Polymorphism in Eta via Type Erasure

Main-Decompiled.java

public static class id extends Function {
  public Closure enter(StgContext context) {
    // context.R(2) is the first argument of the function
    // All id simply does is evaluate the argument -
    // it cannot do much more!
    return context.R(2).evaluate(context);
  }

  public int arity() { return 1; }
}

public static class showWithParens extends Function {
  public Closure enter(StgContext context) {
    // Dictionary for (Show a)
    Closure showA = context.R(2);
    Closure x     = context.R(3);
    // Thunk for (show x ++ ")")
    Main.sat_s2D0 localsat_s2D0 = new Main.sat_s2D0(showA, x);
    // Represents a lazy string: "("
    Main.sat_s2CX localsat_s2CX = new Main.sat_s2CX();
    // Evaluates ("(" ++ (show x ++ ")"))
    return Base.zpzp().applyPP(context, localsat_s2CX, localsat_s2D0);
  }

  public int arity() { return 2; }
}

public static class showWithParensEx extends Function {
  public Closure enter(StgContext context) {
    Closure showable = context.R(2);
    // Evaluate first argument
    Closure localClosure2 = showable.evaluate(context);
    // Cast to Showable data constructor
    Main.ShowableD showable_eval = ((Main.ShowableD)localClosure2);
    // Grab '(Show a)' typeclass dictionary from existential type
    Closure showableDict = showable_eval.x1;
    // Grab 'a' from Showable data constructor
    Closure x = showable_eval.x2;
    // Evaluatate 'show x'
    context.R(2, showableDict);
    return Show.show().enter(context).applyP(context, x);
  }

  public int arity() { return 1; }
}


public static class showWith extends Function {
  public Closure enter(StgContext context) {
    Closure f = context.R(2);
    // sat_s2CQ represents the expression for "SomeString"
    Main.sat_s2CQ someString = new Main.sat_s2CQ();
    // evaluate f "SomeString"
    return f.applyP(context, someString);
  }

  public int arity() { return 1; }
}

Main.hs

-- Parametric Polymorphism
id :: a -> a
id x = x

-- Ad-hoc or bounded Polymorphism
showWithParens :: (Show a) => a -> String
showWithParens x = "(" ++ show x ++ ")" 

-- Existential types
data Showable = Showable (forall a. (Show a) => a)

showWithParensEx :: Showable -> String
showWithParensEx (Showable x) = show x

-- Higher-rank function
showWith :: (forall a. a -> Int) -> Int
showWith f = f "SomeString"