Why aren't variables interpolated in `constant` declarations?

Question

use v6.d;

my Str $foo = 'Hello';
my constant $BAR = "--$foo--";
say $BAR;

OUTPUT:

Use of uninitialized value element of type Str in string context.
Methods .^name, .raku, .gist, or .say can be used to stringify it to something meaningful.
  in block  at deleteme.raku line 4
----

Expected OUTPUT:

--Hello--

The same thing happens without the my or with our in place of my.

[187] > $*DISTRO
macos (12.6)
[188] > $*KERNEL
darwin
[189] > $*RAKU
Raku (6.d)

---

Answer 1

The value assigned to a constant is evaluated at compile time, not runtime. This means that constant values can be calculated as part of the compilation and cached.

Regular assignments happen at runtime. Thus, in:

my Str $foo = 'Hello';
my constant $BAR = "--$foo--";

The assignment to $foo has not taken place at the time "--$foo--" is evaluated. By contrast, if $foo were to be a constant, the value is available at compile time and interpolated, so:

my constant $foo = 'Hello';
my constant $BAR = "--$foo--";
say $BAR;

Produces:

--Hello--