If this is done right it can enable this sort of thing : http://www.davidchristiansen.dk/2014/09/06/pretty-printing-idris/ On Fri, Sep 5, 2014 at 5:11 PM, Alan & Kim Zimmerman wrote:

Hi Neil

Thanks for the feedback.

I am going to start putting together a proof of concept, aiming to identify what annotations are needed to roundtrip source.

The first version will make use of the index into a separate structure scheme, so that it can be used with existing GHC ASTs. Hopefully the information gained will help in understanding what is needed for the changes to the future AST.

The concept I will be working with is a pretty-printer, where relative spacing for the particular elements is derived from the initial SrcSpan information. Any new elements added or changed in the AST can then have only relative information, and the final render should honour the layout from the original.

It may be possible to harmonise this with Chris Done's hindent package, which is a code-specific pretty printer for haskell-src-exts.

Alan

On Sat, Aug 30, 2014 at 11:18 PM, Neil Mitchell wrote:

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Since Alan is trying to do something for HaRe that I want for HLint on top of haskell-src-exts, he asked me for my opinions on the proposal. There seem to be two approaches to take:

* Add SrcSpan's throughout. The HSE approach of having a list of inner source spans is nasty - the details of which source space goes where is entirely undocumented and hard to discover. Even worse, for things like instance, which may or may not have a where after, the number of inner SrcSpan's changes. Simon's idea of hsdo_do_loc is much cleaner, and easily extends to Maybe SrcSpan if the keyword is optional.

* Having the annotation be a type parameter gives much greater flexibility. In particular, it would let you mark certain nodes as being added/deleted. However, since SrcSpan has an Int in it, you can always pass around a separate IntMap and make the SrcSpan really be an index into more detailed information. It's nasty, but only the people who use it pay for it.

Both approaches have disadvantages. You could always combine both ideas, and have a SrcSpan and entirely separately an annotation (which defaults to (), rather than SrcSpanInfo), but maybe that's too much extra baggage on the AST.

Thanks, Neil

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A further use case would be to be able to convert all the locations to be relative, or include a relative portion, so that as tools manipulate

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by adding or removing parts the layout can be preserved.

I think I may need to make a wip branch for this and experiment, it is always easier to comment on concrete things.

Alan

On Thu, Aug 28, 2014 at 10:38 PM, Simon Peyton Jones < simonpj@microsoft.com> wrote:

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I thiink the key question is whether it is acceptable to sprinkle this kind of information throughout the AST. For someone interested in source-to-source conversions (like me) this is great, others may find

it

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intrusive.

It’s probably not too bad if you use record syntax; thus

| HsDo { hsdo_do_loc :: SrcSpan -- of the word "do"

, hsdo_blocks :: BlockSrcSpans

, hsdo_ctxt :: HsStmtContext Name

, hsdo_stmts :: [ExprLStmt id]

, hsdo_type :: PostTcType }

Simon

From: Alan & Kim Zimmerman [mailto:alan.zimm@gmail.com] Sent: 28 August 2014 19:35 To: Richard Eisenberg Cc: Simon Peyton Jones; ghc-devs@haskell.org Subject: Re: GHC AST Annotations

This does have the advantage of being explicit. I modelled the initial proposal on HSE as a proven solution, and I think that they were

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keep it non-invasive, to allow both an annotated and non-annoted AST.

I thiink the key question is whether it is acceptable to sprinkle this kind of information throughout the AST. For someone interested in source-to-source conversions (like me) this is great, others may find it intrusive.

The other question, which is probably orthogonal to this, is whether we want the annotation to be a parameter to the AST, which allows it to be overridden by various tools for various purposes, or fixed as in Richard's suggestion.

A parameterised annotation allows the annotations to be manipulated via something like for HSE:

-- |AST nodes are annotated, and this class allows manipulation of the annotations. class Functor ast => Annotated ast where

-- |Retrieve the annotation of an AST node. ann :: ast l -> l

-- |Change the annotation of an AST node. Note that only the annotation of the node itself is affected, and not -- the annotations of any child nodes. if all nodes in the AST tree are to be affected, use fmap.

amap :: (l -> l) -> ast l -> ast l

Alan

On Thu, Aug 28, 2014 at 7:11 PM, Richard Eisenberg wrote:

For what it's worth, my thought is not to use SrcSpanInfo (which, to me, is the wrong way to slice the abstraction) but instead to add SrcSpan fields to the relevant nodes. For example:

| HsDo SrcSpan -- of the word "do" BlockSrcSpans (HsStmtContext Name) -- The parameterisation is unimportant -- because in this context we never use -- the PatGuard or ParStmt variant [ExprLStmt id] -- "do":one or more stmts PostTcType -- Type of the whole expression

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data BlockSrcSpans = LayoutBlock Int -- the parameter is the indentation level ... -- stuff to track the appearance of any semicolons | BracesBlock ... -- stuff to track the braces and semicolons

The way I understand it, the SrcSpanInfo proposal means that we would have lots of empty SrcSpanInfos, no? Most interior nodes don't need one, I

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Popping up a level, I do support the idea of including this info in the AST.

Richard

On Aug 28, 2014, at 11:54 AM, Simon Peyton Jones <

simonpj@microsoft.com>

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wrote:

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In general I’m fine with this direction of travel. Some specifics:

· You’d have to be careful to document, for every data constructor in HsSyn, what the association between the [SrcSpan] in

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SrcSpanInfo and the “sub-entities” · Many of the sub-entities will have their own SrcSpanInfo wrapped around them, so there’s some unhelpful duplication. Maybe you only want the SrcSpanInfo to list the [SrcSpan]s for the sub-entities (like the syntactic keywords) that do not show up as children in the syntax

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Anyway do by all means create a GHC Trac wiki page to describe your proposed design, concretely.

Simon

From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Alan & Kim Zimmerman Sent: 28 August 2014 15:00 To: ghc-devs@haskell.org Subject: GHC AST Annotations

Now that the landmines have hopefully been cleared from the AST via [1] I would like to propose changing the location information in the AST.

Right now the locations of syntactic markers such as do/let/where/in/of in the source are discarded from the AST, although they are retained in the rich token stream.

The haskell-src-exts package deals with this by means of using the SrcSpanInfo data type [2] which contains the SrcSpan as per the current GHC Located type but also has a list of SrcSpan s for the syntactic markers, depending on the particular AST fragment being annotated.

In addition, the annotation type is provided as a parameter to the AST, so that it can be changed as required, see [3].

The motivation for this change is then

1. Simplify the roundtripping and modification of source by explicitly capturing the missing location information for the syntactic markers.

2. Allow the annotation to be a parameter so that it can be replaced with a different one in tools, for example HaRe would include the tokens for the AST fragment leaves.

3. Aim for some level compatibility with haskell-src-exts so that tools developed for it could be easily ported to GHC, for example exactprint [4].

I would like feedback as to whether this would be acceptable, or if

On Sat, Aug 30, 2014 at 3:32 PM, Alan & Kim Zimmerman wrote: the AST trying to think. the tree? the

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same goals should be achieved a different way.

Regards

Alan

[1] https://phabricator.haskell.org/D157

[2]

http://hackage.haskell.org/package/haskell-src-exts-1.15.0.1/docs/Language-H...

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[3]

http://hackage.haskell.org/package/haskell-src-exts-1.15.0.1/docs/Language-H...

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[4]

http://hackage.haskell.org/package/haskell-src-exts-1.15.0.1/docs/Language-H...

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