Support default Kotlin values

[smyrick]

📝 Description

Add support for Kotlin default values. By calling functions with the callBy methods instead we can pass a map of KParameters. Then if one of the params is optional and it is not sent a value from GraphQL input we can exclude that parameter from the map, which mean the default value will be used.

Kotlin

fun print(message: String? = "hello"): String? = message

GraphQL

query PrintMessages {
    first: print(message = "foo") # foo
    second: print(message = null) # null
    third: print # hello
}

This info will not be conveyed in the schema, but it simplifies the logic needed for optional arguments. If you still need to have logic for the default values I recommend using the OptionalInput wrapper, since you will still not be able to determine the difference when the client didn't pass a value or it passed in the same value as the default.

This is a breaking change because we are changing the function signature that is open for FunctionDataFetcher

🔗 Related Issues

Fixes #53

[smyrick]

TODO - update docs

Updated in 173b331

[rowi1de]

Is this change intended: https://kotlinlang.slack.com/archives/CQLNT7B29/p1616235196005400

[smyrick]

I could make this change to the code in FunctionDataFetcher.getParameters to just do a map instead of mapNotNull then pass in null when there is no argument specified.

But then this unit test no longer works and neither will default values.
https://github.com/smyrick/graphql-kotlin/blob/c5b8fdc3739930f712a114b9abc15dee8fc[…]diagroup/graphql/generator/execution/FunctionDataFetcherTest.kt

Since we can’t inspect if a KParameter has a default value, we can only support one use case or the other.

• Support Kotlin default values but require them for nullable input
• Do not support Kotlin default values and always pass null for input if argument is missing in request