Additional fixups for extract command

- Rework command line options
- Make specifying a version optional
- Remove stdout option and require a tap to be specified
- Do not allow user to extract into homebrew/core
- Rework new class name generation to use existing Formulary tools

Library/Homebrew/dev-cmd/extract.rb

@@ -1,14 +1,15 @@
-#:  * `extract` [`--force`] [`--stdout`] <formula> <version> [<tap>]:
+#:  * `extract` [`--force`] <formula> `--tap=`<tap> [`--version=`<version>]:
 #:    Looks through repository history to find the <version> of <formula> and
 #:    creates a copy in <tap>/Formula/<formula>@<version>.rb. If the tap is
 #:    not installed yet, attempts to install/clone the tap before continuing.
+#:    A tap must be passed through `--tap` in order for `extract` to work.
 #:
 #:    If `--force` is passed, the file at the destination will be overwritten
 #:    if it already exists. Otherwise, existing files will be preserved.
 #:
-#:    If `--stdout` is passed, the file will be written to stdout on the
-#:    terminal instead of written to a file. A <tap> cannot be passed when
-#:    using `--stdout`.
+#:    If an argument is passed through `--version`, <version> of <formula>
+#:    will be extracted and placed in the destination tap. Otherwise, the most
+#:    recent version that can be found will be used.
 
 require "utils/git"
 require "formula_versions"
@@ -20,51 +21,65 @@ module Homebrew
 
   def extract
     Homebrew::CLI::Parser.parse do
-      switch      "--stdout", description: "Output to stdout on terminal instead of file"
-      switch      :debug
-      switch      :force
+      flag   "--tap="
+      flag   "--version="
+      switch :debug
+      switch :force
     end
 
-    odie "Cannot use a tap and --stdout at the same time!" if ARGV.named.length == 3 && args.stdout?
-    raise UsageError unless (ARGV.named.length == 3 && !args.stdout?) || (ARGV.named.length == 2 && args.stdout?)
+    # If no formula args are given, ask specifically for a formula to be specified
+    raise FormulaUnspecifiedError if ARGV.named.empty?
+
+    # If some other number of args are given, provide generic usage information
+    raise UsageError if ARGV.named.length != 1
+
+    odie "The tap to which the formula is extracted must be specified!" if args.tap.nil?
 
     formula = Formulary.factory(ARGV.named.first)
-    version = ARGV.named[1]
-    destination_tap = Tap.fetch(ARGV.named[2]) unless args.stdout?
-    destination_tap.install unless destination_tap.installed? || args.stdout?
+    if args.version.nil?
+      version = formula.version
+    else
+      version = args.version
+    end
+    destination_tap = Tap.fetch(args.tap)
+    destination_tap.install unless destination_tap.installed?
 
-    unless args.stdout?
-      path = Pathname.new("#{destination_tap.path}/Formula/#{formula}@#{version}.rb")
-      if path.exist?
-        unless ARGV.force?
-          odie <<~EOS
-            Destination formula already exists: #{path}
-            To overwrite it and continue anyways, run `brew extract #{formula} #{version} #{destination_tap.name} --force`.
-          EOS
-        end
-        ohai "Overwriting existing formula at #{path}" if ARGV.debug?
-        path.delete
+    odie "Cannot extract formula to homebrew/core!" if destination_tap.name == "homebrew/core"
+
+    path = Pathname.new("#{destination_tap.path}/Formula/#{formula}@#{version}.rb")
+    if path.exist?
+      unless ARGV.force?
+        odie <<~EOS
+          Destination formula already exists: #{path}
+          To overwrite it and continue anyways, run:
+            `brew extract #{formula} --version=#{version} --tap=#{destination_tap.name} --force`
+        EOS
       end
+      ohai "Overwriting existing formula at #{path}" if ARGV.debug?
+      path.delete
     end
 
-    rev = "HEAD"
-    version_resolver = FormulaVersions.new(formula)
-    until version_resolver.formula_at_revision(rev) { |f| version_matches?(f, version, rev) || rev.empty? } do
-      rev = Git.last_revision_commit_of_file(formula.path.parent.parent, formula.path, before_commit: "#{rev}~1")
+    if args.version.nil?
+      rev = Git.last_revision_commit_of_file(formula.path.parent.parent, formula.path)
+      odie "Could not find #{formula} #{version}!" if rev.empty?
+      version_resolver = FormulaVersions.new(formula)
+    else
+      rev = "HEAD"
+      version_resolver = FormulaVersions.new(formula)
+      until version_resolver.formula_at_revision(rev) { |f| version_matches?(f, version, rev) || rev.empty? } do
+        rev = Git.last_revision_commit_of_file(formula.path.parent.parent, formula.path, before_commit: "#{rev}~1")
+      end
+      odie "Could not find #{formula} #{version}!" if rev.empty?
     end
 
-    odie "Could not find #{formula} #{version}!" if rev.empty?
-
     result = version_resolver.file_contents_at_revision(rev)
 
     # The class name has to be renamed to match the new filename, e.g. Foo version 1.2.3 becomes FooAT123 and resides in Foo@1.2.3.rb.
-    result.gsub!("class #{formula.name.capitalize} < Formula", "class #{formula.name.capitalize}AT#{version.gsub(/[^0-9a-z ]/i, "")} < Formula")
-    if args.stdout?
-      puts result if args.stdout?
-    else
-      ohai "Writing formula for #{formula} from #{rev} to #{path}"
-      path.write result
-    end
+    name = formula.name.capitalize
+    versioned_name = Formulary.class_s("#{name}@#{version}")
+    result.gsub!("class #{name} < Formula", "class #{versioned_name} < Formula")
+    ohai "Writing formula for #{formula} from #{rev} to #{path}"
+    path.write result
   end
 
   # @private