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Update design notes for CPP.

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Facundo Domínguez 2020-04-24 17:34:08 -03:00 committed by Mark Karpov
parent fc64eada5c
commit 5bffeffa05
2 changed files with 15 additions and 14 deletions
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22
DESIGN.md
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@ -24,7 +24,7 @@ It also includes recommendations for future implementers.
We set for the following goals (mostly taken from
[brittany](https://github.com/lspitzner/brittany)):
* Preserve the meaning of the formatted functions;
* Preserve the meaning of the formatted functions when no CPP is used;
* Make reasonable use of screen space;
* Use linear space and computation time on the size of the input;
* Preserve comments;
@ -140,8 +140,10 @@ 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.
We allow CPP directives in the input, but we forgo the goal to preserve the
meaning of the formatted functions in that case.
Instead of supporting CPP better, we hope for a solution to replace CPP to
do conditional compilation.
There are the following challenges when formatting a module with CPP:
@ -180,14 +182,15 @@ 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:
or Ormolu produces the same output we would get if the CPP directives
were considered comments:
```
$ hindent --version
hindent 5.2.7
$ ormolu --version
ormolu 0.0.5.0 HEAD fc64eada5c4da7a5b07d2872e253671b48aec115
using ghc-lib-parser 8.10.1.20200412
$ hindent test.hs
$ ormolu --mode inplace test.hs
$ cat test.hs
{-# LANGUAGE CPP #-}
@ -227,9 +230,6 @@ 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

7
README.md
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@ -116,8 +116,9 @@ independent top-level definitions.
* CPP support is experimental. CPP is virtually impossible to handle
correctly, so we process them as a sort of unchangeable snippets. This
works only in simple cases when CPP conditionals are self-contained. Use
Ormolu with CPP at your own risk.
works only in simple cases when CPP conditionals surround top-level
declarations. See the [CPP][design-cpp] section in the design notes for
a discussion of the dangers.
* Input modules should be parsable by Haddock, which is a bit stricter
criterion than just being valid Haskell modules.
* Various minor idempotence issues, most of them are related to comments.
@ -154,7 +155,7 @@ See [LICENSE.md][license].
Copyright © 2018–present Tweag I/O
[design]: https://github.com/tweag/ormolu/blob/master/DESIGN.md#cpp
[design-cpp]: https://github.com/tweag/ormolu/blob/master/DESIGN.md#cpp
[contributing]: https://github.com/tweag/ormolu/blob/master/CONTRIBUTING.md
[license]: https://github.com/tweag/ormolu/blob/master/LICENSE.md
[haskell-src-exts]: https://hackage.haskell.org/package/haskell-src-exts