Fun and generic things to do with EMGM at the London HUG

I just gave a talk about EMGM at the London HUG. The slides can be found on Scribd or on Github.

Thanks to everybody who showed up! I'm very sorry I was so late. Thanks to Neil for filling in until I got there.

Experiments with EMGM: Emacs org files

I've been meaning to write some things about EMGM for a while, but I hadn't found one of those round tuits as of yet. Until now.

David Miani is working on a Haskell library for interacting with emacs org files. "For those that do not know, an org file is a structured outline style file that has nested headings, text, tables and other elements," says David. He has a collection of datatypes for building and manipulating these files.

David seeks a better way to do what he's doing. (It's a noble goal. I hope you keep doing it.) To return to his words: "While writing an OrgFile is fairly easy, reading (and accessing inner parts) of an org file is very tedious, and modifying them is horrendous." He goes on to give an example that I'll describe more below.

When I read the above statement, I was expecting that generic programming could help him out. When I saw his code, I knew it was a perfect use case. That's what inspired this entry, the first use case for EMGM from Haskell Café.

First, this is a literate Haskell post, so we run through the usual preliminaries.

> {-# LANGUAGE TemplateHaskell       #-}
> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE FlexibleContexts      #-}
> {-# LANGUAGE FlexibleInstances     #-}
> {-# LANGUAGE OverlappingInstances  #-}
> {-# LANGUAGE UndecidableInstances  #-}
> 
> module Org where
> 
> import Text.Regex.Posix

We import Generics.EMGM.Derive for the deriving portion of EMGM. This is not exported from the main body of the library, because it has a lot of symbols only needed for building a representation. We'd rather not clog up your symbol list if possible.

> import Generics.EMGM.Derive

In general, I'd recommend doing the deriving in a separate module and only export the datatype and generated type class instances. Then, in other modules, you can use EMGM functions or write your own. However, this being a demonstration, we will also import Generics.EMGM here to use the available functions.

> import qualified Generics.EMGM as G

The following collection of types are copied from David's post. They describe the structure of an org file.

> type Line = Int
> type Column = Int
> 
> data FilePosition = FilePosition Line Column
> 
> data WithPos a = WithPos { filePos :: FilePosition, innerValue :: a }
> 
> data OrgTableP = OrgTableP [WithPos OrgTableRow]
> 
> data OrgFileElementP
>   = TableP OrgTableP
>   | ParagraphP String
>   | HeadingP OrgHeadingP
> 
> data OrgHeadingP = OrgHeadingP Int String [WithPos OrgFileElementP]
> 
> data OrgFileP = OrgFileP [WithPos OrgFileElementP]
> 
> data OrgTableRow
>   = OrgTableRow [String]
>   | OrgTableRowSep 

In order to use EMGM, we must generate the values and instances used by the library. This is simple with one Template Haskell (TH).

> $(deriveMany 
>   [ ''FilePosition
>   , ''WithPos
>   , ''OrgTableP
>   , ''OrgHeadingP
>   , ''OrgFileElementP
>   , ''OrgFileP
>   , ''OrgTableRow
>   ])

Note that in this case, we had to use deriveMany for a list of type names. For the most part, we'd probably use derive; however, the datatypes OrgHeadingP and OrgFileElementP are mutually recursive. If we use derive for each type, then some values are generated that are naturally also muturally recursive. Apparently, TH expects all symbols to be available on a per-splice basis. This means that we can't $(derive ''OrgFileElementP) and then $(derive ''OrgHeadingP) or vice versa. We have to derive them simultaneously, so that both sets of symbols are available at the same time.

David gives the example of reading "the description line for the project named 'Project14'" in the following file:

* 2007 Projects
** Project 1
Description: 1
Tags: None
** Project 2
Tags: asdf,fdsa
Description: hello
* 2008 Projects
* 2009 Projects
** Project14
Tags: RightProject
Description: we want this

He then provides some messy code to perform it. (No offense meant. Mine would've looked no better.) I'll skip the code, since I couldn't get it to compile as provided.

