcfunction(L, "cfunc", &cfunc); //This will definitely work.
lua_pushcfunction(L, "dfunc", &dfunc); //Will this work?
If I can only use cfunc, why? I don't need to do anything like that in C++. I can just pass the address of a C++ function to C and everything just works.
Yes, the function must be declared as extern (C).
The calling convention of functions in C and D are different, so you must tell the compiler to use the C convention with extern (C). I don't know why you don't have to do this in C++.
See here for more information on interfacing with C.
It's also worth noting that you can use the C style for declaring function arguments.
Yes, your typedef translation is correct. OTOH have you looked at the htod tool?
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.