Definition.hs: add and export new lookupMeta* functions

[mb21]

lookupMetaInlines, lookupMetaBlocks and lookupMetaBool

This is an API change.

Needed for jgm/pandoc#4907.

[jgm]

Instead of having these different functions, we could introduce a version of lookupMeta with a more general type:

lookupMeta' :: FromMetaValue a => Meta -> Maybe a

and have a FromMetaValue class. That might be more elegant, no?

[mb21]

Hm.. since none of the three added functions return a Maybe, it would have to be:

lookupMeta' :: FromMetaValue a => String -> Meta -> a

But then we'd need to enable FlexibleInstances. Or we go with:

lookupMeta' :: FromMetaValue a => String -> Meta -> [a]

But that leaves the Bool case uncovered.

[mb21]

Or I can simply add the three new functions to Text.Pandoc.Writers.Shared instead. Although they are indeed very similar to those existing in pandoc-types: lookupMeta, docTitle, docAuthors and docDate.

[jgm]

Yeah, let's do it that way, at least for now. Mauro Bieg <notifications@github.com> writes:

…

Or I can simply add the three new functions to `Text.Pandoc.Writers.Shared` instead. Although they are indeed very similar to those existing in pandoc-types: `lookupMeta`, `docTitle`, `docAuthors` and `docDate`. -- You are receiving this because you commented. Reply to this email directly or view it on GitHub: #51 (comment)

[mb21]

Okay, I'll leave this pull open in case you want to move the functions over from Text.Pandoc.Writers.Shared.

Maybe we should add something to the docstring of lookupMeta, like:

see also the lookupMeta* functions in @Text.Pandoc.Writers.Shared@