Reserve 'on'.

compiler/src/Lexer.hs

@@ -33,6 +33,7 @@ import Token
 keywords =
     [ (InKeyword, "in")
     , (OfKeyword, "of")
+    , (OnKeyword, "on")
     , (AsKeyword, "as")
     , (WithKeyword, "with")
     , (FromKeyword, "from")

compiler/src/Parser.hs

@@ -49,6 +49,7 @@ tokenErrorString MinusSign = "\"-\""
 tokenErrorString ExclamationPoint = "\"!\""
 tokenErrorString InKeyword = "keyword \"in\""
 tokenErrorString OfKeyword = "keyword \"of\""
+tokenErrorString OnKeyword = "keyword \"on\""
 tokenErrorString AsKeyword = "keyword \"as\""
 tokenErrorString WithKeyword = "keyword \"with\""
 tokenErrorString FromKeyword = "keyword \"from\""

compiler/src/Token.hs

@@ -50,7 +50,8 @@ data Token = Identifier String
            | MinusSign
            | ExclamationPoint
            | InKeyword
-           | OfKeyword
+           | OfKeyword    -- We reserve some common, short English words for use as future keywords.
+           | OnKeyword
            | AsKeyword
            | WithKeyword
            | FromKeyword