How to load & parse an HI (interface) file?

[Saurabh Nanda]

(GHC newbie alert -- is this the right mailing list for these kind of
questions?)

I"m writing some code to figure out all the instances of particular
type-classes and after exploring a lot of options (hlint, haskell-src-exts,
annotations, doctests, etc), I realized that the compiler had already
figured it out and written it to disk for me!

More digging led me to
https://www.stackage.org/haddock/lts-9.0/ghc-8.0.2/LoadIface.html#v:loadSrcInterface
after which I got stuck. How does one call this function? Specifically:

* What is SDoc and how to construct a reasonable value for this argument?
* IsBootInterface would mostly be False, right?
* What does `Maybe FastString` represent and how does one construct it?
* Finally how does one evaluate the resulting monadic action to get access
to the underlying `ModIface`?

-- Saurabh.
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