Solidity Invalid implicit conversion from string memory to bytes memory requested

Question

pragma solidity ^0.5

contract ProofOfExistence {

    // ... some code here

    function proofFor(string memory document) public view returns(bytes32) {
        return sha256(document);
    }

    // ... some more code here
}

Gives following error :

TypeError: Invalid type for argument in function call. Invalid implicit conversion from string memory to bytes memory requested.

I am unable to understand why this error is coming.

UPDATE :

pragma solidity ^0.5;
// Proof of Existence contract, version 3
contract ProofOfExistence3 {
  mapping (bytes32 => bool) private proofs;
  // store a proof of existence in the contract state
  function storeProof(bytes32 proof)  public{
    proofs[proof] = true;
  }
  // calculate and store the proof for a document
  function notarize(string memory document) public {
   **bytes32 proof = proofFor(document);**
    storeProof(proof);
  }
  // helper function to get a document's sha256
  function proofFor(bytes memory document) public pure returns (bytes32)  {
    return sha256(document);
  }
  // check if a document has been notarized
  function checkDocument(string memory document) view public returns (bool) {
    **bytes32 proof = proofFor(document);**
    return hasProof(proof);
  }
  // returns true if proof is stored
  function hasProof(bytes32 proof) view public returns(bool) {
    return proofs[proof];

UPDATE ERROR After the update as suggested I am getting the same error but on the lines having ** ...** this time. Why is it coming now? Thanks

Answer 1

Argument to sha256 should be in bytes. But you're passing document which is a string. Following would work.

function proofFor(bytes memory document) public pure returns(bytes32) {
    return sha256(document);
}

And a side note - your function doesn't need to be view type since it is not viewing any storage items. This can be made a pure function which has neither side effects nor storage viewing. More details can be found here.

UPDATE :

If changing string to bytes gives errors at other function calls you can either convert them to bytes as well or else you can cast string to bytes as @Mikhail suggested. Then that would be;

function proofFor(string memory document) public pure returns(bytes32) {
    return sha256(bytes (document));
}

Answer 2

Should probably be

return sha256(bytes (document));