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I received an error by the compiler regarding the following solidity code and I would like to know what's this error and what's the fix?

    interface tokenRecipient { function   receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) external; }

Error in

    TypeError: Data location must be "calldata" for parameter in external function, but none was given.
    interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) external; }                                                                                                                                                                                       
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  • It gives no error in Remix at least. Where do you get the error? – Lauri Peltonen Sep 28 '18 at 10:27
  • In Remix compiler version:0.5.0 – hibanl Sep 28 '18 at 11:11
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There are some breaking changes in Solidity version 0.5. More details at: https://solidity.readthedocs.io/en/latest/050-breaking-changes.html

One of these breaking changes is:

Explicit data location for all variables of struct, array or mapping types is now mandatory. This is also applied to function parameters and return variables. For example, change uint[] x = m_x to uint[] storage x = m_x, and function f(uint[][] x) to function f(uint[][] memory x) where memory is the data location and might be replaced by storage or calldata accordingly. Note that external functions require parameters with a data location of calldata.

This explicitly states that if your function is external the location type must be calldata - but the location is only required for structs, arrays and mappings. As bytes is almost the same as byte[] it is considered as an array (https://solidity.readthedocs.io/en/v0.4.21/types.html#arrays).

If you're wondering what is calldata, this is also explained at https://solidity.readthedocs.io/en/v0.4.21/types.html#data-location

There is also a third data location, calldata, which is a non-modifiable, non-persistent area where function arguments are stored. Function parameters (not return parameters) of external functions are forced to calldata and behave mostly like memory.

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