What do you mean by `Free.monad()` Today, i tried ...

# arrow

[PhBastiani]

What do you mean by Free.monad()
Today, i tried to translate de Scala Cats sample into Kotlin (see https://gist.github.com/PBastiani/3598c838cbfdd982238360244b7550ed)
Something seems wrong in the interpreter : i understand that in my FREE program FreeKVStore<Unit> is in conflict with Kind<F, A> ! But, I don't know how to fix that...

[raulraja]

should A be covariant?

[PhBastiani]

Could you be more precise ?

The Scala version of the interpreter is more intuitive to me. And, the use of the Kind type is for me +/- nebulous ....

[raulraja]

What is the error you are getting when compiling?. I ment make 'A' be 'out A'. Sorry on my phone

[PhBastiani]

Why A should be covariant ?

I fixed the compilation warning.

But the execution of something like that

`val res  = KVStoreA.binding() {
       put("A", 10).bind()  //-> `java.lang.ClassCastException`
       // delete("A").bind()   //->  `kotlin.TypeCastException`
       // get("A").bind()  // -> is OK. i.e. returns None
       //   return@binding
}.fix().foldMap(impureInterpreter(), Option.monad())

gives me an exception ! The conflict coming from FreeKVStore<Unit>

[PhBastiani]

I think that FunctionK<KVStoreA.F, F> is not the right way to declare the natural transformation ! We do not specify any context to interpret the AST

[raulraja]

Looks like the unit returning expressions in your interpreter are not lifted to Option and you are casting to it nonetheless

[raulraja]

F in the interpreter should be concrete to ForOption and when you return an operation it should be lifted to Option

[raulraja]

Otherwise you can make the interested depend on a Monad you use to lift to via just and pass the option Monad manually

[raulraja]

So the cast exception is coming from the interpreter right?

[PhBastiani]

Thx Raul for your help... I updated my interpreter with ForId. You mean something like that ?

https://gist.github.com/PBastiani/3598c838cbfdd982238360244b7550ed

Remains a warning due to the use of Any?

I think I can improve the type checking by writing
fun <F> impureInterpreter() : FunctionK<KVStoreA.F, ForId> = object : FunctionK<KVStoreA.F,Id<A>> i'm in the right way ?

[raulraja]

On my phone but I don't think you need the type prameter F if your interpreter is fixed to ForId

[raulraja]

https://github.com/arrow-kt/arrow/blob/master/modules/free/arrow-free-data/src/test/kotlin/arrow/free/Interpreters.kt

[raulraja]

Those examples should help

[raulraja]

The problem with casting is an inference issue in the Kotlin compiler

[raulraja]

Despite proving you cover all cases of the GADT it's unable to prove that at typechecking and you need to cast it at the end of the when expression

[raulraja]

I plan on submitting a fix to the Kotlin compiler for GADT inference enhancements once I'm done with the arrow compiler plugins, which should remove the need to cast

[raulraja]

FuntionK is after all a natural transformation that proves you can go from Al states of F to G and that casts should have not been necessary in the first place if the compiler infers properly the RHS

[PhBastiani]

Thank you for your patience. I updated the Gist...
Excellent news for your proposal. Type checking is a brain teaser.
In fact here, it seems to me that I can put any type in Id without neither compilation warning nor execution error 🤔

[PhBastiani]

Next step for this KATA : code generation with @higherkind annotation;
and, the use of the State monad... for sure, with other headaches due to type checking