Is there a list of improvements that we will have in ERC223?
Are there examples of contracts?
Is there a migration path for existing ERC20 tokens?
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ERC223 tokens are backwards compatible with ERC20 tokens. It means that ERC223 supports every ERC20 functionality and contracts or services working with ERC20 tokens will work with ERC223 tokens correctly.
Sources: https://github.com/Dexaran/ERC23-tokens/tree/Recommended#erc23-token-standard (ERC23 and 223 are the same thing). To read the full discussion about ERC20 and ERC223, follow this link: https://github.com/ethereum/EIPs/issues/223
Problems of ERC20 that ERC223 will solve:
ERC223 token transaction costs 2 times less than
transferFrom of original ERC20.
There is a very nice explanation on https://docs.google.com/document/d/1Feh5sP6oQL1-1NHi-X1dbgT3ch2WdhbXRevDN681Jv4
In case the doc goes bust, let me summarize here. Technically, there are two different ways to deposit ERC20 tokens to a contract: 1. transfer 2. approve + transferFrom
If you use the 1 step transfer function to a contract, the receiver contract can't reject/recognize the transaction. So, essentially the tokens get lost in a sink. Sending to a wallet using transfer is fine.