Unresolved implicit parameter

[TimWhiting]

Discussed in #467

^{Originally posted by woojamon February 8, 2024}
May I ask why the show function is not implicity resolved in the example below?

// main.kk

type myType
  MyCase(value:int)

fun show(result:error<myType>) : pure string
  match result
    Ok(m) -> "MyCase"
    Error(x) -> exn/show(x)

fun eq(left:error<myType>, right:error<myType>) : pure bool
  match (left,right)
    (Ok(_),Ok(_)) -> True
    (Error(l), Error(r)) -> l.message == r.message
    _ -> False

pub fun myFun(p1:a, p2:a, ?eq:(a,a) -> pure bool, ?show:a -> pure string) 
  "hello"

fun main()
  myFun(Ok(MyCase(1)),Ok(MyCase(2)))

main.kk(19,36): error: cannot resolve implicit parameter
context : myFun(Ok(MyCase(1)),Ok(MyCase(2)))
parameter : ?show : (error) -> pure string
candidates: ...
hint : add a (implicit) parameter to the function signature?

This seems like a bug

[TimWhiting]

@woojamon

Explicitly adding ,?show=show works, but the above example without the explicit show should ideally work as well.

Ahh, I think I figured it out. The main function isn't in the same recursive definition group as show because it doesn't use show explicitly. This means that it could be type checked prior to show is defined. You can move the datatype / helper function to its own file, or even do something as simple as calling show("") and it type checks fine.

[woojamon]

Ok, I'll move some things to their own files. Thanks!

[woojamon]

Wait, why doesn't eq suffer the same problem then?

[TimWhiting]

It just happens to be typed checked prior to main, or is uniquely named.