Implement `filter_map` from Ruby2.6.

mrbgems/mruby-enum-ext/mrblib/enum.rb

@@ -826,5 +826,17 @@ module Enumerable
     end
     hash.values
   end
+
+  def filter_map(&blk)
+    return to_enum(:find_index, val) unless blk
+
+    ary = []
+    self.each do |x|
+      x = blk.call(x)
+      ary.push x if x
+    end
+    ary
+  end
+
   alias filter select
 end

mrbgems/mruby-enum-ext/test/enum.rb

@@ -188,3 +188,8 @@ assert("Enumerable#to_h") do
   assert_equal h0, h
   assert_equal({1=>4,3=>8}, c.new.to_h{|k,v|[k,v*2]})
 end
+
+
+assert("Enumerable#filter_map") do
+  assert_equal [4, 8, 12, 16, 20], (1..10).filter_map{|i| i * 2 if i%2==0}
+end