Feature #11781


Would it be possible to alias .prepend() towards .unshift() for class Array by default?

Added by shevegen (Robert A. Heiler) almost 7 years ago. Updated almost 7 years ago.

Target version:



For Strings we can do:

abc = 'world!'
abc[0,0] = 'Hello '
abc # => "Hello world!"

For Arrays we can do:

abc = ['world!']
abc[0,0] = 'Hello '
abc # => ["Hello ", "world!"]

This is nice.

For Strings we can also use .prepend() to add to the beginning.

For Arrays, we have to use .unshift().

I have a hard time remembering .unshift though, .prepend() seems
to be easier for me to remember.

I'd like to use both .prepend for Strings and Arrays; right now
I have to use different names. I could alias prepend to unshift
for class Array, but then I'd have to carry these modifications
into my projects, which is not so good - I would prefer to just
stick to what MRI is doing.

Could we have the alias .prepend() for class Array, meaning
.unshift() too? That way I could use .prepend() for both Arrays
and Strings.

Thanks for reading!

Updated by danielpclark (Daniel P. Clark) almost 7 years ago

prepend is already a keyword in the Ruby language for prepending a module in the ancestry chain:

Updated by matz (Yukihiro Matsumoto) almost 7 years ago

It sounds nice. Since it is right before 2.3 release, it may be too late for it.


Updated by normalperson (Eric Wong) almost 7 years ago

I prefer we avoid introducing needless aliases.

It increases human cognitive overhead for reviewing/auditing
code and also wastes machine memory + CPU cycles at startup.

Updated by nobu (Nobuyoshi Nakada) almost 7 years ago

Their arities differ.
Let String#prepend take any number of arguments?

diff --git i/array.c w/array.c
index dd14837..b98142d 100644
--- i/array.c
+++ w/array.c
@@ -5832,6 +5832,7 @@ Init_Array(void)
     rb_define_method(rb_cArray, "pop", rb_ary_pop_m, -1);
     rb_define_method(rb_cArray, "shift", rb_ary_shift_m, -1);
     rb_define_method(rb_cArray, "unshift", rb_ary_unshift_m, -1);
+    rb_define_method(rb_cArray, "prepend", rb_ary_unshift_m, -1);
     rb_define_method(rb_cArray, "insert", rb_ary_insert, -1);
     rb_define_method(rb_cArray, "each", rb_ary_each, 0);
     rb_define_method(rb_cArray, "each_index", rb_ary_each_index, 0);
diff --git i/string.c w/string.c
index e6df91d..d0f6454 100644
--- i/string.c
+++ w/string.c
@@ -2691,21 +2691,28 @@ rb_str_concat(VALUE str1, VALUE str2)
  *  call-seq:
- *     str.prepend(other_str)  -> str
+ *     str.prepend(other_str, ...)  -> str
  *  Prepend---Prepend the given string to <i>str</i>.
  *     a = "world"
  *     a.prepend("hello ") #=> "hello world"
  *     a                   #=> "hello world"
+ *
+ *     a = "world"
+ *     a.prepend("hello", " ") #=> "hello world"
+ *     a                       #=> "hello world"
 static VALUE
-rb_str_prepend(VALUE str, VALUE str2)
+rb_str_prepend(int argc, VALUE *argv, VALUE str)
-    StringValue(str2);
-    StringValue(str);
-    rb_str_update(str, 0L, 0L, str2);
+    int i;
+    for (i = argc; i > 0;) {
+	VALUE str2 = argv[--i];
+	StringValue(str2);
+	rb_str_update(str, 0L, 0L, str2);
+    }
     return str;
@@ -9379,7 +9386,8 @@ 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, "prepend", rb_str_prepend, -1);
+    rb_define_method(rb_cString, "unshift", rb_str_prepend, -1);
     rb_define_method(rb_cString, "crypt", rb_str_crypt, 1);
     rb_define_method(rb_cString, "intern", rb_str_intern, 0); /* in symbol.c */
     rb_define_method(rb_cString, "to_sym", rb_str_intern, 0); /* in symbol.c */
diff --git i/test/ruby/test_string.rb w/test/ruby/test_string.rb
index eed7c69..853ef7c 100644
--- i/test/ruby/test_string.rb
+++ w/test/ruby/test_string.rb
@@ -2194,6 +2194,14 @@
     assert_equal(S("hello world"), a)
     assert_equal(S("hello "), b)
+    a = S("world")
+    b = S("hel")
+    c = S("lo ")
+    a.prepend(b, c)
+    assert_equal(S("hello world"), a)
+    assert_equal(S("hel"), b)
+    assert_equal(S("lo "), c)
   def u(str)

Updated by sawa (Tsuyoshi Sawada) almost 7 years ago

String#prepend and String#concat are a pair that work similarly (with just the difference on where the new substring is inserted). And since there is already Array#concat, introducing Array#prepend would suggest that they should work similarly. However, while Array#concat takes an array and uses that as sub-array of the original array, the proposed Array#prepend takes an object and uses that as an element of the original array. This is confusing. I don't think the proposal is good.

Updated by matthewd (Matthew Draper) almost 7 years ago

I too would expect Array#prepend to be the opposite of Array#concat, taking other_ary as a parameter, because that seems to more closely match the relationship between String#prepend and String#concat.

It sounds potentially useful in its own right.


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