When is an IComparer<T> a subtype of a different IComparer<T1>? When T1 is a subtype of T. IComparer is contravariant in its T parameter, which means that IComparer's subtype relationship goes in the opposite direction as T's.
class Animal { /* ... */ }
class Dog : Animal { /* ... */ }...
Incorrect usage:
In the following snippet, the last match will never be used:
let x = 4
match x with
| 1 -> printfn "x is 1"
| _ -> printfn "x is anything that wasn't listed above"
| 4 -> printfn "x is 4"
prints
x is anything that wasn't listed abov...
type Person = {
Age : int
PassedDriversTest : bool }
let someone = { Age = 19; PassedDriversTest = true }
match someone.PassedDriversTest with
| true when someone.Age >= 16 -> printfn "congrats"
| true -> printfn "wait until you are 16"
| false -> p...
As the static keyword is used for accessing fields and methods without an instantiated class, it can be used to declare constants for use in other classes. These variables will remain constant across every instantiation of the class. By convention, static variables are always ALL_CAPS and use unders...
Jagged arrays are arrays that instead of primitive types, contain arrays (or other collections). It's like an array of arrays - each array element contains another array.
They are similar to multidimensional arrays, but have a slight difference - as multidimensional arrays are limited to a fixed nu...
You can also use regular expressions to split a string. For example,
import re
data = re.split(r'\s+', 'James 94 Samantha 417 Scarlett 74')
print( data )
# Output: ['James', '94', 'Samantha', '417', 'Scarlett', '74']
IList<T> is never a subtype of a different IList<T1>. IList is invariant in its type parameter.
class Animal { /* ... */ }
class Dog : Animal { /* ... */ }
IList<Dog> dogs = new List<Dog>();
IList<Animal> animals = dogs; // type error
There is no subtype relat...
By default, Django renders ForeignKey fields as a <select> input. This can cause pages to be load really slowly if you have thousands or tens of thousand entries in the referenced table. And even if you have only hundreds of entries, it is quite uncomfortable to look for a particular entry amo...
Interfaces may have variant type parameters.
interface IEnumerable<out T>
{
// ...
}
interface IComparer<in T>
{
// ...
}
but classes and structures may not
class BadClass<in T1, out T2> // not allowed
{
}
struct BadStruct<in T1, out T2> // not allo...
Delegates may have variant type parameters.
delegate void Action<in T>(T t); // T is an input
delegate T Func<out T>(); // T is an output
delegate T2 Func<in T1, out T2>(); // T1 is an input, T2 is an output
This follows from the Liskov Substitution Principle, w...
If a covariant type appears as an output, the containing type is covariant. Producing a producer of Ts is like producing Ts.
interface IReturnCovariant<out T>
{
IEnumerable<T> GetTs();
}
If a contravariant type appears as an output, the containing type is contravariant. Produc...
The case modifier causes the Scala compiler to automatically generate common boilerplate code for the class. Implementing this code manually is tedious and a source of errors. The following case class definition:
case class Person(name: String, age: Int)
... will have the following code automati...
F|forbidden
Similar to Deny, this flag forces the server to immediately return a 403 Forbidden status code to the requesting browser or client for the request.
Example: Deny access to requests that end with exe:
RewriteRule .exe$ - [F]
G|gone
If a requested resource was available in the past,...
Unity works with hierarchies in order to keep your project organized. You can assign objects a place in the hierarchy using the editor but you can also do this through code.
Parenting
You can set an object's parent with the following methods
var other = GetOtherGameObject();
other.transform.Se...
Destructurling works in many places, as well as in the param list of an fn:
(defn my-func [[_ a b]]
(+ a b))
(my-func [1 2 3]) ;= 5
(my-func (range 5)) ;= 3
Destructuring also works for the & rest construct in the param list:
(defn my-func2 [& [_ a b]]
(+ a b))
(my-func2 1 ...
In oracle, the difference (in days and/or fractions thereof) between two DATEs can be found using subtraction:
SELECT DATE '2016-03-23' - DATE '2015-12-25' AS difference FROM DUAL;
Outputs the number of days between the two dates:
DIFFERENCE
----------
89
And:
SELECT TO_DATE( '201...
Our example array:
arr=(a b c d e f)
Using a for..in loop:
for i in "${arr[@]}"; do
echo "$i"
done
2.04
Using C-style for loop:
for ((i=0;i<${#arr[@]};i++)); do
echo "${arr[$i]}"
done
Using while loop:
i=0
while [ $i -lt ${#arr[@]} ]; do...
