Gfyuki

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

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));

}

Should probably be

return sha256(bytes (document));