Our solution using EMGM follows:

> projDesc :: String -> OrgFileP -> Maybe String
> projDesc name file = do
>   hdg <- G.firstr (headings name file)
>   para <- firstPara hdg
>   if para =~ "Description" then return para else Nothing
> 
> headings :: String -> OrgFileP -> [OrgHeadingP]
> headings name = filter check . G.collect
>   where
>     check (OrgHeadingP _ possible _) = name == possible
> 
> firstPara :: OrgHeadingP -> Maybe String
> firstPara hdg = paraStr =<< G.firstr (G.collect hdg)
>   where
>     paraStr (ParagraphP str) = Just str
>     paraStr _                = Nothing

Primarily, we take advantage of the functions collect and firstr. Here, collect is the key. It's type is collect :: (Rep (Collect b) a) => a -> [b], and it returns a list of bs stored somewhere in a value of type a. This allows us to collect the OrgHeadingPs in an OrgFileP (headings) and the OrgFileElementPs in an OrgHeadingP (firstPara). Now, we don't have to build a bunch of functions that break down each of these types to get to their components.

Our use of firstr is simply the same as we would use the Prelude function head, except that firstr returns a Maybe: unlike head, it's a total function.

David's top-level function would now become this:

> get14 :: OrgFileP -> Maybe String
> get14 = projDesc "Project14"

Well, this was a fun experiment with generic programming. I hope to do more in the future.

I want to thank David for bringing up this problem in the mailing list. Not only did I get to play more with EMGM, I also released an update to the library when I discovered the issue requiring deriveMany.

Update 2008-03-30: The source code for this entry is now available at GitHub.

Template Haskell 2.3 or Cabal 1.2? EMGM can't have both!

Updates below!

I'm about ready to give up. I would like EMGM to support both GHC 6.8 and 6.10 to allow for more potential uses. This effectively means it should build with Cabal version 1.2 (which is distributed with GHC 6.8) and 1.6 (distributed with GHC 6.10). I didn't think this would be a difficult problem, — shows you what little I know — but it has turned into a reasonably sized annoyance.

As of version 0.2, EMGM supports generating code for type representations using Template Haskell (TH). Since TH is now a dependency, I thought it would be handy to provide the representation values and instances for it. And so I do that here. If you look at the linked file, you'll notice an #ifdef surrounding $(derive ''Loc). This is because the template-haskell package includes a new Loc datatype in version 2.3, and in order to support versions 2.2 (for GHC 6.8) and 2.3 (for GHC 6.10), I needed to do some funky C preprocessor (CPP) stuff.

Well, I followed the advice given in this thread started by Jason Ducek with useful responses from Duncan Coutts. The result was this emgm.cabal file with a flag for determining which version of template-haskell was available and whether or not to define the necessary macro. I later learned that this does't work when attempting to cabal-install GHC 6.8, because template-haskell-2.3 fails to correctly specify the version of base or GHC that it requires.

Now, to work around this problem, Duncan described how to hook into Cabal to get the actual package dependencies and insert the CPP option into the build info. It was not too difficult to figure this out. And in fact, the code for my Setup.lhs is in the appendix of this article in case it is later useful for me or someone else.

Unfortunately, as soon as I had this implemented, I discovered it didn't work in GHC 6.8.3/Cabal 1.2. There's a very minor difference in Cabal that simply breaks my code, and I don't know how to work around it. The difference is in PackageIdentifier:

Cabal 1.2:

> data PackageIdentifier = PackageIdentifier { pkgName :: String, pkgVersion :: Version }

Cabal 1.6:

> data PackageIdentifier = PackageIdentifier { pkgName :: PackageName, pkgVersion :: Version }
> newtype PackageName = PackageName String

I need PackageIdentifier to determine which version of template-haskell is being used as a dependency. But I either use a String or a PackageName depending on which version of Cabal is used. I don't think there's a way to know which version of Cabal is used when building a Setup.lhs file.

