Unit is F#’s value for the absence of a meaningful value. It is written as (), has the type unit, and lets every expression and function still produce a value. A function that prints, writes, or otherwise performs an action commonly returns unit rather than a useful result. section 4.1.6 section 6.1.1
This differs from void: unit is an ordinary value that can be returned or bound to a name. F# converts between unit and void when calling .NET Framework Class Library APIs, so void-returning .NET methods appear as unit-returning functions in F#. section 4.1.6
Use unit in two complementary places:
- As an output, signal that a function completed an action but has no meaningful result. A signature such as int -> unit means the function accepts an integer and performs an effect such as console output. section 4.1.6
- As an input, write () when a function needs no specific argument but must run anew on each call. This distinguishes a callable computation from a value calculated once during binding. section 4.1.6
- When deliberately discarding a meaningful result in a sequence of actions, use ignore. It turns any value into unit and makes that discard explicit to the compiler. section 4.1.7
Recalculate current time on each call
let getTheCurrentTime = System.DateTime.Now binds one time value, so later uses copy that original value. Define let getTheCurrentTime () = System.DateTime.Now instead, then call getTheCurrentTime () each time a fresh evaluation is wanted. The () input says no data is required, not that no call occurs. section 4.1.6
Seeing unit in an input or output is a useful clue that code interacts with the outside world or exists to sequence actions. Prefer returning useful values when later code needs them; expression-oriented code composes those results directly. For a unit-returning conditional action, F# can omit an else () branch. section 4.1.4 section 8.2.5
let getTheCurrentTime () = System.DateTime.Now #1 let x = getTheCurrentTime () #2 let y = getTheCurrentTime () #2
Compare a bound DateTime value with a function taking (). The latter is called explicitly, making repeated evaluation visible at each use. section 4.1.6
