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Unit ‚Äč

The sp.unit type in SmartPy represents a unique value, often used to signify the absence of a meaningful result or value. It corresponds to the unit type in Michelson.

Example: Contract parameters

On Tezos every contract has a parameter type. For example, a contract of type sp.contract[sp.int] can be called with an argument of type sp.int. To do this from within any entrypoint one would write (assuming c has type sp.contract[sp.int]):

smartpy
sp.transfer(42, sp.mutez(0), c)

But what if a contract doesn't require any arguments? In this case it can simply specify an expected parameter of type sp.unit, giving the contract the type sp.contract[sp.unit].

In order to call such a contract from an entrypoint, one would then use the unit value () (assuming c has type sp.contract[sp.unit]):

sp.transfer((), sp.mutez(0), c)

See also: sp.transfer, contracts

Example: Variants

SmartPy variants can be used to represent alternative cases with extra information, e.g.:

smartpy
shape : type = sp.variant(
   Circle=sp.int,
   Rectangle=sp.pair[sp.int, sp.int]
)

As you can see, a circle comes with only one number (its radius) whereas a rectangle carries two numbers (length and width).

But what about cases that don't require any extra info? sp.unit to the rescue! Here is a type that would model events coming from a physical sensor:

smartpy
senser_event : type = sp.variant(
    TemperatureChange=sp.int,
    HumidityChange=sp.int,
    MotionDetected=sp.unit
)

Temperature and humidity changes each come with a number, whereas for detected motion there is no extra information, hence sp.unit.