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1. -- related to https://stackoverflow.com/questions/66370965/polymorphic-input-function-with-multiple-argument-types/66371082?noredirect=1#comment117368103_66371082
2.  
3. {-# LANGUAGE RankNTypes #-}
4.  
5. main = putStrLn (foo id)
6.  
7. foo :: (forall a. Show a => a -> String) -> String
8. foo fn =
9.     let
10.         val1 = fn "hey"
11.         val2 = fn 42
12.     in
13.         val1 <> val2
14.  
15.  
16. -- main.hs:3:22: error:
17. --    * Couldn't match type `a' with `String'
18. --      `a' is a rigid type variable bound by
19. --        a type expected by the context:
20. --          forall a. Show a => a -> String
21. --        at main.hs:3:18-23
22. --      Expected type: a -> String
23. --        Actual type: String -> String
24. --    * In the first argument of `foo', namely `id'
25. --      In the first argument of `putStrLn', namely `(foo id)'
26. --      In the expression: putStrLn (foo id)
27. --  |
28. --3 | main = putStrLn (foo id)
29. --  |                      ^^
30. --compiler exit status 1