Add hints for type errors where the user might have meant {=} instead of {}

[timbertson]

I thought dhall-lang/dhall-lang#1024 would make for a good first use case of context-specific hints next to errors, much like rustc. The idea is that if you have a type error involving {}, and it's a context where {=} could work, we include a hint in the error output:

$ echo '{ x = 1 } // {}' | ./dist/build/Dhall/dhall

Use "dhall --explain" for detailed errors

Error: You can only override records

Hint: {} is the empty record type, use {=} for the empty record value

1│ { x = 1 } // {}

(input):1:1

This hint appears for the following type errors:

• ({Type = { x: Natural }, default={ x = 1 }})::{}
• {} // { x = 1 }
• {} /\ { x = 1 }
• (\(x: Natural) -> x + 1) {}

The hint is only shown when the erroneous value in question is literally {} so it should have a fairly high relevance. In the final example {=} would also be the wrong type though, so it's not 100%.

The only issue I had is when {} is on the right hand side of a combine operation, i.e. { x = 1 } /\ {}

The --explain text for this one actually looks incorrect to me, it ends in:

You supplied this expression as one of the arguments:

↳ { x = 1 }

... which is not a record, but is actually a:

↳ Type

My hint shows up when I reverse the operands, so it seems like maybe the code constructing this error is always passing in the left hand side, rather than the expression matching the non-record type.

[sjakobi]

@timbertson Thanks for the report of the incorrect type error explanation! I have created a fix in #2244.

[sjakobi]

I like it! :)

[Gabriella439]

Thank you for doing this! 🙂