Using Reality.eth from a contract

Fetching information

The simplest way use for Reality.eth is to fetch some information for a question that has already been posted using the DApp.

Hovering your mouse over the “…” mark in the upper right-hand corner of a question will show you the question ID and the hash of the question content.

Fetching the answer to a particular question

Each question can be referred to by its question_id, which is a bytes32.

There is only one question with any given question_id, and there can only be one corresponding answer.

You can fetch the final answer for a question by calling

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bytes32 response = resultFor(bytes32 question_id);

This will return bytes32 data, or throw an error (revert) if the question does not exist or has not been finalized. You often want to cast the resulting bytes32 to a uint256 when you process it. By convention all supported types use 0xff...ff (aka bytes32(type(uint256).max)) to mean “invalid”.

Interpreting the result of a question

Questions have a type, which decides what format the answer should take, and tells the Reality.eth dapp what kind of UI it should display to submit answers. The dapp recognizes the following types:

bool

A bool type represents a yes/no question. The result will be 1, 0, or 0xff...ff for invalid, for example:

Did Donald Trump win the 2016 US presidential election?
  • 1: Yes

  • 0: No

  • 0xff..ff: Invalid

uint

A uint type represents a number. The question may have a decimals field by which it should be divided. Negative numbers are not currently supported.

How many electoral votes did Hillary Clinton win in the 2016 US presidential election?

single-select

A single-select type represents a list of outcomes, producing a select box in the dapp. It is expressed as a zero-indexed, json-encoded array.

Which party’s nominee won the 2016 US presidential election?

  • 0: Democrat

  • 1: Republican

  • 2: Libertarian

The outcomes are supplied in a field called outcomes. There is also an invalid option (0xff..ff) which is added automatically.

["Democrat", "Republican", "Libertarian"]

multiple-select

The multiple-select type represents a list of outcomes, producing checkboxes in the dapp. It is Expressed as a one-indexed, json-encoded array.

Which party’s nominee won the seats the House in the 2016 congressional elections?

  • 1: Democrat

  • 2: Republican

  • 4: Libertarian

These are added together, so 0 would mean than none were selected, (1 + 2) = 3 would indicate Democrat and Republican, (1 + 4) = 5 would indicate Democrat and Libertarian, etc. There is also an 0xff..ff: Invalid option.

The outcomes are supplied in a field called outcomes, as with single-select. ["Democrat", "Republican", "Libertarian"]

datetime

A datetime type represents a question that is answered with a date/time. This will be represented as a date picker in the UI, and expressed as a unix timestamp (seconds since 1970), of 0xff..ff for invalid.

The question may also specify a precision indicating whether it expects a datetime to the nearest year (Y), month (m), day (d), hour (H), minute (i) or second (s). If no precision is specified, it should be assumed to be d, ie the answer should be a date (in UTC) with no time. An answer with a greater precision than that specified should be considered wrong.

How questions are structured

Questions are expressed as a JSON object. This should specify a title and a type, and may also include a category and lang field. It may include other fields depending on the type, such as outcomes to specify the possible outcomes in a single-select or multiple-select, or decimals in a uint type.

Example: {"title": "Did Trump win the 2016 US presidential election?", "type": "bool", "category": "politics", "lang": "en"}

Templates

To avoid duplication and resulting gas fees, parts common to many questions are included in a pre-defined template. The template includes placeholders, and each question only needs to pass in the data necessary to replace the placeholders.

The following templates are built in, one for each question type:

  • 0: {"title": "%s", "type": "bool", "category": "%s", "lang": "%s"}

  • 1: {"title": "%s", "type": "uint", "decimals": 18, "category": "%s", "lang": "%s"}

  • 2: {"title": "%s", "type": "single-select", "outcomes": [%s], "category": "%s", "lang": "%s"}

  • 3: {"title": "%s", "type": "multiple-select", "outcomes": [%s], "category": "%s", "lang": "%s"}

  • 4: {"title": "%s", "type": "datetime", "category": "%s", "lang": "%s"}

See Custom templates for how and why you can create your own custom template.

Questions are creating by combining the ID of the template (for example, 2 for the built-in single-select template) with the unicode delimiter characther “␟”.

For example, a simple boolean question intended for the category “news-politics” might use the string: Did Donald Trump win the 2016 US presidential election?␟news-politics␟en

A question with multiple outcomes will need the JSON-encoded outcome list passed in, eg Which party's nominee won the 2016 US presidential election?␟"Democrat","Republican","Libertarian"␟news-politics␟en

Asking questions

You can ask a new question by calling the askQuestion() function.

The content of the question defined as a combination of a numerical template_id and a string of parameters.

