This is known as eta-expansion : in a functional language,
let f a = g a
Is equivalent to
let f = g
This makes mathematical sense : if the two functions are equal for every input, then they're equal.
In your example, g is addTwoNumbers 5 and the code you wrote is entirely equivalent to:
let add5toNumber y = addTwoNumbers 5 y
There are a few situations where they are different:
- In some situations, the type system may not recognize
y as universally quantified if you omit it.
- If
addTwoNumbers 5 (with one parameter only) has a side-effect (such as printing 5 to the console) then the eta-expanded version would print 5 every time it's called while the eta-reduced version would print it when it's defined. This may also have performance consequences, if addTwoNumbers 5 involved heavy calculations that can be done only once.
- Eta-reduction is not very friendly to labels and optional arguments (but they don't exist in F#, so that's fine).
And, of course, unless your new function name is extremely readable, providing the names of the omitted arguments is always a great help for the reader.
