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Take a look4 at the things the community is working on to get a sense of the many useful ideas that can be run as decentralized applications. 2.2.1 Ether Although Ethereum brings general computations to the blockchain, it still makes use of a “coin”. Its coin is called “ether”, and, as any coin, it is a number that can be stored into account addresses and can be spent or received as part of transactions or block generation. To run certain transactions, users must spend Ether. But why is this the case? A Turing-complete language5 is a language that, by definition, can perform any computation. In other words, if there is an algorithm for something, it can express it. Ethereum scripts, called smart contracts, can thus run any computation. Computations are run as part of a transaction. This means each node in the network must run computations. Any machine capable of running a Turing-complete language (i.e. a Turing machine) has one problem: the halting problem6. The halting problem essentially states that no Turing machine can determine beforehand whether a program run in it will either terminate (halt) or run forever. In other words, the only way of finding out if a piece of code loops forever or not is by running that code. This poses a big problem for Ethereum: no single node can get caught up in an infinite loop running a program. Doing so would essentially stop the evolution of the blockchain and halt all transactions. But there is a way around that. Since computation is costly, and it is in fact rewarded by giving nodes that produce blocks ether (like Bitcoin), what better way to limit computations than by requiring ether for running them. Thus Ethereum solves the problem of denial of service attacks through malicious (or bugged) scripts that run forever. Every time a script is run, the user requesting the script to run must set a limit of ether to spend in it. Ether is consumed by the script as it runs. This is ensured by the virtual machine that runs the scripts. If the script cannot complete before running out of ether, it is halted at that point. In Ethereum the ether assigned to an script as a limit is known as gas (as in gasoline). As ether represents value, it can be converted to other coins. Exchanges exist to trade ether for other coins. This gives ether a real money valuation7, much like coins from Bitcoin. 2.2.2 Smart Contracts Smart contracts are the key element of Ethereum. In them any algorithm can be encoded. Smart contracts can carry arbitrary state and can perform any arbitrary computations. They are even able to call other smart contracts. This gives the scripting facilities of Ethereum tremendous flexibility. Smart contracts are run by each node as part of the block creation process. Just like Bitcoin, block creation is the moment where transactions actually take place, in the sense that once a transaction takes place inside a block, global blockchain state is changed. Ordering affects state changes, and 4 http://dapps.ethercasts.com 5 https://en.wikipedia.org/wiki/Turing_completeness 6 https://en.wikipedia.org/wiki/Halting_problem 7 https://coinmarketcap.com/currencies/ethereum/ 31PDF Image | Intro to Ethereum and Smart Contracts
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