"append","delete","copy" in Go is parametric polymorphism hardcoded in the compiler. append can be called with many different array types. There is nothing complicated with that.

go can iterate over arrays irregardless of the element type the same way with the same construct using "range", therefore "range" is polymorphic.

The real debate here is should the compiler be "privileged" or should there be API in userland to implement these behavior.

> Today it exists in Standard ML, OCaml, F#, Ada, Haskell, Mercury, Visual Prolog, Scala, Julia, and others.

> Java, C#, Visual Basic .NET and Delphi have each introduced "generics" for parametric polymorphism.

https://en.wikipedia.org/wiki/Parametric_polymorphism

A bunch of low-reach languages have true parametric polymorphism. A bunch of common languages have polymorphism that may be parametric or may be bounded by runtime type information checks, and you can't tell the difference from the types.

Neither of those contradict my assertion that most programmers have never done work in a language where the type system can make guarantees that they can trust.

This is a very pedantic point

The only programmers who haven't used a language with parametric polymorphism are those who have only programmed in C, Go, or dynamic languages at this point.

Outside of Java syntax noise, I really have my doubts that Java's parametric polymorphism is too complicated for the average programmer to understand.

  public static <A> A identity(A a) { return a; }

Java does not have parametric polymorphism because instanceof exists in the language. That reduces what you can say about something from the type alone to "maybe parametric, maybe bounded by runtime checks that aren't visible in the type."

How many languages provide you a way to guarantee at the type level that the behavior of a function cannot be influenced by instantiation of type variables? I don't believe that to be true of anything that runs in the JVM or in .NET, for instance, as runtime type information is always available. C++ has dynamic_cast. What common language actually has parametricity expressed in the type system?

That’s true..but Haskell has bottom and Scala has bottom + instanceOf checks and JVM reflection as well. And they clearly have usable parametric polymorphism despite things being “broken.”

Saying Java doesn’t have parametric polymorphism because you can cheat is like saying Haskell isn’t pure because of unsafePerformIO.

Agreed. There is always a point where it's the developer's responsibility to write good code, even in languages where it's very difficult to shoot yourself in the foot.