Add syntax aware contexts to errors

[Marwes]

We should have better, more human readable error messages. Elm has had a fair amount of praise for its error messages so stealing some ideas from it should be a good idea http://elm-lang.org/blog/compiler-errors-for-humans. From the blog post we can see that the error messages contain not only row and column information about where the error occurs but also that the error occurred in the 1st argument.

To do this we may add an enum as below in addition the the current Span which would meaning changing the signature of unify_span to take an AstLocation as well which each caller needs to supply with the extra information (which might need to be optional, at least to start with).

enum AstLocation {
    Argument { position: u32 },
    Array { position: u32 },
    Pattern { position : .. },
    IfElsePredicate,
    // ... more
}

This can be a bit bigger project but just adding a few AstLocation's may be good starting project.

[Marwes]

The above is just an idea, might be a good idea to look at how Elm represents it as well

[brendanzab]

This seems to be the root of the error reporting in the elm compiler: https://github.com/elm-lang/elm-compiler/tree/master/src/Reporting

[Marwes]

I did the the setup needed for this a while ago (see

gluon/check/src/typecheck.rs

Lines 156 to 158 in c450475

	pub enum Help {
	UndefinedFlatMapInDo,
	}

).

To continue on this just add another variant to enum Help which contains AstLocation. Then do as explained in #145 (comment)