sets: adding ?adjoin.

core/sets/sets-docs.factor

@@ -87,6 +87,11 @@ HELP: adjoin
 }
 { $side-effects "set" } ;
 
+HELP: ?adjoin
+{ $values { "elt" object } { "set" set } { "?" "a boolean" } }
+{ $description "A version of " { $link adjoin } " which returns whether the element was added to the set." }
+{ $notes "This is slightly less efficient than " { $link adjoin } " due to the initial membership test." } ;
+
 HELP: delete
 { $values { "elt" object } { "set" set } }
 { $description "Destructively removes " { $snippet "elt" } " from " { $snippet "set" } ". If the element is not present, this does nothing." $nl "Each mutable set type is expected to implement a method on this generic word." }

core/sets/sets.factor

@@ -132,6 +132,9 @@ M: sequence cardinality
 : without ( seq set -- subseq )
     tester [ not ] compose filter ;
 
+: ?adjoin ( elt set -- ? )
+    2dup in? [ 2drop f ] [ adjoin t ] if ; inline
+
 ! Temporarily for compatibility
 
 : unique ( seq -- assoc )