Operating on HsSyn

[Alan & Kim Zimmerman]

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 at 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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