How can I determine if something is a Perl 6 container?

Question

I have no useful reason to know this, but I was wondering if I have a name like $x if it was a mutable thingy or just a name for something (an immutable thing), I'm not trying anything to do; this is curiosity.

I played with something like this:

 my @a = 'a', ('b', 'c' ); my @b = 'd', 'e', 'f', @a; my @ab = @a, @b, ( 1, 2 ); for @ab { put "name: {.^name} VAR: { .VAR.^name }"; when .VAR.can('FETCH') { put "Container: $_" } when .VAR.can('STORE') { put "Container: $_" } when Scalar { put "Scalar container: $_" } when Array { put "Array Container: $_" } default { put "Object: $_" } } 

I get:

 name: Array VAR: Scalar Array Container: abc name: Array VAR: Scalar Array Container: defabc name: List VAR: Scalar Object: 1 2 

I see that I can match Array type types, but I thought that checking that something could be FETCH or STORE would be a valid match.

Answer 1

Not sure what you are asking here, but given your input, the result looks as expected.

A simplified example to illustrate:

 my (@a, @b, @c); my @ab = @a, @b, @c; for @ab { say "{.^name} contained in a {.VAR.^name}"; } 

which will output

 Array contained in a Scalar 

three times, regardless of the contents of @a , @b and @c .

Remember that in Perl6 there is no implicit smoothing, and the mutmility of an array (unrelated arrays) is realized by placing its elements in scalar containers.

---

Given the change in your question, perhaps the following code will again explain what happens:

 # make argument raw so binding won't mess with it sub info($_ is raw) { dd $_; put " value type: {.^name}"; put " VAR type: {.VAR.^name}"; put " can fetch: {so .VAR.can('FETCH')}"; put " can store: {so .VAR.can('STORE')}"; } my \values = 42, my $ = 42, [1, 2], (1, 2); info $_ for values; put "\n"; # put values in array, which wraps them in a scalar container info $_ for [values]; 

Note that only lists and arrays can STORE (but not FETCH ), while scalars cannot.

This may come as a surprise: first, we could expect all assignees to provide STORE .

Looking at the implementation, we see that for variables with the sigil @ and % we make a STORE call , but not so for variables with the sigil $ . If we rode along the rabbit hole, we would end up specifying a VM-level container , that is, as assigning scalar containers (or calling the STORE object's Proxy method).