ormolu/DESIGN.md

Ormolu

• Analysis of the existing solutions
  • Brittany
  • Hindent
  • Stylish Haskell
  • Haskell formatter
• Proposed design
  • Parsing
  • CPP
  • Printing
  • Configuration
  • Handling of language extensions
  • Testing
  • Functionality of executable
  • Why not contribute to/fork Hindent or Brittany?
• Roadmap

This document describes design of a new formatter for Haskell source code. It also includes recommendations for future implementers.

We set for the following goals (mostly taken from brittany):

• Preserve the meaning of the formatted functions;
• Make reasonable use of screen space;
• Use linear space and computation time on the size of the input;
• Preserve comments;
• Be idempotent.

Analysis of the existing solutions

In order to design a new formatter we need to study the existing solutions so we can borrow the good bits and avoid making the same mistakes.

Brittany

Brittany builds on top of ghc-exactprint—a library that uses parser of GHC itself for parsing and thus it guarantees that at least parsing phase is bug-free (which is admittedly the cause of majority of bugs in other projects, see below).

ghc-exactprint is capable of dealing not only with vanilla Haskell source code, but also with CPP. However it does so by running the preprocessor with parameters provided by user (such as values of #defines). It doesn't parse actual preprocessor directives in a way that would allow their reconstruction, thus Brittany still lacks support for handling source code with CPP in it. That said, we'll show later supporting CPP correctly is virtually impossible anyway.

After parsing, Haskell AST and a collection of annotations are available. The annotations are there because Haskell AST doesn't provide enough information to reconstruct source code (for example it doesn't include comments). Brittany's approach then amounts to manipulation of these entities using a stateful monad in which very low-level transformations are described.

After the manipulations a few functions from ghc-exactprint are used for rendering of code together with some custom logic. I do not fully understand why the function exactPrint from ghc-exactprint is not used directly.

The code is hard to read and is buggy too, as one would expect with such approach. There are enough bugs in Brittany at the moment so that it's hardly usable although it's now 2 years old. Looking at the opened bugs it's clear that almost all of them are because of the too-low-level approach which seems to be very fragile.

Hindent

Hindent uses haskell-src-exts for parsing like all older projects. haskell-src-exts does not use parser of GHC itself, and is a source of endless parsing bugs. Hindent is affected by these upstream issues as well as Stylish Haskell and Haskell formatter (see below). This already makes all these projects unusable with some valid Haskell source code, but let's continue studying Hindent anyway.

Hindent is quite different from Brittany in the sense that it does not attempt to do manipulations on source code and annotations to print them afterwards, instead it just prints the parsed code straight away. This means that maybe 70-80% of the code does printing, the package is essentially is about pretty-printing parsed Haskell code.

Perhaps surprisingly, this approach seems to be better (IMO), for several reasons:

1. Brittany cannot guarantee that the output will be “canonical”. With given configuration, if we pass it different inputs expressing identical Haskell programs modulo spacing and indentation, by design it may produce different outputs:

   input0   input1   ...   inputN
      |       |              |
   output0  output1  ...   outputN
   

   While with the approach taken by Hindent we get rather this:

   input0   input1   ...   inputN
     |        |              |
      \       |             /
       \      |            /
        \     |           /
         \    |          /
          \   |         /
           \  |        /
        canonical output
   

   Because there is one way to pretty print parsed program. This means that the users won't need to think about the layout at all because it'll be 100% determined by the pretty-printer after the transformation.

2. The code is easier to read and debug. Pretty-printing functions are very straightforward. If there is a bug (in pretty-printer, not in parser which Hindent cannot control), it's easy to fix AFAIU.

Hindent is also notable for its ability to handle CPP and inputs that do not constitute complete modules. It splits input stream into so-called “code blocks” recognizing CPP macros and then only pretty-prints “normal code” without touching CPP directives. After that CPP is inserted between pretty-printed blocks of source code. The approach fails when CPP breaks code in such a way that separate blocks do not form valid Haskell expressions, see this for example.

