Add source position information for missing imports

[Gabriella439]

Related to #561

This adds source position information to missing imports

Before:

$ dhall <<< './foo'

↳ ./foo

Error: Missing file …/foo

After:

$ dhall <<< './foo'

↳ ./foo

Error: Missing file …/foo

(stdin):1:1

[Profpatsch]

Awesome, I think I can work with this. Now my next request would be to show a NonEmpty of all missing imports. I’d like to see you do that with exceptions. :P

[Gabriella439]

@Profpatsch: Yeah, that will follow in another pull request (possibly replacing exceptions with something like ExceptT along the way)

[Profpatsch]

possibly replacing exceptions with something like ExceptT along the way

I approve of this. It’s so darn hard to follow the exception codepaths.

[Profpatsch]

Ah, the position of the imports in the source files are not yet available though, I get 1:1:(source) when running this patch:

foo.dhall

{ aetiglnetglineeglseglvcnvfglcnlvfgneqlenlvqgenlvqgenleqgvnle =
	{ xyc = ./abc }
}

$ dhall-flycheck foo.dhall
[{"filename":"foo.dhall","line":1,"column":1,"message":"\n\n↳ ./abc\n\nError: Missing file /home/philip/kot/dhall/flycheck/abc\n\n"}]

I would expect the error position to point to the file name on line 2 of foo.dhall. That way I can show the problematic file.

Oh, and if I have

bar.dhall

./foo.dhall

$ dhall-flycheck bar.dhall
[{"filename":"bar.dhall","line":1,"column":1,"message":"\n\n↳ ./foo.dhall\n  ↳ ./abc\n\nError: Missing file /home/philip/kot/dhall/flycheck/abc\n\n"}]

I need some way to point to the import of ./foo.dhall in bar.dhall to highlight the transitive import problem (so that the user can follow the import chain though until the import of the missing file itself is highlighted).

[Profpatsch]

Something like { foo = ./foo.dhall, abc = ./abc } should highlight both ./foo.dhall (because of the transitive import error of abc) as well as ./abc (because of the direct import error) of course.