Short, sharp Solidity: pure vs view vs call

Jul 9, 2020 16:19 · 618 words · 3 minute read ethereum truffle solidity

Introduction

A quick worked example demonstrating the differences between the read-only Solidity elements pure, view and .call() in the Truffle console.

At time of writing I’m using: Truffle v5.1.30 (core: 5.1.30), Solidity v0.5.16 (solc-js), Node v14.4.0, Web3.js v1.2.1, and Ubuntu 20.04 LTS (Regolith flavour).

The smart contract

If you want to follow along then after running truffle init in a working folder, copy the following code into contracts/CallDemo.sol:

// SPDX-License-Identifier: Unlicencse
pragma solidity ^0.5.16;

contract CallDemo {
  uint private sampleNumber;

  constructor() public {
      sampleNumber = 163;
  }

  // Declared as a view function
  // i.e. will not modify state
  function viewRetrieve() public view returns (uint) {
      return sampleNumber;
  }

  // Declared as a pure function
  // i.e. will not modify or read from state
  function pureRetrieve() public pure returns (uint) {
      return 163;
  }

  // Not declared as either pure or view,
  // will flag as warning in compiler
  function nonViewRetrieve() public returns (uint) {
      return sampleNumber;
  }
  
}

and then in migrations/2_deploy_contract.js the following:

var CallDemo = artifacts.require("CallDemo");

module.exports = function(deployer) {
  // Arguments are: contract
  deployer.deploy(CallDemo);
};

Truffle compile and deploy

Running truffle deploy on the above will successfully compile and deploy, but with one warning regarding the last function:

    /home/call-truffle-demo/contracts/CallDemo.sol:19:3: Warning: Function state mutability can be restricted to view
  function nonViewRetrieve() public returns (uint) {
  ^ (Relevant source part starts here and spans across multiple lines).

We’ll get to this later.

Interrogating via truffle console

Dropping into truffle console let’s retrieve the data from the first function:

let app = await CallDemo.deployed()
let viewReturn = await app.viewRetrieve()
viewReturn.words[0]

which gives us, correctly, 163.

So let’s run the same on our second function:

let pureReturn = await app.pureRetrieve()
pureReturn.words[0]

again: 163. So by that logic:

let nonViewReturn = await app.nonViewRetrieve()
nonViewReturn.words[0]

Should give us 163, right? Wrong. We get Uncaught TypeError: Cannot read property '0' of undefined.

Alternatively if we just run nonViewReturn we get transaction details with no sign of 163:

{
 tx: '0xf383bc9a345686c40dbb52eff8424e72fb921ad44a2aca3f3f20682ec071a05a',
 receipt: {
   transactionHash: '0xf383bc9a345686c40dbb52eff8424e72fb921ad44a2aca3f3f20682ec071a05a',
   transactionIndex: 0,
   blockHash: '0xc840932201f90053a4d17b832a80762122560910b42dbb71eeff78438643a91d',
   blockNumber: 21,
   from: '0x852209101adbaa71516b99197e0cba0f8d102d58',
   to: '0x780a64a641cf32f293b61526abac6859fdc325ad',
   gasUsed: 22077,
   cumulativeGasUsed: 22077,
   contractAddress: null,
   logs: [],
   status: true,
   logsBloom: '0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
   rawLogs: []
 },
 logs: []
}

So how do we get our number? We need to add .call() to the end of the function:

let nonViewReturnCall = await app.nonViewRetrieve.call()
nonViewReturnCall.words[0]

And this gives us 163.

What’s happening?

In essence all three functions are read-only; we are not changing anything on the Ethereum blockchain, we are simply drawing data from it. The first two functions however are declared respectively as:

  • View: This declares that no state will be changed. In other words the function is simply returning state (sampleNumber), but not making any changes to the data currently on the blockchain.
  • Pure: Declares that no state variable will be changed or read. This is an even more stringent declaration; we are not even reading any data outside of the function itself. Had we attempted to return the variable sampleNumber within a function declare pure the contract would have failed to compile.

As the last function was not defined as either of these, and subsequently generated a warning, then the .call() function has to be used. To quote the Solidity documentation:

In order to interface with contracts that do not adhere to the ABI, or to get more direct control over the encoding, the functions call, delegatecall and staticcall are provided.

For a more detailed explanation I would recommend Calls vs. transactions in Ethereum smart contracts.

As an aside you will notice that if you copy and paste CallDemo.sol into the online Remix IDE you don’t have to worry about the .call() on the last function - the IDE figures out it is needed all by itself.

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