Looking at the bug tracker there are many bugs. Part of them is because of the use of haskell-src-exts, the other part is because the maintainer doesn't care (anymore?) and doesn't fix them. Well it's as simple as that, with any sort of commercial backing the bugs in pretty printer would be fixed long time ago.

Stylish Haskell

Stylish Haskell also uses haskell-src-exts and suffers from the same upstream problems. I haven't studied the transformations it performs, but it looks like it transforms the parsed source code partially by manipulating AST and partially by manipulating raw text (e.g. to drop trailing whitspace from each line). CPP Macros are just filtered out silently as a preprocessing step before feeding the code to haskell-src-exts.

Stylish Haskell is not so invasive as the other formatters and most reported bugs are about parsing issues and CPP. As I understand it, people mostly use it to screw their import lists.

Haskell formatter

Haskell formatter is an older project that didn't get much traction. It also uses haskell-src-ext and also tries to do manipulations on the parsed AST. The issue tracker doesn't have many issues probably because it never got popular enough (only 15 stars on GitHub). All the issues are about upstream problems with haskell-src-exts.

Proposed design

This section describes a solution that combines all the good things from the projects above.

Parsing and CPP

It is clear that ghc-exactprint is better than haskell-src-exts, so we should use that. If we go with ghc-exactprint though, we'll need to specify which parser to use, e.g. the parser that parses whole module or the one which parsers declarations/expressions/etc. It seems that in general it's better to use the parser for modules because it should work with all input files containing complete modules, while with other parsers it's impossible to guess what they'll be called on.

CPP

Formatting a module which uses CPP directives won't be supported. Instead, we hope for a solution to replace CPP to do conditional compilation.

There are the following challenges when formatting a module with CPP:

• GHC parser won't accept anything but a valid, complete module.

• To preserve CPP directives they must be either removed from input and saved in some sort of structure or turned into a special sort of comments and restored later, but that is not easy because then the formatting logic itself should somehow be aware of CPP directives (mainly conditiionals) and avoid performing transformations that may lead to incorrect Haskell in the end. This is much, much harder than simple pretty-printing we want to do.

Here is an example that proves that CPP is hard to support if possible at all. Let's suppose that we want to format the following snippet of code:

f = f1
  where
        f1 = g
#if C1
g = g1
  where
    g1 = g2
      where
        g2 = False
#else
        g = True
#end

After commenting CPP directives we get:

f = f1
  where
        f1 = g
{-[#if C1]-}
g = g1
  where
    g1 = g2
      where
        g2 = False
{-[#else]-}
        g = True
{-[#end]-}

Then, after formatting we get:

f = f1
  where
    f1 = g
{-[#if C1]-}
g = g1
  where
    g1 = g2
      where
        g2 = False
{-[#else]-}
        g = True
{-[#end]-}

And finally, we uncomment CPP directives:

f = f1
  where
    f1 = g
#if C1
g = g1
  where
    g1 = g2
      where
        g2 = False
#else
        g = True
#end

Now the definition of f is broken when C1 doesn't hold.

Therefore, CPP should not be supported. If the CPP extension is enabled, we should signal an error right away.

Printing

Just pretty-printing code (following the approach of Hindent) seems sane. It is straightforward and when complemented with enough tests (see the section about testing below), it should work all right.

Implementation can be just a Reader monad producing something like text builder. The context of Reader can store current indentation and configuration options.

As a pretty-printing library we can use Outputable (and SDoc) from the ghc package itself. The benefit is that AST components that we'll want to print are already instances of Outputable, so we'll get correct renderings for free.

Configuration

There should be some configuration but at this point I'll leave this section empty. Personally I'm OK with imposing “one good style” and only allowing users to tweak things like page width in columns and indentation levels. Others will probably disagree. I generally like the philosophy of this post by Chris Done (the author of Hindent) which says that as long as the default style is conventional and good it doesn't really matter how code gets formatted. Consistency is more important.

