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to know your claims. For instance, you can set a claim called “email”, so that anyone that wants to perform a transaction with you can get your email address. This is useful, since an Ethereum address is not bound to an identity (or email address), only to its private-key. The contract is as simple as possible. First there is the contract keyword that signals the beginning of a contract. Then comes OwnerClaims, the contract name. Inside the contract there are two types of elements: variables and functions. Among variables there are two types as well: constants and writable variables. Constants are just that: they can never be changed. Writable variables, however, save state in the blockchain. It is these variables that encode the state saved in the blockchain, nothing more. Functions are pieces of code that can either read or modify state. Read-only functions are also marked as constant in the code and do not require gas to run. On the other hand, functions that mutate state require gas, since state transitions must be encoded in new blocks of the blockchain (and these cost work to produce). Values returned from functions are returned to the caller. The owners variable in our contract is a map14, also known as associative array or dictionary. It matches a key to a value. In our case, the key is an address. Addresses in Ethereum are the identifiers of either normal accounts (usually managed by users) or other contracts. When an owner of an address decides to set a claim, it is this mapping from address to a claim that we are interested in. In fact, we are not simply mapping an address to a claim, but to a group of key-values that constitute a group of claims (in the form of another map). This is convenient because an address owner might want to make several details about himself known to others. In other words, address owners might want to make their email address and their cellphone number available. To do so, they might create two claims: one under the “email” key, and the other under the “phone” key. The contract leaves to each owner to decide what entries to create, so the names of the keys are not known in advance. For this reason, a special “default” key is available, so any reader might know at least one claim if he doesn’t know what keys are available. In truth, this key is also in place for a different reason: Solidity does not make it practical to return bulk data from functions. In other words, it is not easy to return all claims connected to an address in a single function call. In fact, the mapping type does not even have an iteration operation (although one can be coded if needed), so it is not possible to know what keys are inside a mapping. It is left as an exercise for the reader to find ways to improve this if needed. 2.2.3 Current and Potential Uses What we just saw with our simple example gave us a taste of what is possible with Ethereum. Do note it has nothing to do with exchanging money! Although ether is necessary to perform mutations on the network, our contract is strictly concerned with securely establishing a series of claims connected to an Ethereum address. Nothing more. Not only the result is mathematically verifiable (no other person other than the owner of the address can set claims), but is also very hard to erase: it is recorded in a globally distributed database with no central node! 14 https://en.wikipedia.org/wiki/Associative_array 37PDF Image | Intro to Ethereum and Smart Contracts
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