When are F# function calls evaluated; lazily or immediately?

Question

Curried functions in F#. I get the bit where passing in a subset of parameters yields a function with presets. I just wondered if passing all of the parameters is any different. For example:

let addTwo x y = x + y
let incr a = addTwo 1
let added = addTwo 2 2

incr is a function taking one argument. Is added an int or a function? I can imagine an implementation where "added" is evaluated lazily only on use (like Schroedinger's Cat on opening the box). Is there any guarantee of when the addition is performed?

Answer 1

added is not a function; it is just a value that is calculated and bound to the name on the spot. A function always needs at least one parameter; if there is nothing useful to pass, that would be the unit value ():

let added () = addTwo 2 2

Answer 2

F# is an eagerly evaluated language, so an expression like addTwo 2 2 will immediately be evaluated to a value of the int type.

Haskell, by contrast, is lazily evaluated. An expression like addTwo 2 2 will not be evaluated until the value is needed. The type of the expression would still be a single integer, though. Even so, such an expression is, despite its laziness, not regarded as a function; in Haskell, such an unevaluated expression is called a thunk. That basically just means 'an arbitrarily complex expression that's not yet evaluated'.