show returns undefined inside FFI/JS #4564

Description

Following official documentation, to call show upon a data value inside JS I just need to do that, it should Just Work™.

But for some reason show returns undefined. More over, if I do spago bundle-app and look at the resulting index.js, I don't even see the string for the Show instance to produce, so if there's some odd trick required, it's impossible to derive from the output.

This came up as part of forum discussion.

To Reproduce

1. Create src/Main.purs as:

   module Main where
   
   import Prelude
   
   import Effect (Effect)
   
   data Foo = Foo
   instance fooShow :: Show Foo where
     show Foo = "Foo"
   
   -- | re-export of `show`
   show_ :: ∀ a. Show a => a -> String
   show_ = show
   
   foreign import printFoo :: Foo -> Effect Unit
   
   main :: Effect Unit
   main = printFoo Foo -- same as `log $ show Foo`

   and src/Main.js as:

   import { show_ } from '../Main/index.js';
   
   export const printFoo = function(foo) {
       return function () {
           console.log(show_(foo));
       };
   };

2. Execute spago run

Expected behavior

It should output Foo, not undefined

PureScript version

0.15.15