Port KeySet from niecza

src/core/Set.pm

@@ -59,3 +59,51 @@ my class Set is Iterable does Associative {
 sub set(*@args) {
     Set.new(@args);
 }
+
+class KeySet is Iterable does Associative {
+    has %!elems;
+
+    method keys { %!elems.keys }
+    method values { %!elems.values }
+    method elems returns Int { %!elems.elems }
+    method exists($a) returns Bool { %!elems.exists($a) && %!elems{$a} }
+    method Bool { %!elems.Bool }
+    method Numeric { %!elems.Numeric }
+    method hash { %!elems.hash }
+    method at_key($k) {
+        Proxy.new(FETCH => { %!elems.exists($k) ?? True !! False },
+                  STORE => -> $, $value { if $value { %!elems{$k} = True } else { %!elems.delete($k) }});
+    }
+    method exists_key($k) { self.exists($k) }
+    method delete_key($k) { %!elems.delete($k) }
+
+    # Constructor
+    method new(*@args --> KeySet) {
+        my %e;
+        sub register-arg($arg) {
+            given $arg {
+                when Pair { %e{.key} = True; }
+                when Set | KeySet { for .keys -> $key { %e{$key} = True; } }
+                when Associative { for .pairs -> $p { register-arg($p); } }
+                when Positional { for .list -> $p { register-arg($p); } }
+                default { %e{$_} = True; }
+            }
+        }
+
+        for @args {
+            register-arg($_);
+        }
+        self.bless(*, :elems(%e));
+    }
+
+    submethod BUILD (:%!elems) { }
+
+    submethod Str(Any:D $ : --> Str) { "keyset(< %!elems.keys() >)" }
+    submethod gist(Any:D $ : --> Str) { "keyset({ %!elems.keys».gist.join(', ') })" }
+    submethod perl(Any:D $ : --> Str) { 'KeySet.new(' ~ join(', ', map { .perl }, %!elems.keys) ~ ')' }
+
+    method iterator() { %!elems.keys.iterator }
+    method list() { %!elems.keys }
+    method pick($count = 1) { %!elems.keys.pick($count) }
+    method roll($count = 1) { %!elems.keys.roll($count) }
+}

src/core/stubs.pm

@@ -6,7 +6,7 @@
 my class Whatever { ... }
 my class Bag { ... }
 my class KeyBag { ... }
-my class KeySet is Iterable does Associative { }
+my class KeySet { ... }
 my class KeyHash is Iterable does Associative { }
 my class Seq is List does Positional { }
 my class Exception { ... }