* Explain more of the rationale for excluding CPP. * Change the CPP example to change the meaning. Use hindent. * Fix a typo
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Ormolu
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:
-
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.
-
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. Therefore, formatting the Haskell code between CPP directives is not an option.
-
Ignoring the CPP directives and formatting the Haskell code can change the meaning of the Haskell code. An example follows.
Let's suppose that we want to format the following snippet of code:
$ cat test.hs
main = print (g && f1)
where
f1 = h
where
h = True
#ifdef C1
g = g1
where
g1 = g2
where
g2 = False
#else
g = True
#endif
#ifndef C1
g = False
#endif
$ runhaskell -XCPP test.hs
True
At the time of this writing, formatting this program with hindent
produces the same output we would get if the CPP
directives were
considered comments:
$ hindent test.hs
$ cat test.hs
main = print (g && f1)
where
f1 = h
where
h = True
#ifdef C1
g = g1
where
g1 = g2
where
g2 = False
#else
g = True
#endif
#ifndef C1
g = False
#endif
$ runhaskell -XCPP test.hs
False
Running the formatter causes the output of the program to change
from True
to False
when C1
is not defined.
A solution could be to make the formatter more careful with CPP directives, constraining how directives can be inserted in Haskell code to avoid changing the meaning by reformatting. But this would introduce additional complexity, and the problem would need to be solved repeteadly for every tool out there which wants to parse Haskell modules. If CPP is replaced with some language extension or mechanism to do conditional compilation, all tools will benefit from it.
Therefore, CPP won't 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:
- Given input snippet of source code parse it and pretty print it.
- 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.
- Check the output against expected output. Thus all tests should include two files: input and expected output.
- 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
withghc
(orghc-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).