Specify Haskell project templates and generate files

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In my last blog post, I discussed one aspect of my work with FP Complete: the goal of creating a Haskell IDE. Since then, I’ve gotten lots of incredible feedback from the community, and in particular have been in email discussion with some of the major players in the Haskell IDE scene. I think it’s safe to say that we all agree that there’s going to be a large amount of overlap in our efforts, and we will be coordinating to try and minimize duplicated work as much as possible, while still providing for the unique goals of each IDE project.

In response to all this, I’ve created a Wiki on Github to keep track of our goals, and a Haskell IDE Google Group for discussion. I strongly recommend joining if you’re interested in any more sophisticated Haskell code editing tools.

We have a lot of topics to cover, and a single blog post won’t be nearly enough to even scratch the surface. For now, I’d like to focus on one specific feature. I haven’t chosen to start here because it’s the most important feature, but because I think it’s a problem that we can solve relatively easily and thoroughly.

Project Templates

Most (all?) IDEs provide the concept of a project template: instead of writing all of the code for a project from scratch, you select a template, answer a few questions, and a bunch of files are automatically generated. We already have this in the Haskell world: Yesod provides the project scaffolded (via the yesod init command), and I believe Snap provides something like this as well. But these are just two examples. I’m sure we could easily come up with a dozen other possible templates: a GTK+ application, a web services client, or a console app.

Currently, there’s no standard for how this should work in the Haskell world or, to my knowledge, in the non-Haskell world either. (If there is, please let me know, I’d like to be able to build on existing work.) The Yesod scaffolding (and Snap’s I believe) are both generated via specialized command line tools. I’m sure each IDE would be fully capable of building wrappers for for two tools, but that quickly becomes an existential complexity issue. It also makes it much more difficult for someone to start providing a new scaffolding. I know we suffer from this already in the Yesod world, where innovation is definitely stifled by having the One True Blessed Scaffolding.

So here are my goals for the ideal templating system:

  • A single file to represent a template. This can be some kind of archive (ZIP file, tarball, etc), I don’t really care, but single file systems simplify things greatly.

  • Provide a Haskell library for both generating and consuming these templates. We can have a command line tool as a wrapper around the library, but the library should be the primary means of interacting. (You’ll see this as a pattern as I talk more about the IDE world.)

  • Build on top of commonly used formats as much as possible. The reasoning here is that, even though we’ll be providing a canonical Haskell library, not all IDEs are written in Haskell (yet). If someone is writing an IDE in Python and wants to provide Haskell support, we should make it as easy as possible.

    • By the way, it’s worth pointing out that, as described, there’s nothing Haskell-centric about my proposal here. I’ve been going in the direction of creating language-agnostic tools and formats as much as possible (e.g., keter, which can host web apps written in any language).
  • I’m guessing that the most common way that people will want to actually provide a template is as a Git(hub)/Darcs repo. It would be great if we could provide a web service that takes a repo and automatically generates a template file. Then users of an IDE could theoretically just type in a repo URL to some text box and automatically get the most recent code available.

  • Similarly, we should provide a simple command-line tool that takes a folder and generates a template file.

A semi-concrete proposal

As many of you know, I normally prefer to discuss actual working code/ideas than to discuss theoretical ideas. In this case, however, I think it’s worth fleshing out the idea a bit before jumping in and implementing something. So I’m going to lay out my proposal here, and ask for everyone’s input and recommendations before we start implementation. I recommend the discussion be targeted at the Haskell IDE Google Group as much as possible.

For file format: let’s use JSON. I’m not worried about file size: these project template files will likely be transferred over HTTP most of the time, and compression can be performed at that level. As for binary files, we’ll base64-encode the contents.

The JSON file needs to have three sections:

  1. Metadata describing the project template itself. This would be the name of the template, a description, author, homepage, and maybe a version. (Version could be automatically generated as the date it was created.) This is all pretty boring.

  2. Data that needs to be collected from the user. In the Yesod scaffolding, we ask for the user’s name, the project name, and the database backend to use. The first two are (mostly) free-form text, while the third is an enumeration. I think we’ll need to support a few basic datatypes:

    • Text, with a regex for validation.
    • Booleans
    • Enumerations

    We can also allow default values. So to model the Yesod scaffolding, perhaps something like this:

        [ {"name":"user-name","type":"text","validation":".+","description":"Your name"}
        , {"name":"project-name","type":"text","validation":"[\w_]+","description":"Name of your project"}
        , {"name":"database-backend","type":"enumerator","choices":
            "description":"Name of your project"}
  3. The files that will be generated. We need to take into account some issues:

    1. Some files will be generated conditionally based on the input from the user.
    2. Some of the files will be named based on the user input (e.g., the name of the cabal file).
    3. The actual contents of the file will depend on the user input (e.g., the contents of the cabal file).
    4. We want to support both textual and binary files. Binary files need not have any conditional aspect to them.

    For the first issue, we’ll need to have a basic expression language. I think equality, inequality, and, or, parantheses and variables should be sufficient. So to say that the file config/postgres.yml should only be generated if the database backend is postgresql, we could have something like:

     "contents":"We'll discuss this in a moment...",
     "condition":"database-backend == 'postgresql'"

    For the conditional file naming, how about something like this:


    In order to solve the third point, we’ll use a combination of what we’ve established for points 1 and 2:

        [ {"content":"name: "},
          {"content":"\n     , postgresql-simple >= 0.3 && < 0.4","condition":"database-backend == 'postgresql'"}

    The last one is easiest to solve: each file can have a field encoding which is either “text” or “base64”.


Once we have the file format figured out, the library is relatively simple. Let’s describe a simple consumption API:

data CodeTemplate
instance FromJSON CodeTemplate

data UserInputType = UIText (Maybe Regex) | UIEnumeration [(Text, Text)] | UIBool
data UserInput = UserInput
    { uiType :: UserInputType
    , uiName :: Text
    , uiDescription :: Text

userInputs :: CodeTemplate -> [UserInput]
generateFiles :: CodeTemplate -> Map Text Text -> Map FilePath LByteString

Setting up a generation API for dealing with completely static files should be simple. It will be a bit more involved to deal with conditionals, but with properly defined ADTs it shouldn’t be too bad.

Next steps

I think the most important next step is to determine what use cases my proposal doesn’t cover. The file creating code specifically doesn’t allow many common text generation techniques, like looping, as I simply see no use case for it, but perhaps I’m mistaken. I’m also curious to hear what other ideas people have for project templates.


  • Use throwM instead of monadThrow
  • Support for conduit 1.3.0


  • Drop system-filepath