Built-in Functions

panic

_10

fun panic(_ message: String): Never

Terminates the program unconditionally and reports a message which explains why the unrecoverable error occurred.

_10

panic("something went wrong: ...")

assert

_10

fun assert(_ condition: Bool, message: String)

Terminates the program if the given condition is false, and reports a message which explains how the condition is false. Use this function for internal sanity checks.

The message argument is optional.

revertibleRandom

_10

fun revertibleRandom(): UInt64

info

unsafeRandom() function currently behaves the same way as revertibleRandom(). unsafeRandom will be deprecated in the next release of Cadence.

Returns a pseudo-random number.

The random number returned is safe as it is backed by a distributed source of randomness, that is unbiased, unpredictable and verifiable.

The random number cannot be biased or predicted by miners or by other transactions or code running before a call to revertibleRandom.

Moreover, the sequence of returned random numbers is different per block, and per transaction within the same block.

warning

There is an additional safety-relevant aspect that developers need to be mindful about.

The function is not immune to post-selection manipulation by a non-trusted party where a random number is sampled and then rejected.

A transaction can atomically revert all its actions. Therefore, it is possible for a transaction calling into a smart contract to post-select favorable results and revert the transaction for unfavorable results.

The function remains totally safe when called by a trusted party.

This limitation is inherent to any smart contract platform that allows transactions to roll back atomically and cannot be solved through safe randomness alone. Flow protocol has suggested a solution to implement safe commit-reveal schemes and address this limitation.

RLP

Recursive Length Prefix (RLP) serialization allows the encoding of arbitrarily nested arrays of binary data.

Cadence provides RLP decoding functions in the built-in RLP contract, which does not need to be imported.

decodeString

_10

fun decodeString(_ input: [UInt8]): [UInt8]

Decodes an RLP-encoded byte array. RLP calls this a "string." The byte array should only contain of a single encoded value for a string. If the encoded value type does not match, or it has trailing unnecessary bytes, the program aborts. If the function encounters any error while decoding, the program aborts.

decodeList

_10

fun decodeList(_ input: [UInt8]): [[UInt8]]`

Decodes an RLP-encoded list into an array of RLP-encoded items.

Note that this function does not recursively decode, so each element of the resulting array is RLP-encoded data. The byte array should only contain of a single encoded value for a list. If the encoded value type does not match, or it has trailing unnecessary bytes, the program aborts. If the function encounters any error while decoding, the program aborts.