Move (some) of the logic of the dhall executable into Dhall

[ocharles]

dhall takes a file, parsers it, resolves imports, type checks, normalizes and then prints - along with a bunch of error checking. A lot of these steps would be useful to have in Dhall. I find myself often playing around in GHCI and wanting a way to take a FilePath and have a Expr s X that is type checked and normalized, but not extracted.

I propose adding

load :: FilePath -> IO (Expr s X)

and having dhall use this. The error checking that dhall does probably gets moved into a new Exception type, with a handler that knows how to print out specific errors.

[Gabriella439]

Sure! My main suggestions are:

• Change the argument to a Text value representing Dhall source code. Importing a file then becomes a special case of Dhall's import system, plus you can then load URLs or inline Dhall expressions
• I recommend naming this rawOutput because there is a similar rawInput function in the Dhall module

[Gabriella439]

This was fixed by #418, which added new Dhall.inputExpr and Dhall.inputExprWith utilities that do exactly what you requested