Shall I use immutable or const while passing parameters to functions

[John Colvin via Digitalmars-d-learn]

On Tuesday, 7 April 2015 at 15:11:39 UTC, tcak wrote:
> I have data in memory, and I want a function to take a part of 
> data for processing only. It will only read and won't change.
>
> char[] importantData;
>
>
> With Immutable,
>
> void dataProcessor( string giveMeAllYourData ){}
>
> dataProcessor( cast( immutable )( importantData[5 .. 14] ) );
>
>
>
> With Const,
>
> void dataProcessor( in char[] giveMeAllYourData ){}
>
> dataProcessor( cast( const )( importantData[5 .. 14] ) );
>
>
> (Code with const wasn't tested)
>
> Which one is better? I didn't understand it very well in 
> http://dlang.org/const3.html this page.

const means "I will not modify this via this variable" and 
immutable means "no one will modify this from anywhere"

Don't cast to immutable unless you are sure what you are doing. 
Immutable is a strong guarantee, it's easy to break it by 
accident if you're casting things. The safest way of getting an 
immutable array is with the .idup property.

You don't have to cast to const most of the time. mutable is 
implicitly convertible to const:

void foo(const int[] a){}

void main()
{
	int[] a = [1,2,3,4];
	foo(a);
	foo(a[0..2]);
}

Also, see https://archive.org/details/dconf2013-day01-talk02

In general, immutable in an API allows greater freedom for the 
implementation, const allows for greater freedom/convenience for 
the API user. Which is more important depends on your exact 
problem.