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
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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