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In the ERC-721 spec interface, the functions safeTransferFrom, transferFrom and approve all have the payable modifier. What is the reason for this? As far as I can tell, none of these functions require it, and I'm not sure what I should be doing with the value.

function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;

function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;

function transferFrom(address _from, address _to, uint256 _tokenId) external payable;

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It appears the answer is to do with mutability, from the "Caveats" section of the ERC721 spec,

Solidity issue #3412: The above interfaces include explicit mutability guarantees for each function. Mutability guarantees are, in order weak to strong: payable, implicit nonpayable, view, and pure. Your implementation MUST meet the mutability guarantee in this interface and you MAY meet a stronger guarantee. For example, a payable function in this interface may be implemented as nonpayble (no state mutability specified) in your contract. We expect a later Solidity release will allow your stricter contract to inherit from this interface, but a workaround for version 0.4.20 is that you can edit this interface to add stricter mutability before inheriting from your contract.

So the functions are marked payable as a way of explicitly not requiring that these functions are view or pure.

Here's a link to the discussion of issue #3412, which discusses mutability guarantees.

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