`?　reciever.method` => メソッドのドキュメント表示

map はc言語で実装されてるらしい

[1] pry(main)> ? [1,2,3].map

From: array.c (C Method):
Owner: Array
Visibility: public
Signature: map()
Number of lines: 9

Invokes block once for each element of self. Creates a
new array containing the values returned by the block.
See also Enumerable#collect.

If no block is given, an enumerator is returned instead.

   a = [ "a", "b", "c", "d" ]
   a.collect {|x| x + "!" }   #=> ["a!", "b!", "c!", "d!"]
   a                          #=> ["a", "b", "c", "d"]

presence は便利メソッドがいっぱいあるgemactivesupportで定義されている。
説明もわかりやすい。

[2] pry(main)> ? "".presence

From: /$HOME/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/activesupport-5.1.4/lib/active_support/core_ext/object/blank.rb @ line 27:
Owner: Object
Visibility: public
Signature: presence()
Number of lines: 16

Returns the receiver if it's present otherwise returns nil.
object.presence is equivalent to

   object.present? ? object : nil

For example, something like

  state   = params[:state]   if params[:state].present?
  country = params[:country] if params[:country].present?
  region  = state || country || 'US'

becomes

  region = params[:state].presence || params[:country].presence || 'US'

return [Object]

`$　reciever.method` => メソッドのソースコード表示

さっき見た通りc言語で実装されているmapメソッド

[3] pry(main)> $ [1,2,3].map

From: array.c (C Method):
Owner: Array
Visibility: public
Number of lines: 13

static VALUE
rb_ary_collect(VALUE ary)
{
    long i;
    VALUE collect;

    RETURN_ENUMERATOR(ary, 0, 0);
    collect = rb_ary_new2(RARRAY_LEN(ary));
    for (i = 0; i < RARRAY_LEN(ary); i++) {
	rb_ary_push(collect, rb_yield(RARRAY_PTR(ary)[i]));
    }
    return collect;
}

presence もしっかり定義されている

[4] pry(main)> $ "".presence

From: /$HOME/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/activesupport-5.1.4/lib/active_support/core_ext/object/blank.rb @ line 43:
Owner: Object
Visibility: public
Number of lines: 3

def presence
  self if present?
end