Trouble using Arbitrary to generate a BigDecimal

[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' }

I asked this on the fast-check repo and they said it was to do with how the Effect library maps BigDecimal to an Arbitrary. Thread here

[gcanti]

I can't reproduce it, I'm seeing large values here

import { Arbitrary, BigDecimal, FastCheck, Schema } from "effect"

const schema = Schema.Struct({
  money: Schema.BigDecimal.pipe(Schema.betweenBigDecimal(
    BigDecimal.fromBigInt(0n),
    BigDecimal.fromBigInt(100000000n)
  ))
})

const arbitrary = Arbitrary.make(schema)

console.log(FastCheck.sample(arbitrary, 100).map((x) => BigDecimal.unsafeToNumber(x.money).toString()))
/*
[
  '65000',   '1200',      '15',      '3.5e-11', '26000000', '2100',
  '2.2',     '0.67',      '7.7e-14', '7.2e-7',  '7200',     '0.0000059',
  '1.8e-13', '0.0000062', '5800',    '580',     '2.5e-13',  '5e-14',
  '1700',    '3700',      '2.3e-16', '4.1e-17', '140',      '38',
  '2.3e-7',  '49000000',  '5e-16',   '480000',  '4e-11',    '600',
  '4.1e-17', '5.1e-16',   '4300',    '0.6',     '550',      '3e-8',
  '310000',  '4.8',       '6.9e-12', '610',     '0.74',     '0',
  '6.3e-9',  '0.0074',    '4.4e-12', '7.7e-17', '0.042',    '0.00051',
  '4.1e-15', '7.5e-11',   '3.4',     '0.043',   '27000000', '1.1e-16',
  '1600000', '3000000',   '7.6e-12', '60000',   '760000',   '0.0013',
  '0',       '1.7e-12',   '1.8e-7',  '0.00068', '620000',   '0.002',
  '0.57',    '0.4',       '36000',   '4.2e-10', '2e-18',    '0.33',
  '30',      '5.8e-9',    '5600',    '3500',    '2e-14',    '0.000076',
  '6e-16',   '410',       '7e-11',   '58',      '6.6e-9',   '100000000',
  '0.032',   '1e-9',      '3.5e-11', '0.003',   '450',      '0.19',
  '0.59',    '0.0067',    '0.004',   '0.00035', '0.008',    '110',
  '5000',    '0.009',     '1.3e-16', '37'
]
*/

[tobold]

interesting, thank you.

I've run the exact same code and get this:

[
  '0.006',     '0.62',      '0.0058',    '0.009',     '7.4',      '0.00035',
  '65',        '0.0049',    '0.25',      '6.8',       '0.6',      '0.071',
  '0.014',     '0.071',     '3.4',       '0.018',     '0.74',     '0.043',
  '0.0066',    '60',        '2.4',       '0.074',     '0.45',     '49',
  '0.0000073', '0.0012',    '0.0068',    '0.033',     '0.000071', '3.3',
  '0.42',      '7.6',       '0.43',      '0.000067',  '0.051',    '0.000031',
  '4',         '0.0033',    '57',        '6',         '0.00001',  '76',
  '0.00062',   '1',         '0.00064',   '0.064',     '1.7',      '3.8',
  '0.0042',    '68',        '0.0000074', '0.0000049', '41',       '3.5',
  '14',        '7',         '0.45',      '0.0064',    '0.063',    '64',
  '44',        '0.0074',    '7.4',       '0.77',      '0.0002',   '0.028',
  '0.00057',   '2.3',       '0.0024',    '0.62',      '0.77',     '0.000027',
  '37',        '0.0000017', '55',        '0.0000033', '0.51',     '0.51',
  '0.5',       '0.05',      '43',        '42',        '0.000023', '0.00057',
  '0.0019',    '36',        '7.4',       '12',        '0.00002',  '0.000064',
  '0.0066',    '0.001',     '11',        '0.00009',   '3.6',      '50',
  '5.3',       '0.000022',  '0.4',       '0.0000014'
]

[gcanti]

That's because I was being dumb, and while trying to reproduce the issue I accidentally changed the way values were being generated.

So the code responsible is here:

effect/packages/effect/src/Schema.ts

Line 7890 in fb53370

const bigDecimalArbitrary = (): LazyArbitrary<bigDecimal_.BigDecimal> => (fc) =>

Here fc.integer({ min: 0, max: 18 }) generates the scale.

const bigDecimalArbitrary = (): LazyArbitrary<bigDecimal_.BigDecimal> => (fc) =>
  fc.tuple(fc.bigInt(), fc.integer({ min: 0, max: 18 }))
    .map(([value, scale]) => bigDecimal_.make(value, scale))

I guess we could tweak that, maybe to fc.integer({ min: -18, max: 18 })?

[tobold]

Ok that makes sense. Yes that change would be appreciated.

Is there a way I can configure the BigDecimal arbitrary generation locally (without changing the Effect package code)?

[gcanti]

You can provide an override through an annotation

import { BigDecimal, Schema } from "effect"

const MyBigDecimal = Schema.BigDecimal.annotations({
  arbitrary: () => (fc) => {
    return fc.tuple(fc.bigInt(), fc.integer({ min: -18, max: 18 }))
      .map(([value, scale]) => BigDecimal.make(value, scale))
  }
})

const schema = Schema.Struct({
  money: MyBigDecimal.pipe(Schema.betweenBigDecimal(
    BigDecimal.fromBigInt(0n),
    BigDecimal.fromBigInt(100000000n)
  ))
})

[gcanti]

#5875

[tobold]

Thanks, version 3.19.9 of Effect works.