Stream: beginners

Topic: How do you define a function that takes no parameters?

Sanjay Chouksey (Dec 25 2023 at 19:58):

I am playing around with Roc and thought of a way to get trivial OO semantics using function closures and records:

math = \n ->  {
    addn: \a, b ->
        a + b + n,
    subn: \a, b -> a - b + n,
    foo: \_ -> n
}
p = math 10
expect p.addn 1 2 == 13
expect p.subn 5 2 == 13
expect p.foo 1 == 10

But I could not specify functions that take no parameters. I had to use a placeholder, foo: \_ -> n to do so. I tried foo: \ -> n, but the compiler gets stuck with: "TODO provide more context."

Karakatiza (Dec 25 2023 at 20:15):

A pure function with no parameters cannot change its output and so is a constant =)
So foo: n
To work around this the convention I think is to pass an empty value - {}, and pattern match it in the argument to {}
foo: \ {} -> n
But I think the only advantage this gives in Roc is deferring the execution of the body until needed

Brendan Hansknecht (Dec 25 2023 at 20:48):

Yep

Brendan Hansknecht (Dec 25 2023 at 20:49):

And it allows more polymorphisms of the type. A value becomes a concrete type. A function can return something that can become one of multiple possible types.

Brendan Hansknecht (Dec 25 2023 at 20:50):

As in List a will become a concrete type when used like maybe List U32. A function that returns a List a can generate a different list type on each call. So maybe List U32 one time and List U8 another time.

Last updated: Jul 26 2025 at 12:14 UTC