Lambdas

The type of functions in SmartPy is sp.lambda_[t1, t2] where t1 is the argument type and t2 the result type. It corresponds to the lambda type in Michelson.

Lambdas can be defined either using Python's lambda x: ... syntax or, when they have a name, using def f(x): ....

For example, the definition

f = lambda x: x + 1

is equivalent to:

smartpy

def f(x):
    return x + 1

Here f has type sp.lambda_[sp.int,sp.int].

To call a lambda, simply pass it its argument in parentheses. For example, with the above definition:

smartpy

assert f(1) == 2

Effects

Lambdas can be allowed to have so-called effects. These are specified by adding the @sp.effects(...) decorator to a function definition.

Possible effects are:

@sp.effects(with_storage="read-only"): Allows a lambda to read, but not modify a contract's storage.
@sp.effects(with_storage="read-write"): Allows a lambda to both read and modify a contract's storage.
@sp.effects(with_operations=True): Allows a lambda to emit operations (e.g. transfers to other contracts). It can be either True or False.

Effects can also be combined, e.g. @sp.effects(with_storage="read-write", with_operations=True).

Example: Modifying storage

Here is a function that increments a counter every time it is called. It returns the updated value of the counter. Because the counter is kept in storage and modified, the read-write storage effect needs to be declared.

smartpy

@sp.effects(with_storage="read-write")
def fresh_id():
    self.data.counter += 1
    return self.data.counter

Example: Transfers from inside a lambda

Here is a function that sends one mutez to each contract in a given list. It returns the number of contracts called. Because sending (sp.transfer) is an operation, the with_storage effect needs to be declared.

smartpy

@sp.effects(with_operations=True)
def send_one_mutez_to_each(winners):
    count = 0
    for w in winners:
        count += 1
        sp.transfer((), sp.mutez(1), w)
    return count

Lambdas ​

Effects ​

Lambdas

Effects