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Version: 17.3.0

Smart contract interaction

Taquito allows developers to interact with Smart Contracts as if they are "Plain Old Javascript Objects."

The "Machine Language" of Tezos Smart Contracts is named Michelson. Michelson is a stack-based language that is human-readable. It's possible to author Smart-Contracts directly in Michelson. However, developers can use High-Level Languages (such as Ligo or SmartPy) to write smart contracts.

Taquito makes developing applications (dApps or traditional programs) around a Tezos Smart Contract easy. Taquito can also "originate" (create) a new Smart Contract to the Tezos Blockchain.

Michelson is a somewhat specialized language that isn't typical in Javascript or Typescript development contexts. Taquito helps to bridge the gap between the Tezos blockchain and a standard Javascript or Typescript development environment.

Taquito's Smart Contract Abstraction​

Taquito assists developers by reading the Michelson code for a given contract from the blockchain. Based on the retrieved Michelson code, Taquito generates a contract javascript object with methods and storage that correspond to the contract's Michelson entry points, storage definitions, and values.

The Counter Contract​

In this guide, we use a straightforward "counter" smart contract to illustrate how Taquito works.

The counter contract has two entry points named increment and decrement. Taquito uses these entrypoints to generate corresponding javascript methods available to the developer.

The counter contract's storage is a simple integer that gets increased or decreased based on the calls to the entrypoints.

Counter Contract in JSLIGO​

type storage = int;


type parameter =
| ["Increment", int]
| ["Decrement", int]
| ["Reset"];


type return_ = [list <operation>, storage];


/* Two entrypoints */
const add = ([store, delta] : [storage, int]) : storage => store + delta;
const sub = ([store, delta] : [storage, int]) : storage => store - delta;


/* Main access point that dispatches to the entrypoints according to
   the smart contract parameter. */
const main = ([action, store] : [parameter, storage]) : return_ => {
 return [
   (list([]) as list <operation>),    // No operations
   (match (action, {
    Increment: (n: int) => add ([store, n]),
    Decrement: (n: int) => sub ([store, n]),
    Reset:     ()  => 0}))
  ]
};

You can view this contract and deploy it to a testnet using the Ligo WebIDE

Counter Contract Michelson source code​

{ parameter (or (or (int %decrement) (int %increment)) (unit %reset)) ;
  storage int ;
  code { UNPAIR ;
         IF_LEFT { IF_LEFT { SWAP ; SUB } { ADD } } { DROP 2 ; PUSH int 0 } ;
         NIL operation ;
         PAIR } }

Loading the contract in Taquito​

To load the contract from the Tezos Blockchain, we use the Tezos.contract.at method. We can inspect the contract methods and data types using the c.parameterSchema.ExtractSignatures() method.

The following example shows how to load the contract and view the methods on that contract.

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The at() method causes Taquito to query a Tezos nodes RPC API for the contracts "script" and "entrypoints." From these two inputs, Taquito builds an ordinary JavaScript object with methods that correspond to the Smart Contracts entrypoints.

The at method returns a representation of the contract as a plain old javascript object. Taquito dynamically creates an increment and decrement method that the developer can call as follows:

  • contract.methods.increment()
  • contract.methods.decrement()

In Tezos, to call an entrypoint on a contract, one must send a transfer operation. In the counter contract case, the transfer value can be 0 as the contract does not expect to receive any tokens. The transfer must have the appropriate Michelson values specified as "params" to call the increment entrypoint.

We can inspect the transfer params produced by Taquito using the toTransferParams() method:

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Calling the Increment function​

In the next example, we call the send() method. This example requires a different ceremony for getting a temporary key for signing.

We call the send() method on the increment() method. Taquito then forges this operation into a transfer operation (with a transfer value of zero), signs the operation with our testing key, and injects or broadcasts the operation to the Tezos RPC node.

Then we wait for the confirmation(3) to complete. The 3 number tells Taquito how many confirmations to wait for before resolving the promise. 3 is a good value for this type of demonstration, but we recommend a higher value if you are dealing with mainnet transactions.

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Choosing between the methods or methodsObject members to interact with smart contracts​

note

Since Taquito version 10.2.0, the parameter can be passed in an object format when calling a smart contract entry point.

The ContractAbstraction class has a new member called methodsObject, which serves the same purpose as the methods member. The format expected by the smart contract method differs: methods expects flattened arguments while methodsObject expects an object.

It is at the user's discretion to use their preferred representation. We wanted to provide Taquito users with a way to pass an object when calling a contract entry point using a format similar to that used by the storage parameter when deploying a contract.

An example showing the difference is provided below.

In the following example, a contract's set_child_record method will be called by passing the arguments using the flattened representation. The methods member of the ContractAbstraction class allows doing so. First, it is possible to obtain details about the signature of the set_child_record entry point by using the getSignature method as follow:

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The preceding example returns an array which contains the different possible signatures. Different signatures are possible as the set_child_record method contains some optional arguments. In the following example the set_child_record method is called by passing the arguments in the flattened way:

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