Add functions liftPandocPure and liftPandocIO, with a HasPandocError class

[ddssff]

I'm not sure whether these are generally useful, or they just fit into a design pattern I've been using.

[jgm]

Could you give a little explanation of how you use these (and maybe an example)?

[ddssff]

Generally speaking, if you are building a system of any size you start to accumulate a lot of error types, and these might turn into a heirarchy, where a FileError type might have one constructor to hold an IOException, another to hold a PandocError, and so on. To keep the code in different parts of the system uncoupled, you give each subsystem's error type a class such as

class HasPandocError e where fromPandocError :: PandocError -> e

Now your pandoc code can throw an error of some type it doesn't know about, as long as that error type has a HasPandocError instance.

The idea of liftPandocPure and liftPandocIO is to use the fromPandocError function to transform the error type, which in the pandoc library is always PandocError. So if I want to call pandoc from inside a LaTeXT monad I can say

latexify :: AppAbbrevs s e m => Markup -> LaTeXT m ()
latexify x = latexFromMarkup (liftPandocPure . pandocFromMarkdown) x

pandocFromMarkdown :: PandocMonad m => Text -> m Pandoc
pandocFromMarkdown t = softBreakToLineBreak <$> P.readMarkdown readOptsForMarkdown (removeCarriageReturns t)

latexFromMarkup :: AppAbbrevs s e m => (Text -> LaTeXT m Pandoc) -> Markup -> LaTeXT m ()
...

So pandocFromMarkdown gets lifted into LaTeXT m.