Haskell – null instead of ==

coding-stylehaskelloperatorstypeclass

I have just started to learn Haskell out of interest. I follow learnyouahaskell.com.

There I found this:

null checks if a list is empty. If it
is, it returns True, otherwise it
returns False. Use this function
instead of xs == [] (if you have a
list called xs)

Why is that? Why should we use null instead of == when both produce the same result?

Thanks.

Best Answer

Comparing the lists with == requires elements to be comparable (denoted as Eq a).

Prelude> :t (==[])
(==[]) :: (Eq a) => [a] -> Bool

For example, [sin] == [] won't work, since you can't compare functions. It might seem stupid, but the type system must judge type of an expression without looking at its value.

An alternate check would be length xs == 0, this doesn't require equality but won't stop if your list is infinite (try length [1..] == 0). That's why there's a dedicated function.

null [] = True
null _ = False

Prelude> :t null
null :: [a] -> Bool     -- Notice lack of (Eq a).

Related Solutions

C# – Can’t operator == be applied to generic types in C#

As others have said, it will only work when T is constrained to be a reference type. Without any constraints, you can compare with null, but only null - and that comparison will always be false for non-nullable value types.

Instead of calling Equals, it's better to use an IComparer<T> - and if you have no more information, EqualityComparer<T>.Default is a good choice:

public bool Compare<T>(T x, T y)
{
    return EqualityComparer<T>.Default.Equals(x, y);
}

Aside from anything else, this avoids boxing/casting.

Javascript – “null coalescing” operator in JavaScript

Update

JavaScript now supports the nullish coalescing operator (??). It returns its right-hand-side operand when its left-hand-side operand is null or undefined, and otherwise returns its left-hand-side operand.

Please check compatibility before using it.

The JavaScript equivalent of the C# null coalescing operator (??) is using a logical OR (||):

var whatIWant = someString || "Cookies!";

There are cases (clarified below) that the behaviour won't match that of C#, but this is the general, terse way of assigning default/alternative values in JavaScript.

Clarification

Regardless of the type of the first operand, if casting it to a Boolean results in false, the assignment will use the second operand. Beware of all the cases below:

alert(Boolean(null)); // false
alert(Boolean(undefined)); // false
alert(Boolean(0)); // false
alert(Boolean("")); // false
alert(Boolean("false")); // true -- gotcha! :)

This means:

var whatIWant = null || new ShinyObject(); // is a new shiny object
var whatIWant = undefined || "well defined"; // is "well defined"
var whatIWant = 0 || 42; // is 42
var whatIWant = "" || "a million bucks"; // is "a million bucks"
var whatIWant = "false" || "no way"; // is "false"

Best Answer

Related Solutions

C# – Can’t operator == be applied to generic types in C#

Javascript – “null coalescing” operator in JavaScript

Clarification

Related Topic