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
_______________________________________________
ghc-devs mailing list
ghc-devs@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
