Calling precompiled contracts from a pure function

Question

I'm trying to call some of the new precompiled contract (ecmul) from a pure function but it fails with:

browser/ballot.sol:11:23: TypeError: Function declared as pure, but this expression (potentially) modifies the state and thus requires non-payable (the default) or payable.
            if iszero(call(not(0), 0x07, 0, input, 0x60, p, 0x40)) {
                      ^--^

This is the sample contract (requires solc 0.4.19):

pragma solidity ^0.4.19;

contract Ballot {
    function ecmul(uint256 x, uint256 y, uint256 scalar) public pure returns (uint256[2] p) {
        // With a public key (x, y), this computes p = scalar * (x, y).
        uint256[3] memory input;
        input[0] = x;
        input[1] = y;
        input[2] = scalar;
        assembly {
            // call ecmul precompile
            if iszero(call(not(0), 0x07, 0, input, 0x60, p, 0x40)) {
                revert(0, 0)
            }
        }
    }
}

Taken from the article precompiles & solidity.

If I declare the function as view it will work but seems as unnecessary since it is a precompiled contract it should not modify the storage of any contract.

Answer 1

Firstly note that the view modifier, as seen in the doc here doesn't allow modifying of the state, it only allows reading of the state and behaves the same as constant (the modifier used for these code snippets in the article you've linked to). The pure modifier, in the doc here prevents both modifying and reading of the state.

To answer your question, it seems like using call() with any arguments raises this error when the pure modifier, which prevents reading of the state, is used. Which makes sense as call() needs to read the state to get the contract code at the address specified so it can execute a function on it.