Trouble using Arbitrary to generate a BigDecimal (from the Effect library)

[tobold]

We're using Fast-check to run property tests against our Effect Schema but Fast-check is struggling to generate values that match the schema for BigDecimal values.

Our code looks like this:

const schema = Schema.Struct({
  money: Schema.BigDecimal.pipe(Schema.betweenBigDecimal($`0`, $`100_000_000`))
}

const arbitrary = Arbitrary.make(schema)

FastCheck.asyncProperty(arbitrary, async (generated) => {
  console.log({money: BigDecimal.unsafeToNumber(generated.money).toString() });
}

When we log out the generated values, they are all very small decimals (all less than 100 I believe). It feels like they are the wrong order of magnitude (schema defines it as between 0 and 100,000,000).

e.g.:

{ money: '5.4e-7' }
{ money: '0.0001' }
{ money: '5.7' }
{ money: '0.0064' }
{ money: '1.3e-12' }
{ money: '3.9e-17' }
{ money: '4.1e-8' }
{ money: '0.000032' }
{ money: '0.0073' }
{ money: '0' }
{ money: '0.000039' }
{ money: '3.6e-14' }
{ money: '0.0000061' }
{ money: '0.062' }
{ money: '9e-11' }
{ money: '0.00031' }
{ money: '1.7e-8' }
{ money: '0.00025' }

Is there a way we can configure Fast-check to understand how to generate BigDecimals? I thought because Fast-check is bundled with Effect, it would be able to handle it.

[gruhn]

You're talking about this effect library right? As far as I know, fast-check is not directly affiliated with this project. The issue might be with their usage pattern. I had a quick look at their codebase but at a glance I can't tell how they map their BigDecimal representation to a fast-check Arbitrary. I would recommend to file in issue at their repository first.

[tobold]

Yes, that's the library. I will raise an issue with them.

But if I wanted to use fast-check to generate an Arbitrary of a custom class, how would I do it? Is there a way to configure fast-check to generate any Arbitrary I need?

[gruhn]

I'm not familiar with effect, so not sure what is possible through their interface. If you use fast-check directly it is possible. For example, if you have a class that has a string argument, you can derive an Arbitrary for that class from the string Arbitrary:

import fc from "fast-check"

fc.string().map(str => new MyClass(str))