FAQ: How to code the law?
Booleans and conditions
Money and rounding
In catala, monetary values are represented as an integer number of cents.
A calculation with the catala money type always result in an amount rounded to the nearest cent. This means, that, when performing intermediate computations on money, rounding must be considered by the programmer at each step. This aims at making review by domain experts easier, since for each intermediate value, they can follow along and perform example computations with a simple desk calculator.
The round of builtin will round to the nearest monetary unit (e.g. euro or dollar).
To round an amount to an arbitrary multiple of a cent, perform a multiplication with that multiple, round the amount and divide it by the same amount.
Example: rounding to the nearest multiple of 10 cents
declaration scope Rounding10Cents:
output result content money
scope Rounding10Cents:
definition result equals
round of ($4.13 * 10.) / 10.
$ clerk run round.catala_en --scope Rounding10Cents
[0/1] <catala> interpret Rounding10Cents ⇐ _build/round.catala_en
┌─[RESULT]─
│ result = $4.10
└─
Rounding up or down
To round up or down, add or subtract half a unit before performing the computation and rounding.
Example
To compute a tax amount that is defined as 0.5% of a gross amount (here $149.26) rounded down to the nearest cent
declaration scope Rounding:
output tax_amount content money
scope Rounding:
definition tax_amount equals
money of
(
round of
(
((decimal of $149.26) * 0.5% * 100.0) - 0.5
)
/ 100.
)
This is a bit of a mouthful, but can be made more readable by introducing a function.
declaration scope Rounding:
output tax_amount content money
internal round_down_cents content money depends on amount_decimal content decimal
scope Rounding:
definition tax_amount equals
round_down_cents of (decimal of $149.26 * 0.5%)
definition round_down_cents of amount_decimal equals
money of ((amount_decimal * 100.0 - 0.5) / 100.)
Besides being more readable, this has the benefit of making it more obvious that any computation that requires keeping a fractional number of cents should be performed on decimals, and converted to a monetary amount at the end.