Derivation fails for Named Tuples

[vkorenev]

Read.derived[(String, Int)] compiles just fine while Read.derived[(a: String, b: Int)] fails with an error:

//> using scala 3.7.3
//> using dep "org.tpolecat::doobie-core::1.0.0-RC10"
import doobie.*
import doobie.implicits.*

val _ = Read.derived[(String, Int)]

val _ = Read.derived[(a: String, b: Int)] 

@main
def main() = {}

Compiling project (Scala 3.7.3, JVM (21))
[error] ./sc.scala:8:22
[error] Type argument (a : String, b : Int) does not conform to upper bound Product
[error] val _ = Read.derived[(a: String, b: Int)] 
[error]                      ^
Error compiling project (Scala 3.7.3, JVM (21))

[AyushPrakash414]

Hi

I noticed the pending work on resolving the Read.derived issue with named tuples in Scala, and I’d like to take it up. I’m confident I can handle it efficiently and ensure a solid outcome.

Please assign this task to me so I can get started.

Best,
AyushPrakash

[jatcwang]

Hi @AyushPrakash414 thanks for offering to help. Please go ahead!

[AyushPrakash414]

Thanks for assigning this. I’ll review it shortly.

[AyushPrakash414]

@jatcwang I verified the behavior on the Scala 3 ReadPlatform.

At the moment, Read.derived is defined as:

inline def derived[A <: Product](using Mirror.ProductOf[A]): Read[A]

Named Tuples in Scala 3 do not extend Product, which explains why:

Read.derived[(String, Int)] // works

while derivation fails for named tuples, even though they do have a Mirror.

Supporting named tuples would require relaxing the Product bound and
reworking derivation to rely on Mirror.Of[A] instead. This appears to be
a current design limitation rather than a simple bug.

[jatcwang]

@AyushPrakash414 Great! If that's the case then we can relax the constraint and prove it with a test 👍