added Semigroup instance for Parser a where a is a Semigroup (#38)

* added Semigroup instance for Parser a where a is a Semigroup

* added @paf31's suggestions

Author: dwhitney

src/Text/Parsing/StringParser.purs

@@ -3,6 +3,8 @@
 module Text.Parsing.StringParser where
 
 import Prelude
+
+import Control.Apply (lift2)
 import Control.MonadPlus (class MonadPlus, class MonadZero, class Alternative)
 import Control.Monad.Rec.Class (class MonadRec, tailRecM, Step(..))
 import Control.Plus (class Plus, class Alt)
@@ -98,3 +100,6 @@ fail msg = Parser \{ pos } -> Left { pos, error: ParseError msg }
 -- | `try p` backtracks even if input was consumed.
 try :: forall a. Parser a -> Parser a
 try (Parser p) = Parser \(s@{ pos }) -> lmap (_ { pos = pos}) (p s)
+
+instance semigroupParser :: Semigroup a => Semigroup (Parser a) where
+  append = lift2 append

test/Main.purs

@@ -51,8 +51,9 @@ exprTest = buildExprParser [ [Infix (string "/" >>= \_ -> pure div) AssocRight]
 
 tryTest :: Parser String
             -- reduce the possible array of matches to 0 or 1 elements to aid Array pattern matching
-tryTest = try ((<>) <$> string "aa" <*> string "bb") <|>
-          (<>) <$> string "aa" <*> string "cc"
+tryTest = 
+  try (string "aa" <> string "bb") <|> 
+      (string "aa" <> string "cc")
 
 canParse :: forall a. Parser a -> String -> Boolean
 canParse p input = isRight $ runParser p input