As far as I can tell, my options are the following:

1. Hack more on the Setup.lhs to figure out a different way of dealing with the template-haskell issue.
2. Release for GHC 6.10 only. Note that the problem only occurs when mixing cabal-install and template-haskell. EMGM builds fine with GHC 6.8 in general.
3. Remove the TH deriving code and the CPP macro.
4. Leave things as they are and warn people about the issue. If/when template-haskell gets patched, it may fix the problem.

I'm probably going to go with the last option for now.

Appendix

This is a literate Haskell Setup.lhs containing a build hook for passing a CPP option when a version of the template-haskell package greater than or equal to 2.3 is specified as a dependency. You might find it a useful example of using part of the Cabal library.

> module Main (main) where

> import System.Cmd
> import System.FilePath

> import Data.Version

> import Distribution.Simple
> import Distribution.Simple.LocalBuildInfo
> import Distribution.Simple.Program
> import Distribution.Simple.Setup

> import Distribution.Package
> import Distribution.PackageDescription

> main :: IO ()
> main = defaultMainWithHooks hooks
>   where
>     hooks = simpleUserHooks { buildHook = buildHook' }

> -- Insert CPP flag for building with template-haskell versions >= 2.3.
> buildHook' :: PackageDescription -> LocalBuildInfo -> UserHooks -> BuildFlags -> IO ()
> buildHook' pkg lbi hooks flags =
>   buildHook simpleUserHooks pkg (lbi { localPkgDescr = newPkgDescr }) hooks flags
>   where

>     -- Old local package description
>     oldPkgDescr = localPkgDescr lbi

>     -- New local package description
>     newPkgDescr =
>       case thVersion of
>         Nothing ->
>           oldPkgDescr
>         Just version ->
>           if version >= Version [2,3] []
>           then
>             oldPkgDescr
>               { library = addThCppToLibrary (library oldPkgDescr)
>               , executables = map addThCppToExec (executables oldPkgDescr)
>               }
>           else
>             oldPkgDescr

>     -- Template Haskell package name
>     thPackageName = "template-haskell"

>     -- template-haskell version
>     thVersion = findThVersion (packageDeps lbi)

>     -- CPP options for template-haskell >= 2.3
>     thCppOpt = "-DTH_LOC_DERIVEREP"

>     -- Find the version of the template-haskell package
>     findThVersion []          = Nothing
>     findThVersion (PackageIdentifier (PackageName name) version:ps)
>       | name == thPackageName = Just version
>       | otherwise             = findThVersion ps

>     -- Add the template-haskell CPP flag to a BuildInfo
>     addThCppToBuildInfo :: BuildInfo -> BuildInfo
>     addThCppToBuildInfo bi =
>       bi { cppOptions = thCppOpt : cppOptions bi }

>     -- Add the template-haskell CPP flag to a library package description
>     addThCppToLibrary :: Maybe Library -> Maybe Library
>     addThCppToLibrary ml = do
>       lib <- ml
>       return (lib { libBuildInfo = addThCppToBuildInfo (libBuildInfo lib) })

>     -- Add the template-haskell CPP flag to an executable package description
>     addThCppToExec :: Executable -> Executable
>     addThCppToExec exec =
>       exec { buildInfo = addThCppToBuildInfo (buildInfo exec) }

P.S. I used the recently released pandoc 1.2 for this article. Nice highlighting, eh?

Update #1: Apparently, Blogger's feed and/or Planet Haskell aren't ready for the games I played with pandoc. Thus, I have removed the syntax highlighting. That's unfortunate, because it looked good on the web.

Update #2: Thanks to the magic of the internets and the fact that there are people much smarter than I, there is a solution. In hindsight, it's obvious (of course): dynamically typed programming!

Neil Mitchell sent me this little piece of code which does just the trick:

> mkPackageName :: (Read a) => String -> a
> mkPackageName nm =
>   fst $ head $ reads shownNm ++ reads ("PackageName " ++ shownNm)
>   where
>     shownNm = show nm

So, I plugged this into thPackageName, removed PackageName from the pattern in findThVersion, and—voilà!—it worked.