Feature #3675: String#prepend, String#>>
http://redmine.ruby-lang.org/issues/show/3675

起票者: Sora Harakami
ステータス: Open, 優先度: Normal

sorahです.

通常, Stringでの文字列を手前に破壊的に加えるのは, a = "foo" + a 等で行いますが,
直感で読みにくく, 記号などで書きにくくなっていると思います.

なので, String#prependと String#>> を追加するというのはどうでしょうか.

それぞれ以下のように機能します.

a = "bar"
a.prepend("foo") #=> "foobar"
p a              #=> "foobar"

b = "bar"
"foo" >> b       #=> "foobar"
p b              #=> "foobar"

patchはこちらです (git diff --no-prefixにて生成):


diff --git string.c string.c
index 0aa2e6c..6c9be57 100644
--- string.c
+++ string.c
@@ -2055,6 +2055,43 @@ rb_str_concat(VALUE str1, VALUE str2)
     }
 }
 
+/*
+ *  call-seq:
+ *     str.prepend(other_str)  -> str
+ *
+ *  Prepend---Prepend the given string to <i>str</i>.
+ *
+ *  a = "world"
+ *  a.prepend("hello ") #=> "hello world"
+ *  a                   #=> "hello world"
+ */
+
+static VALUE
+rb_str_prepend(VALUE str, VALUE str2)
+{
+    StringValue(str2);
+    StringValue(str);
+    rb_str_update(str, 0L, 0L, str2);
+    return str;
+}
+
+/*
+ *  call-seq:
+ *     str >> other_str -> other_str
+ *
+ *  Prepend---Prepend self string to <i>other_str</i>.
+ *
+ *  a = "world"
+ *  "hello " >> a  #=> "hello world"
+ *  a              #=> "hello world"
+ */
+
+static VALUE
+rb_str_prepend_to_another(VALUE str1, VALUE str2)
+{
+  return rb_str_prepend(str2, str1);
+}
+
 st_index_t
 rb_memhash(const void *ptr, long len)
 {
@@ -7524,6 +7561,8 @@ Init_String(void)
     rb_define_method(rb_cString, "reverse!", rb_str_reverse_bang, 0);
     rb_define_method(rb_cString, "concat", rb_str_concat, 1);
     rb_define_method(rb_cString, "<<", rb_str_concat, 1);
+    rb_define_method(rb_cString, "prepend", rb_str_prepend, 1);
+    rb_define_method(rb_cString, ">>", rb_str_prepend_to_another, 1);
     rb_define_method(rb_cString, "crypt", rb_str_crypt, 1);
     rb_define_method(rb_cString, "intern", rb_str_intern, 0);
     rb_define_method(rb_cString, "to_sym", rb_str_intern, 0);
diff --git test/ruby/test_string.rb test/ruby/test_string.rb
index 02271ce..da2477f 100644
--- test/ruby/test_string.rb
+++ test/ruby/test_string.rb
@@ -1882,4 +1882,36 @@ class TestString < Test::Unit::TestCase
       assert_equal('"\\u3042\\u3044\\u3046"', "\u3042\u3044\u3046".encode(e).inspect)
     end
   end
+
+  def test_prepend_to_another
+    assert_equal(S("hello world"), "hello " >> "world")
+
+    foo = Object.new
+    def foo.to_str
+      "b"
+    end
+    assert_equal(S("ab"), "a" >> foo)
+
+    a = S("world!")
+    b = S("hello ")
+    b >> a
+    assert_equal(S("hello world!"), a)
+    assert_equal(S("hello "), b)
+  end
+
+  def test_prepend
+    assert_equal(S("hello world!"), "world!".prepend("hello "))
+
+    foo = Object.new
+    def foo.to_str
+      "b"
+    end
+    assert_equal(S("ba"), "a".prepend(foo))
+
+    a = S("world")
+    b = S("hello ")
+    a.prepend(b)
+    assert_equal(S("hello world"), a)
+    assert_equal(S("hello "), b)
+  end
 end


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