Nicer errors for “type synonyms” about dhall-haskell HOT 9 CLOSED

It sounds like there are several questions being asked here:

• Can Dhall provide more detail by default (without --explain) when there is a type mismatch?

  Yes, I can definitely do that

• If the types differ by a single field, can Dhall just specify which field was wrong?

  It's possible, but I doubt I will implement this any time soon because it adds a lot of complexity

• Can Dhall provide support for type synonyms?

  Probably not, for the reasons outlined here: #10

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Yes, I can definitely do that

Yes, I tested that. It’s just that the default output is unhelpful in that case.

It's possible, but I doubt I will implement this any time soon because it adds a lot of complexity

I was afraid that’s the case, yes.

Probably not, for the reasons outlined here: #10

Trying to grok the answers in that thread (…processing…)

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For the first issue you raised I will implement an intermediate solution for now, which is to put the relevant information at the end of --detailed error messages so that it's easier to obtain: #76

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2. It's possible, but I doubt I will implement this any time soon because it adds a lot of complexity

Isn't it just simple diff? Why should it report only one field and not all?

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@AnthonyJacob: I'm referring to the complexity of computing a diff for arbitrary Dhall types (as opposed to just the diff of two record types). For example, consider this more complex expression:

< foo = { bar = 1, baz = 2 } | qux : Bool > : < foo : { bar : Integer, baz : Double } | qux : Bool >

This would give us the following error:

...
You or the interpreter annotated this expression:

↳ < foo = { bar = 1, baz = 2 } | qux : Bool >

... with this type or kind:

↳ < foo : { bar : Integer, baz : Double } | qux : Bool >

... but the inferred type or kind of the expression is actually:

↳ < foo : { bar : Integer, baz : Integer } | qux : Bool >
...

However, really the error we want is that the baz field had the wrong type (with a trail of breadcrumbs pointing to how to get to that field from the outermost type

Implementing a diff that supports that sort of type error is more complex to implement

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Just an update that Dhall now supports type aliases in master

However, I'm still keeping this open because there is still the issue of providing better error messages

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I've been playing with dhall-to-cabal, and the errors really, really aren't good with larger types. See https://gist.github.com/quasicomputational/330193aabe69abdfc1400b37f6af6670 for an example.

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@quasicomputational: See if you can build dhall-to-cabal against dhall-1.13.0. The latest version of the dhall package added support for concise type diffs explaining why there is a type mismatch (this was a feature implemented specifically with dhall-to-cabal in mind).

See: #336

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I will close this since the original issue was fixed by adding support for type synonyms and the latter issue should be fixed by dhall's support for type diffs in error messages

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