When to use parentheses when calling a function in f#?

Question

I'm learning about f# and I understand you don't need to use parentheses when calling a function.

Ex

let addOne arg1 =
    arg1 + 1

addOne 1

vs

this.GetType()

Why do I have to use parentheses on the second function?

Answer 1

There is a bit of a mismatch between working with .NET libraries and working with F# libraries when it comes to parameters, but you can generally see () not as parentheses, but as a special value of type unit that means "no useful information".

This means that when you say:

addOne 1

You are calling addOne with a value - number 1 - as a parameter. Now, when you apply the same reading to the second example:

this.GetType()

You can read this as calling this.GetType with a value - the special () unit value as a parameter. If you wanted to be consistent, you could write this with space too:

this.GetType ()

In practice, most people will omit the space when calling .NET libraries. When you do not write the space, F# also supports method chaining so you can write e.g. foo().bar().

Many F# functions taking multiple parameters will use the "curried" form, which means that the parameters need to be separated by spaces. For example:

let add a b = a + b
let mul a b = a * b

add 10 (mul 20 3)

Here, you need parentheses around the second expression, so that the compiler knows how to parse the code. This is in contrast with typical .NET methods, which take parameters as a tuple. F# tuples are written as (10, "hello") and so you can see a method call as an ordinary call accepting a tuple:

some.Operation (10, "Hello")

Again, typically you wouldn't write the space here, because you know this is actually a .NET method call, rather than "passing tuple to a function", but conceptually, you can think of it in both ways.

This is the summary - there are a few corner cases where method calls do not really behave like tuples (e.g. when it comes to named parameters), but this way of thinking about it should give you an idea about how things work.