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OP you may have meant to have linked directly to https://soundness.dev/abacist

Not a scala person, but I like the idea of being able to shift between unit/measurement systems. Lots of strange little edge cases. Eg, how does rounding work in imperial, can I get 3.1415926535 meters to the closest 1/64th of an inch?

compare ⊤ and ⊥ in APL (a programming language old enough that the pound was not decimal, but still divided into shillings and pence, at its creation):

https://aplwiki.com/wiki/Encode

https://aplwiki.com/wiki/Decode

This seems similar to the 'ComplexAmount' system I used to use in PHP and JavaScript programs for representing measurements using mixed units. Basically just a map of unit code => quantity. And for extra exactness, quantity would be represented as a rational number.

Was useful for a client that let you place orders using a mixture of USD and CAD (the alternative being a lot of 'canada'/'canadian'/'cad' fields added willy-nilly everywhere a cost needed to be represented; I don't know how people can stand to program that way). But also for generating G-code for a CNC router [1], when sometimes you want to model in mm, and sometimes in inches, and keep all the numbers exact until the last possible moment.

[1] https://github.com/TOGoS/TTSGCG