Functional Programming with D

[Chris Cain]

On Sunday, 14 April 2013 at 00:38:24 UTC, Ali Çehreli wrote:
> 3) "D does not allow you to cast something to immutable"
>
> I think you mean "no implicit conversion" because it is not 
> true if we take what you said literally:
>
>     auto a = [ 1, 2 ];
>     immutable i = cast(immutable)a;  // dangerous
>
>     a[0] = 42;
>     assert(i[0] == 42);  // oops: immutable element has changed!
>
> Ali

Also, the usual way to do this (if you're doing it _properly_, 
this is still vulnerable to being misused) is using 
`assumeUnique` ... the "requirements" for that function are 
implied by its name: it really wants a unique reference that will 
be transformed into an immutable.

```
import std.exception : assumeUnique;

static immutable size_t base = cast(size_t) ('z' - 'a') + 1;

// Remember:
// alias string = immutable(char)[]
string numToString(size_t length, size_t num) {
     char[] result = new char[](length);
     foreach_reverse(ref e; result) {
         auto digit = num % base;
         e = cast(char)('a' + digit);
         num /= base;
     }

     // OK because no other references exist
     return assumeUnique(result);
}
```