A diagnostic representation for "Doc"

[sjakobi]

I often want to see the internal structure of a Doc. I can use the derived Show instance in my fork for this, but I would prefer something more conveniently available.

[sjakobi]

If the existing Show instance for Doc should continue to pretty-print the document, how about a "clone" of Doc with the same constructors. I.e.

data Diag ann =
    Fail
    | Empty
    | Char !Char
    ...
deriving instance Show (Int -> Diag ann) => Show (Diag ann)

diag :: (Show ann, Show (Int -> Diag ann)) => Doc ann -> String
diag = show . docToDiag

Could be nicer, but would already be very useful!

[quchen]

How would this deal with function-based constructors like Column? I like the idea though, and this would make a good addition to the Internal module.

[sjakobi]

How would this deal with function-based constructors like Column?

Users that import Text.Show.Functions from base would see

Column <function>

That's not great, but better than nothing.

I guess we could also "scan" or "sample" these functions – this could even be configurable!

[quchen]

Or we could render the Doc as a Doc, and use the nesting/indentation of the rendering to put into the function fields. Or would that be circular?

[sjakobi]

Since the diagnostic representation should be particularly useful when working on prettyprinter itself, I think it's better not to rely on it for this purpose.