Added detection for Prolog and disambiguation for .pl files

lib/linguist/blob_helper.rb

@@ -262,6 +262,9 @@ module Linguist
         # If it's a .r file, try to guess the language
         r_language ||
 
+        # If it's a .pl file, try to guess the language
+        pl_language ||
+
         # See if there is a Language for the extension
         pathname.language ||
 
@@ -304,6 +307,26 @@ module Linguist
       end
     end
 
+    # Internal: Guess language of .pl files
+    # 
+    # Returns a Language.
+    def pl_language
+      return unless extname == '.pl'
+
+      # The rules for disambiguation are: 
+      #
+      # 1. Many perl files begin with a shebang
+      # 2. Most Prolog source files have a rule somewhere (marked by the :- operator)
+      # 3. Default to Perl, because it is more popular
+      if shebang_script == 'perl'
+        Language['Perl']
+      elsif lines.grep(/:-/).any?
+        Language['Prolog']
+      else
+        Language['Perl']
+      end
+    end
+
     # Internal: Extract the script name from the shebang line
     #
     # Requires Blob#data

lib/linguist/languages.yml

@@ -589,6 +589,12 @@ Perl:
   - .perl
   - .psgi
 
+Prolog:
+  major: true
+  extensions:
+  - .pro
+  - .prolog
+
 Pure Data:
   major: true
   lexer: Text only

test/test_blob.rb

@@ -228,6 +228,11 @@ class TestBlob < Test::Unit::TestCase
     assert_equal Language['R'],           blob("hello-r.R").language
     assert_equal Language['Rebol'],       blob("hello-rebol.r").language
 
+    # .pl disambiguation
+    assert_equal Language['Prolog'],      blob("test-prolog.pl").language
+    assert_equal Language['Perl'],        blob("test-perl.pl").language
+    assert_equal Language['Perl'],        blob("test-perl2.pl").language
+
     # ML
     assert_equal Language['OCaml'],       blob("Foo.ml").language
     assert_equal Language['Standard ML'], blob("Foo.sig").language

test/test_language.rb

@@ -81,6 +81,7 @@ class TestLanguage < Test::Unit::TestCase
     assert_equal Lexer['Ooc'], Language['ooc'].lexer
     assert_equal Lexer['PHP'], Language['PHP'].lexer
     assert_equal Lexer['Perl'], Language['Perl'].lexer
+    assert_equal Lexer['Prolog'], Language['Prolog'].lexer
     assert_equal Lexer['Python Traceback'], Language['Python traceback'].lexer
     assert_equal Lexer['Python'], Language['Python'].lexer
     assert_equal Lexer['REBOL'], Language['Rebol'].lexer
@@ -310,6 +311,7 @@ class TestLanguage < Test::Unit::TestCase
     assert Language['Objective-J'].major?
     assert Language['PHP'].major?
     assert Language['Perl'].major?
+    assert Language['Prolog'].major?
     assert Language['Pure Data'].major?
     assert Language['Python'].major?
     assert Language['R'].major?