Problem

I’m writing a CLI tool - called hmm - in Haskell that manages tmux sessions. By default, I want it to open a session named after the current git context. I.e. if I’m working on branch bar in repo foo I want the session to be named foo-bar. However, on rare occasions, I might want to override either value. To do this, I have optional command line flags

$ hmm

Usage: hmm [-r|--repo REPO] [-b|--branch BRANCH]

  Manages tmux sessions.

Available options:
  -r,--repo REPO           First part of the tmux session name.
  -b,--branch BRANCH       Second part of the tmux session name.
  -h,--help                Show this help text

These options are defined as parsers, then collected into a Options object. Don’t worry about what each cryptic symbol means, just know that branch is a parser that will parse an optional CLI flag into a Maybe String.

branch :: Parser (Maybe String)
branch =
  optional $
    strOption
      ( long "branch"
          <> short 'b'
          <> metavar "BRANCH"
          <> help "Second part of the tmux session name."
      )

data Options = Options
  { optRepo :: Maybe String,
    optBranch :: Maybe String
  }

Importantly, because my defaults are calculated at run-time using sub-process (i.e. git) they are ‘impure’ have the type IO String, not String like the parser expects. This means I cannot add the default values to the parser, meaning it needs to parse to a Maybe String type to implement the functionality I want.

Maybe Sidebar

In Haskell, Maybe is a lot like Rust’s Option<T> (especially if you adjust the formatting a bit).

data Maybe T = 
    Nothing 
    | Just (T)

enum Option<T> {
    None,
    Some(T),
}

These types both represent the computational context of a nullable value. They both encapsulate some value - or the lack there of. They force surrounding code to deal with the potentially null value. Not only that, but they also provide a way to turn Maybe a into Maybe b without caring if the encapsulated value is null.

Pattern Matching solutions

The first and most obvious solution solutions was to pattern match on every combination of inputs, for a total of four function signatures. However, this will quickly grow out of hand as more flags are added - at a rate of $n^2$ where n is the number of optional inputs.

entrypoint :: Options -> IO ()
entrypoint Options (r b) = _ -- Both Options provided.
entrypoint Options (r Nothing) = _ -- Only Repo Provided.
entrypoint Options (Nothing b) = _ -- Only Branch Provided.
entrypoint Options (Nothing Nothing) = _ -- No options provided.

The second solution was to build up the entry point by currying partial functions for each option. In the end, I never implemented this solution because it didn’t seem quite right; how would you elegantly curry functions based on what they’re called with? So in the end I looked for different options.

Solution

The final solution was surprisingly simple. Haskell has a function maybe (lowercase ’m’) that is a lot like Rust’s Option::map_or. The docs for maybe and both definitions are below. Note that both functions sill return a value wrapped in a Maybe/Option.

The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result. - Hackage

maybe :: b -> (a -> b) -> Maybe a -> b
maybe n _ Nothing  = n
maybe _ f (Just x) = f x

pub fn map_or<U, F>(self, default: U, f: F) -> U
where
    F: FnOnce(T) -> U,
{
    match self {
        Some(t) => f(t),
        None => default,
    }
}

So, my entry point is as follows:

hmm :: Options -> IO ()
hmm options = do
  r <- maybe computeDefaultRepo return (optRepo options)
  b <- maybe computeDefaultBranch return (optBranch options)
  let sessionName :: String = r ++ "-" ++ b

A few things to note:

  • r and b have types String
  • computeDefaultXYZ has type IO String

Now, lets break apart maybe computeDefaultRepo return (optRepo options). From the signature above, b corresponds to computeDefaultRepo, so it has type IO String. Next, return is the function that maps (a -> b); It has the signature return :: a -> m a. Lastly, (optRepo options) is the Maybe String monad from the parser. Plugging our known types into the maybe signature we get:

  • b = IO String
  • a = String
  • maybe :: (IO String) -> (String -> (IO String)) -> Maybe String -> IO String

Now, we’ve solved our original problem and converted the default and provided values to the same type! Just one last issue. I said that r and b have types String, but I also said that the maybe function returns a IO String. This is where the do and <- notation come in. They allow us to extract the contents from the IO monad on the condition that we return a IO monad.

Summary

Overall, I really enjoyed writing Haskell. It felt more like programming types and structure rather than programming business logic - a breath of fresh air from the Python I’m used to. At the same time Haskell’s compiler was a lot less picky to work with than Rust’s; although the short and simple nature of this program also probably helped.