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Managing cycles

Usage of the Internet Computer is measured, and paid for, in cycles. This library provides imperative operations for observing cycles, transferring cycles and observing refunds of cycles.

WARNING: This low-level API is experimental and likely to change or even disappear. Dedicated syntactic support for manipulating cycles may be added to the language in future, obsoleting this library.

NOTE: Since cycles measure computational resources, the value of balance() can change from one call to the next.

Value balance

let balance : () -> (amount : Nat)

Returns the actor's current balance of cycles as amount.

Value available

let available : () -> (amount : Nat)

Returns the currently available amount of cycles. The amount available is the amount received in the current call, minus the cumulative amount accepted by this call. On exit from the current shared function or async expression via return or throw any remaining available amount is automatically refunded to the caller/context.

Value accept

let accept : (amount : Nat) -> (accepted : Nat)

Transfers up to amount from available() to balance(). Returns the amount actually transferred, which may be less than requested, for example, if less is available, or if canister balance limits are reached.

Value add

let add : (amount : Nat) -> ()

Indicates additional amount of cycles to be transferred in the next call, that is, evaluation of a shared function call or async expression. Traps if the current total would exceed 2^128 cycles. Upon the call, but not before, the total amount of cycles added since the last call is deducted from balance(). If this total exceeds balance(), the caller traps, aborting the call.

Note: the implicit register of added amounts is reset to zero on entry to a shared function and after each shared function call or resume from an await.

Value refunded

let refunded : () -> (amount : Nat)

Reports amount of cycles refunded in the last await of the current context, or zero if no await has occurred yet. Calling refunded() is solely informational and does not affect balance(). Instead, refunds are automatically added to the current balance, whether or not refunded is used to observe them.