Handling of language extensions

Some language extensions affect how parsing is done. We are going to deal with those in two ways:

• When language pragmas are present in source file, we must parse them before we run the main parser (I guess) and they should determine how the main parsing will be done.
• There also should be configuration file that may enable other language extensions to be used on all files.
• Later we could try to locate Cabal files and fetch the list of extensions that are enabled by default from there.

Testing

It should be possible to add tests incrementally as we develop pretty-printing code and new issues are discovered. For each Haskell module that we want to test, we perform the following steps:

1. Given input snippet of source code parse it and pretty print it.
2. Parse the result of pretty-printing again and make sure that AST is the same as AST of original snippet module span positions. We could make this part of a self-check in the formatter.
3. Check the output against expected output. Thus all tests should include two files: input and expected output.
4. Check that running the formatter on the output produces the same output again (the transformation is idempotent).

In order to grow our testsuite, we would borrow test cases from test suites of existing libraries like Brittany and Hindent. Then we may add test cases for opened issues that Brittany and Hindent have.

When we're confident enough, we can start “mining” new issues by running the program on real source code from Hackage till we don't get new issues anymore. For every issue that we find this way, a test case should be added.

Functionality of executable

• In all cases the program should test if the produced AST is the same as the one we originally parsed and if it differs, an error should be displayed suggesting reporting this on our issue tracker.
• Check mode: return non-zero exit code if any transformations would be applied.
• Modification in place and printing of formatted code to stdout.
• A flag for version/commit information.
• An option to specify location of config file.
• Options to specify parameters that come from config files on command line instead, for example indent levels, maximal line width, etc.

Why not contribute to/fork HIndent or Brittany?

• Forking or contributing to Brittany is not a good idea because this would require re-doing of all transformation logic, which is harder than writing pretty-printing code from scratch. Good documentation would help readability of the code somewhat, but then we would need to spend time either collaborating with the original author or investigating how everything works ourselves.

  Of course with sufficient persistence we could succeed in fixing Brittany's bugs, but the point is that pretty-printing à la Hindent is more maintainable (IMO) and switching to that is equal to re-doing the project.

  In the end, design of Ormolu is going to be simpler and will:

  • make the project more maintainable
  • bugs easier to fix
  • more people will be able to contribute to our project (I couldn't figure out what is going on in Brittany, so I'd not be able to contribute, but I could contribute to Hindent because I understand how it works even though I spent equal amount of time looking at both)

• Forking or contributing to Hindent is not an option because if we replace haskell-src-exts with ghc (or ghc-exact-print) then we'll have to work with a different AST type and all the code in Hindent will become incompatible and there won't be much code to be re-used in that case. It is also possible that we'll find a nicer way to write pretty-printer.

Roadmap

Proposed roadmap (for a single person, about 39 full-time work days):

• Create and setup a repo, setup CI (less than 1 day).
• Implement parsing (2 days).
• Implement basis for pretty-printing of code. We could take this as inspiration, although AST from the ghc package may be slightly different. Also, don't forget about annotations and comments (1 week).
• Implement executable program so we can easier fool around. Try it on simple code samples (2 days).
• Implement location and reading of configuration file in YAML format (2 days).
• Continue writing the pretty-printing code till we cover everything. Start adding tests, see the section about testing. Fix bugs. At this point just repeat this till we cover everything that Brittany and Hindent can do maybe by stealing their tests (at least 2 weeks).
• Start checking how our formatter works on real-world code from Hackage. Fix bugs that we find that way (2 weeks).
• Write a good readme explaining why the project was created and how it is better, etc (2 days).
• Improve the executable by adding more interesting options (2 days).
• Switch CI to Travis and test with multiple GHC versions (1 day).
• Make and announce the first public release on Twitter/Reddit (1 day).
• Figure out how to use the formatter application with major text editors and add the info to the readme (no estimate).