[kmill]

> > program3 fails because runFunction can only run first-order functions and runFunction is a second-order function – a function that takes a first-order function as a parameter. > Here I had no idea that JavaScript implicitly typed `runFunction` that way. That's cool.

In case it's not clear, it's just that it eventually ends up with a run time error (the talk about runFunction being a second-order function is just an explanation for why you should expect it to end up with a run time error). The evaluation is

       program3()
    == runFunction(runFunction)
    == runFunction(1)
    == 1(1)
    == Error: func (with value 1) is not a function

[oldandtired]

Depending on your system, 1(1) is quite sensible, That is applying 1 to a list of parameters just returns the 1st parameter. This is a matter of the semantics that one uses when application is run against any value.

1(1) -> 1

Just because the common idea of application is that it only applies to functions does not mean that application applied to other types is nonsensical and should end up with a runtime error.

If your language allows application to be defined for different types, then the type system should be capable of determining the type that returns from application.

A practical example of a language in which application is valid for integers is Unicon/Icon. Both functions and integers have specific semantics with regards to application. And failure is an option.

[georgewsinger]

I see. The error makes sense now.