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function askQuestion(
   uint256 template_id,
   string question,
   address arbitrator,
   uint32 timeout,
   uint32 opening_ts,
   uint256 nonce
)
returns (bytes32 question_id);

If the arbitrator you have selected charges a per-question fee, you must supply at least this much ETH. Any additonal ETH you send with this call will be assigned as a reward to whoever supplies the final answer to the question. See Fees and payments for more information.

See Templates for the template_id, and how the question parameter is structured.

If you are using a version of the contract that uses an ERC20 token, the same call will work but you cannot use it to supply funds. If you need to supply funds, either for an initial bond or because the arbitrator requires a per-question fee, you should instead call the ERC20 version, askQuestionERC20(). This has an additional parameter for the number of tokens you wish to supply. This will attempt to debit the tokens from the token contract, or revert if they have not been approved.

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function askQuestionERC20(
   uint256 template_id,
   string question,
   address arbitrator,
   uint32 timeout,
   uint32 opening_ts,
   uint256 nonce,
   uint256 tokens,
)
returns (bytes32 question_id);

The arbitrator is the address of the contract that will be able to arbitrate. See Arbitrators for more information. From version 3, this parameter may be left empty. For prior versions, if you wish to make arbitration impossible, you can instead supply the address of the reality.eth contract itself.

The timeout is the time in seconds the question will have after adding an answer before it is automatically finalized. It would typically be around 1 days. The contract sanity-checks set the maximum possible value at 365 days.

The opening_ts is the timestamp for the earliest time at which it will be possible to post an answer to the question. You can use 0 if you intend the question to be answerable immediately.

The nonce is a user-supplied number that can be used to disambiguated deliberate repeated uses of the same question. You can use 0 if you never intend to ask the same question with the same settings twice.

Any ETH or tokens provided with the askQuestion or askQuestionERC20 call will be used as a question reward, minus any fee the specified arbitrator requires when a new question is asked.

The askQuestion call returns a bytes32 ID. This ID is made by hashing the parameters, plus msg.sender. Note that the format will change in version 3.

Note

The Etherscan “write contract” feature has been known to mangle the delimiter character.

As of version 3, you can also specify a minimum bond below which the initial answer will not be accepted. For this, use the askQuestionWithMinBond() or askQuestionWithMinBondERC20() method.

Accepting an answer only if something has happened (pre version 3)

Once a question has been created and the opening date (if set) reached, it can be answered immediately.

In many cases you are not interested in the result of a particular question until it has a particular answer. For example, if you have a contract insuring against my house burning down, you are only interested in the result if my house burned down. You don’t care about all the times in between setting up the policy and claiming when my house didn’t burn down.

You may also want to screen out results indicating “unknown” or “no way to tell” or “hasn’t happened yet”.

One approach is that instead of waiting for the result of a specific question_id, you specify the type of question you want, then wait for a user to send you a question ID with the appropriate content and settings.

To make this easier, we provide a method called getFinalAnswerIfMatches(). This will throw an error not only if the question is not yet answered, but also if the content doesn’t match, the bond or timeout is too low, or the arbitrator is not the one you expect.

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 function getFinalAnswerIfMatches(
     bytes32 question_id,
     bytes32 content_hash,
     address arbitrator,
     uint256 min_timeout,
     uint256 min_bond
 ) returns (bytes32 answer)

You can then screen answer in your contract and only act on results that your contract is interested in.

Accepting an answer only if something has happened (version 3 onwards)

To simplify the process of accepting only questions with an answer, from version 3 you can substitute forResult() with forResultOnceSettled(). This screens the result for an answer representing “not yet settled”, represented as bytes32(-2). Rather than returning this answer, it will revert as it would if the question question had not yet been answered. The contract allows any user to reopen a question in this state, creating a new question, repeatedly if necessary. Once a replacement question has been settled, its result is returned using the ID of the original question. A calling contract can ignore the details of this process and know only that it needs to call forResultOnceSettled().

If using this feature it is also advisable to set a minimum bond.

Custom templates

If you want to create many similar requests, it will be more efficient to create your own template. For example, a flight insurance app might have:

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 {
     "title": "Was flight %s on date %s delayed by more than 3 hours?",
     "type": "bool",
     "category": "flight-information"
 }
This can then by called with a string including only the flight number, the delimiter and the date, eg:

MH17␟2017-12-01

A template can be created by calling createTemplate("template"), where “template” is the JSON template. This returns a numerical ID. If you wish to reference particular template on code running on multiple networks with the same content, you may find it useful to call template_hashes(bytes32 template_hash) to get the numerical ID.