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Numscript templates

Oftentimes transactions will use the same structure over and over again, and you don't want to use hard-coded values in your Numscript. In these cases, you can use a Numscript template to describe a general kind of transaction, and then fill in the template variables at execution time.

First time working through a Numscript guide?

Make sure that you're set up properly: Read through the prerequisites first!. Otherwise the examples below won't work.

Repetitive transactions​

In the game world of Cones of Dunshire, building cones is a central part of play. We want to reward players who successfully build cones with coin.

Building cones

Image that benwyatt builds a cone and, as the ledgerman, we want to reward him with 100 coins for doing so. Here is what that looks like in Numscript:

Simple enough. But consider: benwyatt isn't the only player building conesβ€”ideally all players will be doing that. More, we'd like to encourage cones of varying sizes and quality of materials, so that bigger cones and cones constructed of rarer materials earn larger rewards.

So now barneyvarmn comes along and builds a larger cone of rarer materials. We want to reward his cone building as well:

Notice that to reward barneyvarmn we had to construct a whole new transaction in Numscript. This is a tedious and error-prone process. But we can eliminate these problems by using Numscript templates!

Your first template​

Templates use variables to reflect aspects of a transaction that might change from time to time, in our case the size of the reward and the recipient. Here is a Numscript template that we can use to create cone-building reward transactions. Create a file called cone.num:

There are two things to observe about the template.

First, there is a vars block at the top. This block declares all the variables we are going to use, along with their types. So we have a monetary variable called $amount that can represent quantities of different currencies like [COIN 100] or [USD/2 500]. And we have an account variable called $player that can represent any account in our ledger, either for sending for receiving.

Second, there is a send block that should look largely familiar. It will send an amount of money specified by the variable $amount from the @centralbank account to any account specified by the variable $player.

Variable types

Numscript supports several variable types. Details on the different types and how they are used are available in the Numscript reference docs.

What's great is we can save this template, and use it whenever we need to reward a player for building a cone, or for any transaction from @centralbank to any single account.

Assigning values to variables​

That said, we can't run this Numscript directly, as Formance Ledger can't know what values to use for our variables. When we execute a Numscript template, we also need to specify the values for each variable in a seperate JSON snippet. Suppose that we want to use the template to send 100 coin to benwyatt for building a cone, we can specify the values to plug into our template by creating a file called vars.json with this JSON object:

vars.json
{
"amount": {
"amount": 100,
"asset": "COIN"
},
"player": "@player:benwyatt"
}

As you can see, the names of the keys in the JSON object correspond to the variables in the Numscript template. Note that because monetary is a compound type, we had to specify both a currency and a quantity for the variable named $amount. Note also that the variable names do not have the $ prefix in the JSON snippet.

Running a Numscript template​

A complete transaction consists of a template in Numscript and a specification of the values in JSON. Once we have both, we can pass it to Formance Ledger for execution.

caution

You cannot use the command line to execute Numscript templates.

At least, not at this time. That feature is coming though! In the meantime you can execute Numscript templates using Formance Ledger's built-in API server. If you've never done that before, take a moment to read up on how to use the API server to execute Numscript.

The /{ledger}/script API endpoint expects a request containing a JSON object with two fields: 'plain' which is a string containing the Numscript and vars which is the JSON object containing the values to substitute in:

{
"plain": "NUMSCRIPT TEMPLATE HERE",
"vars": {
"variable1": "value1",
//… etc.
}
}

We can use jd to construct this JSON request from our Numscript template (saved in cone.num) and our value specification (saved in vars.json):

jq --rawfile cone cone.num '{plain: $cone, vars: .}' vars.json

Running this should give the following result

jd output
{
"plain": "vars {\nmonetary $amount\naccount $player\n}\n\nsend $amount (\nsource = @centralbank\ndestination = $player\n)",
"vars": {
"amount": {
"amount": 100,
"asset": "COIN"
},
"player": "@player:benwyatt"
}
}
command not found: jq

If you receive an error like

bash: comand not found: jq

then you do not have the jq command line tool installed. You'll need to install jq to run the examples.

We can now use the output from jd with HTTPie or cURL to send to the Formance Ledger API:

jq --rawfile cone cone.num '{plain: $cone, vars: .}' vars.json \
| http POST http://localhost:3068/dunshire/script
What is HTTPie?

HTTPie is an alternative to cURL for testing REST APIs, designed to have a simpler interface optimized for constructing API test calls. It's pretty cool, and we recommend it over cURL for testing things out. Read more about HTTPie including how to install it.


The API response​

On success

The Formance Ledger API will return a 200 status code on success, and an empty JSON object in the response body.

On failure

The Formance Ledger API will return a 200 status code even on failure, but the JSON object returned in the response body will have more information:

{
"details": "https://play.numscript.org/?payload=eyJlcnJvciI6ImFjY291bnQgaGFkIGluc3VmZmljaWVudCBmdW5kcyJ9",
"err": "account had insufficient funds"
}

The err field will contain a human-readable indication of what went wrong, for example that an account had insufficient funds, or that there was an error in the provided Numscript.

There is also a details field with a URL. When there is an error parsing Numscript, the result can be difficult to readβ€”the provided URL will render the error in an easy-to-read format.

Templates and Metadata​

In addition to specifying the value of variables directly, we can pull those values from metadata associated with accounts.

Let's suppose that we want to take a commission whenever one player transfers coin to another player. We can store the current commission rate as metadata on the @centralbank account, and then pull that rate in dynamically when we execute that kind of transaction with Numscript:

At execution, you'll need to specify the amount to send, and the sender and receiver accounts. Then Formance Ledger will examine the metadata on the @centralbank account, looking for a structure like this:

{
"commission_rate": {
"type": "portion",
"value": "5/100"
}
}

In this case, the metadata specifies a 5% commission on user-to-user transactions.

The advantage is that you can record the variables like your commission rate in a convenient, centralized place, without having to hard code the values.

Going further​

This guide has just been a small taste of what's possible using Numscript templates. Nearly any Numscript transaction can be templatized.

Dig deeper

Want to learn more about creating Numscript templates? The Numscript reference docs have you covered!