MarLinn,
   Thanks for correcting me, and spelling this out. 
   I did mean what Alan mentioned: "re-parsing a pretty printed parse tree gives you back a parse tree identical to the original (ignoring SrcSpans)".
   As I recall, we had to go a bit further to give 'Something' some more structure to take into account things like "(ignoring SrcSpans)" (e.g., to define exact-printers, etc).
   Provided I have failed twice to properly recall the invariant, I refrain from trying to recall the rest tonight :) 

Not diverging from my point above, as far as I understand, an ideal `Outputable` machinery is going to be a bit different from the traditional pretty printers.
I believe with a proper design we can even reuse `Outputable` machinery and provide it as a pretty printer for Haskell terms.
It resembles the scenario in Section 3.7 compared to Section 3.6 of Trees that Grow [1].     
 
Having said all these, we ARE diverging from the original thread, and Simon's questions. 

How about taking printer-design related discussion to the following wiki page (and/or a new ghc-dev thread if needed):
  https://ghc.haskell.org/trac/ghc/wiki/HaskellSyntaxPrinters 

Cheers,
  Shayan

[1] http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf

On Fri, Jul 28, 2017 at 8:43 PM, Alan & Kim Zimmerman <alan.zimm@gmail.com> wrote:
I agree. 4 is the current GHC invariant.

i.e., re-parsing a pretty printed parse tree gives you back a parse tree identical to the original (ignoring SrcSpans)

Alan

On 28 July 2017 at 20:34, MarLinn <monkleyon@gmail.com> wrote:

by

 (parser . prettyPrint . parser) = id  

I meant 

(prettyPrint . parser . prettyPrint) = id

for a valid input.

Simplifying, (parser ∷ String → something), and (prettyPrint ∷ something → String).

Therefore, (parser . prettyPrint . parser ∷ String → something) and (prettyPrint . parser . prettyPrint ∷ something → String).

Therefore, both criteria could only apply for (something ~ String). But as pretty printing adds quotation marks, not even that is true.

There are four formulations that might be applicable:

  1. parser . prettyPrint ≍ id

  2. prettyPrint . parser ≍ id -- ∷ String → String, useless here

  3. prettyPrint . parser . prettyPrint ≍ prettyPrint

  4. parser . prettyPrint . parser ≍ parser

  5. Well, you could go beyond to (prettyPrint . parser . prettyPrint . parser ≍ prettyPrint . parser) etc…

I don't think 1 (or 2) follow from one of the last two. But 1 does imply them. So it is a stronger criterion than both, and therefore probably not the one to choose. Assuming the parser is internally consistent, 3 just says something about the internal consistency of the pretty printer, while 4 says something about the relationship of the pretty printer to the parser. Thus 4 looks like the best candidate for a criterion. Possibly with 3 as a secondary target.

Cheers,
MarLinn


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