dhall-to-json: duplicate entries in an association list

[maxime-didier]

I was editing a dhall configuration file that generates various entries in an association list depending on its inputs, and I accidentally generated two entries with the same key. I only noticed it later when my changes were already in our staging environment.

My problem boils down to the fact that this snippet does not generate an error or a warning :

echo 'toMap { a = 1 } # toMap { a = 2 }' | dhall-to-json
# Output: { "a": 2 }

This is not a huge problem, but a warning would be nice.

[Gabriella439]

Have you tried merging the records before using toMap to convert them to a Map? In other words:

toMap ({ a = 1 } ∧ { a = 2 })

… because if you did that then you'd get an error message.

[maxime-didier]

I cannot apply this suggestion in my actual code. The second operand of my concatenation depends on an Optional and cannot be a naked toMap application.

I could change the type I use from Map.Type Text SomeValue to Map.Type Text (Optional SomeValue), and filter out the Nones though.

PS: Just found unpackOptionals in the prelude, will use it in my code, thanks 👍

[Gabriella439]

There are two aspects of this:

• Should this be a warning or error?
• If so, how would this be implemented?

I'll try to address both.

Neither the language standard nor the Haskell implementation have a system for warnings. There are only errors, which are either parsing errors, import resolution errors, or type errors. Generally speaking, if we were to introduce a system for warnings I believe it would be better to do so at the language standard level rather than at the Haskell implementation level, because we want to promote consistency between implementations. In particular, we want to avoid a situation where the Haskell implementation warns about certain behavior that other implementations accept without warning.

I'm personally reluctant to make this an error because there are situations where we might want to support duplicate keys. For example, even JSON technically permits duplicate keys (albeit it's not a recommended thing to do), and we wouldn't want json-to-dhall to reject valid JSON expressions. However, we can't easily tell from the type alone whether or not the user intends to omit duplicate keys. We could if we were to introduce a new built-in Map type, but that would require enabling Text equality (See: dhall-lang/dhall-lang#88).

However, suppose we were to decide that we do want to forbid duplicate mapKeys in any value that is an association map. That would be pretty difficult to implement, because we can't easily detect at type-checking time if a given Maps will have duplicate mapKeys. That would require pretty sophisticated static analysis to implement.