Maintaining Optparse-Applicative

Posted on February 28, 2017

I’m the current maintainer of optparse-applicative, and have been since about about August 2015. It was already a fantastic library when I took over looking after it, and Paolo Capriotti really deserves all the credit for the nature of the library. It’s been an absolute honour to look after optparse, and I’m very pleased to have been trusted to do so.

In this post, I’ll talk about some of the changes I have made, and my approach to my role as maintainer.

My first priority with optparse-applicative is to keep it simple, not be too eager to merge things, and break as little as possible. As optparse-applicative is used for the creation of applications, it’s very hard to know its penetration and number of users, but it’s quite significant. My second priority is that it remains an exact library, with well defined semantics, which encourage the creation of good CLI interfaces. Finally, I’d like to keep it being the best command line parsing library, not only in Haskell, but for any language out there.

Error Messages

One of my first patches, with 0.12, was good error messages for when the user misses a mandatory term. There used to be particularly nasty situations one could get themselves into when subcommands were involved, where the parse would fail with no error message, and the item they were missing wasn’t even mentioned in the usage line they were shown.

Now when something is missing, we have an error message like

Missing: (-a | -b)

for example.

With 0.13, the implementation of this feature became much nicer. Now, when the parser fails, the error data type is filled with the partially completed, existentially wrapped parser.

data SomeParser = forall a . SomeParser (Parser a)

data ParseError
  = MissingError ... SomeParser
  | ...

This SomeParser can be traversed, and the usage line printed for it printed (including only options with no default value).

In addition to a few extra error message clean ups and additions, I believe that there are now no cases where the parser can fail and one won’t receive a useful error message. From a usability standpoint this is a big deal.

Unix style grouping of short options

Most command line tools allow short options to be grouped together,

tar -xcf

for instance is a valid command. This is pretty much a posix requirement, and it was a bit embarrassing that optparse didn’t support it.

Fixing this was my first patch optparse. It works by popping off the rest of the flags from the command if the first is a flag and not an option. So when considering -xcf, if it notices that -x is a flag, it will parse it, then on the next step of the parse, examine -cf.


This one hasn’t landed in HEAD yet, but was kind of fun. When I realised that I can use the same machinery we use for bash completion and the missing: error message generation it became quite easy to implement cleanly. When an option, flag, or subcommand can’t be parsed, or more specifically no match for it could be found, we include not only the unknown term, but also the existentially wrapped parser in the intermediate error data type.

data ParseError
  = MissingError ... SomeParser
  | UnexpectedError String SomeParser
  | ...

Then, we examine the SomeParser, but instead of including only mandatory options, we include only reachable options. A list of potential terms is generated, and they filtered based on the Damerau–Levenshtein distance to the unrecognised option the user provided.

The end result looks a lot like git’s command completion, and is quite nice I think

Invalid argument `installl'

Did you mean this?
Invalid option `-x'

Did you mean one of these?

I might do a bit more around short options, at the moment the whole term is considered, but it might be good to also compare just the short term too.

Command Groups

This addition allows one to separate out logical groups of subcommands based on some semantic reason. Some potential uses would be to separate common from unusual examples for instance; or safe from dangerous.

This sort of idea has been available with other libraries for a while, so it seemed like time to add it to optparse-applicative as well.

The idea is simple enough, one can specify that this set of subcommands belongs to a particular group. There was however a bit of extra work to ensure that the usage text will remain exactly the same if the user doesn’t provide these terms, no matter how they defined their parser.

Here’s a quick example:

Usage: commands COMMAND

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

Available commands:
  hello                    Print greeting
  goodbye                  Say goodbye

French commands:
  bonjour                  Print greeting
  au-revoir                Say goodbye


These are all pretty minor changes which don’t really change the way that a programmer interacts with the library at all. They just deliver a nicer set of error messages, usability improvements and better and usage help text.