Currently, this is the process I follow to create a new post:

Yeah, I know, the dates are wrong and I just fixed that, but that’s not the worst part. The worst part is that I have to do this each time I want to create a new post. That’s a bummer! I must automate this!

I’m using hakyll to generate static pages for this blog post. This tool provides a set of functions to run the hakyll compiler. The hakyll function takes a set of rules and it’s ready to go:

main :: IO ()
main = hakyll rules

However, hakyll provides a CLI with a predefined set of commands:

$ stack exec site

Missing: COMMAND

Usage: site [-v|--verbose] COMMAND
  site - Static site compiler created with Hakyll

Available options:
  -h,--help                Show this help text
  -v,--verbose             Run in verbose mode

Available commands:
  build                    Generate the site
  check                    Validate the site output
  clean                    Clean up and remove cache
  deploy                   Upload/deploy your site
  preview                  [DEPRECATED] Please use the watch command
  rebuild                  Clean and build again
  server                   Start a preview server
  watch                    Autocompile on changes and start a preview server.
                           You can watch and recompile without running a server

All of those commands are useful, but I’d love to extend the commands with a new one that starts a new post, something like:

$ stack exec site new-post 'Post name'
POST GENERATED: posts/2019-05-28-Post-name.md

And that’s the goal of this post!

Approach #1

My first approach was to extend the commands from hakyll, but it seems it’s not currently possible. So, I decided to try optparse-applicative and create a command line interface (CLI) on my own. However, I would have had to rewrite a lot of code for the hakyll commands. So, I decided to add a new command and then use the rest of hakyll commands:

data Opts = New String deriving Show

Now, let’s define a Parser:

parser :: Parser Opts
parser =
  New <$>
    subparser
      (command "new" (info (option str (long "name")) (progDesc "New post")))

Unfortunately, execParser or customExecParser from optparse-applicative use handleParseResult that exits when the command is not in parsed. I don’t want to exit on failure, because on failure I want to attempt hakyll commands. Fortunately, optparse-applicative provides execParsePure that allows me to handle the parser result as I want. In this case, I attempted this:

main :: IO ()
main = do
  args <- getArgs
  let parseResult =
        execParserPure
          (prefs showHelpOnError)
          (info (helper <*> parser) fullDesc)
          args
  case parseResult of
    Success (New name) -> mkNewPost name >>= createNewPost
    Failure failure    -> do
      progname <- getProgName
      let (msg, _) = renderFailure failure progname
      putStrLn "# CUSTOM COMMANDS"
      hPutStrLn stderr msg
      putStrLn "\n# HAKYLL COMMANDS"
      hakyll rules
    completionInvoke   -> void $ handleParseResult completionInvoke

execParserPure returns a ParserResult and I simply pattern matched it. If the parsing is successful and parses New name it creates a new post and creates a basic templace with the current date and a given name. If it fails, it renders the parsing failure, prints some messages to separate the results from my custom commands and the ones from hakyll, redirects the parsing error to stderr, and then executes any hakyll command. Finally, if the parser result is Completition, I delegate handleParseResult to take care of that, because I don’t know what to do with that.

Now the results:

$ stack exec site

# CUSTOM COMMANDS
Missing: COMMAND

Usage: site COMMAND

Available options:
  -h,--help                Show this help text

Available commands:
  new                      New post

# HAKYLL COMMANDS
Missing: COMMAND

Usage: site [-v|--verbose] COMMAND
  site - Static site compiler created with Hakyll

Available options:
  -h,--help                Show this help text
  -v,--verbose             Run in verbose mode

Available commands:
  build                    Generate the site
  check                    Validate the site output
  ...

Nice! Now, I can do this:

$ stack exec site new --name "some post"
$ ls posts/
...
2019-06-06-some-post.md

And if you want to execute the hakyll commands:

$ stack exec site watch
CUSTOM COMMANDS
Invalid argument `watch'

Usage: site COMMAND

Available options:
  -h,--help                Show this help text

Available commands:
  new                      New post

HAKYLL COMMANDS
Listening on http://127.0.0.1:8000
Initialising...
  Creating store...
  Creating provider...
  Running rules...
Checking for out-of-date items
Compiling
Success

You are probably thinking: Pfff! That’s gonna print the failure of your parser dummy! That’s terrible -_-

I’m probably going to say: Definatelly, I agree with you. I’m going to look for a better approach but for now that will work.

Thanks for reading.

ByE!