[Haskell-cafe] Simple Parsec example, question

14 Sep 2010

      Simple Parsec example, question

I am learning Parsec and have been studying some great reference and
tutorial sites I have found (much thanks to the authors), including:

http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#UserGuide
http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#ReferenceGuide
http://book.realworldhaskell.org/read/using-parsec.html
http://lstephen.wordpress.com/2007/06/19/first-go-with-parsec/
http://jonathan.tang.name/files/scheme_in_48/tutorial/overview.html
http://www.defmacro.org/ramblings/lisp-in-haskell.html

I'm having trouble coding a simple parser to count the number of
lines in a text file.

"lineCount" fails to compile; the compiler error text is below it.

Any advice, code, etc. would be appreciated.

For those using Gtk2Hs + Glade, I have included the glade file after
the Haskell code, in case you want to try it. (You will need to
remove the leading "> " and fix some lines that the email wrapped.)

If you do wish to offer code, feel free to remove or rewrite: eol,
textLines and lineCount entirely. I'm looking for the simplest
way to code this.

Thanks very much,
-- Peter
...
-- A parsing demo, using:
-- Haskell + Gtk2Hs + Glade (GtkBuilder) + Parsec
module Main where
-- import Data.IORef
import Graphics.UI.Gtk
import Graphics.UI.Gtk.Builder
import Graphics.UI.Gtk.Selectors.FileChooser
-- import System.Cmd          -- e.g., for invoking a shell cmd
import System.Glib.GError
import Text.ParserCombinators.Parsec
main :: IO ()
main = do
   initGUI
-- create builder; load UI file
   builder <- builderNew
   handleGError (\(GError dom code msg) -> fail msg) $
      builderAddFromFile builder "demo.glade"
      -- Error message would look something like:
      -- app.exe: user error (Failed to open file 'app.glade':
      --    No such file or directory)
-- get widget handles       (reduce boilerplate?)
   mainWindow     <- builderGetObject builder castToWindow "mainWindow"
   pickFileButton <-
      builderGetObject builder castToFileChooserButton "pickFileButton"
   parseButton    <- builderGetObject builder castToButton "parseButton"
   exitButton     <- builderGetObject builder castToButton "exitButton"
-- signal handlers --
-- parse selected file
   onClicked parseButton $ do
      file <- fileChooserGetFilename pickFileButton
case file of
         Nothing -> do
            putStrLn "\nPlease first select a file."
            return ()
         Just file -> do
            putStrLn $ "\nParsing file: " ++ show file
            result <- parseFromFile lineCount file
            case (result) of
               Left err -> print err
               Right x -> putStrLn $ "Line count = " ++ show x
            return ()
-- exit
   onDestroy mainWindow mainQuit
   onClicked exitButton mainQuit
-- go
   widgetShowAll mainWindow
   mainGUI
-----------------
eol = char '\n'
--      from RWH; perhaps use in future:
-- eol =   try (string "\n\r")
--     <|> try (string "\r\n")
--     <|> string "\n"
--     <|> string "\r"
--      "end of line"
textLines = endBy eol
lineCount :: Parser Int
lineCount = do
   xs <- textLines
   return (length xs)
-- demo.hs:72:3:
--     Couldn't match expected type `GenParser Char () Int'
--            against inferred type `GenParser Char st sep -> b'
--     In a stmt of a 'do' expression: xs <- textLines
--     In the expression:
--         do { xs <- textLines;
--              return (length xs) }
--     In the definition of `lineCount':
--         lineCount = do { xs <- textLines;
--                          return (length xs) }
-- demo.glade follows --
...
<?xml version="1.0"?>
<interface>
  <requires lib="gtk+" version="2.16"/>
  <!-- interface-naming-policy project-wide -->
  <object class="GtkWindow" id="mainWindow">
    <property name="visible">True</property>
    <property name="title" translatable="yes">demo v.8</property>
    <child>
      <object class="GtkVBox" id="vbox1">
        <property name="visible">True</property>
        <property name="border_width">6</property>
        <property name="orientation">vertical</property>
        <property name="spacing">10</property>
        <child>
          <object class="GtkLabel" id="label1">
            <property name="visible">True</property>
            <property name="tooltip_text" translatable="yes">You can hover over the buttons below for some information about them.
You can also resize this window, to make the buttons bigger.
</property>
            <property name="label" translatable="yes">Demo: Haskell + Gtk2Hs + Glade (GtkBuilder) + Parsec
</property>
          </object>
          <packing>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkHBox" id="hbox1">
            <property name="visible">True</property>
            <property name="spacing">5</property>
            <child>
              <object class="GtkFrame" id="pickFileframe">
                <property name="visible">True</property>
                <property name="tooltip_text" translatable="yes">Use this File Chooser widget to select the file to parse.
</property>
                <property name="label_xalign">0</property>
                <property name="shadow_type">none</property>
                <child>
                  <object class="GtkAlignment" id="alignment1">
                    <property name="visible">True</property>
                    <property name="left_padding">12</property>
                    <child>
                      <object class="GtkFileChooserButton" id="pickFileButton">
                        <property name="visible">True</property>
                        <property name="title" translatable="yes">Select file</property>
                      </object>
                    </child>
                  </object>
                </child>
                <child type="label">
                  <object class="GtkLabel" id="pickFileLabel">
                    <property name="visible">True</property>
                    <property name="label" translatable="yes"><b>Select file to parse:</b></property>
                    <property name="use_markup">True</property>
                  </object>
                </child>
              </object>
              <packing>
                <property name="position">0</property>
              </packing>
            </child>
            <child>
              <object class="GtkButton" id="parseButton">
                <property name="label" translatable="yes">P_arse</property>
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">True</property>
                <property name="tooltip_text" translatable="yes">Use this to parse the file you have selected.</property>
                <property name="use_underline">True</property>
              </object>
              <packing>
                <property name="position">1</property>
              </packing>
            </child>
            <child>
              <object class="GtkButton" id="exitButton">
                <property name="label" translatable="yes">E_xit</property>
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">True</property>
                <property name="tooltip_text" translatable="yes">Exit this program and close both windows.</property>
                <property name="use_underline">True</property>
              </object>
              <packing>
                <property name="position">2</property>
              </packing>
            </child>
          </object>
          <packing>
            <property name="position">1</property>
          </packing>
        </child>
      </object>
    </child>
  </object>
</interface>
-- end --

[Haskell-cafe] Simple Parsec example, question

Peter Schmitz