[dastbe]

Canonicalization is tough, and requires you to define some common ordering on your units.

Systems will use an array of unit powers, so that if the array were defined as <Joules, Seconds, Newtons, Meters>, then acceleration would be <0,-2,0,1> and watts would be <1,-1,0,0>. Addition and subtraction require that your arrays are equal, and multiplication and division are pairwise additive/subtractive.

[sigterm]

Is it tough? There are only seven fundamental units. https://en.wikipedia.org/wiki/SI_base_unit

Just represent every unit in terms of them then it's good.

One problem with the array you came up is that the units are not orthogonal, since Joules = Newtons * Meters.

[jcranmer]

Actually, there's more "kinds" of units: radians (versus degrees) and steradians come to mind.

[masklinn]

These are dimensionless derived units in SI equivalence (respectively m/m and m2/m2)

[dastbe]

Sorry! I never really spent much time in the physical sciences, so I didn't know that. You would obviously not want to pick Joules as a fundamental unit.

[jarrettc]

Will the Rust compiler ever understand numbers in types? I.e. will the numbers ever be more than just part of the string that is the type name? If not, then I don't know how possible powers will be. Maybe the solution would be to define some types for commonly-used powers, e.g. PowN4, PowN3, PowN2, PowN1, Pow2, Pow3, Pow4. Users who needed higher powers could define those types themselves, I guess.

[steveklabnik]

We do desire type-level integers, yes. There hasn't been an RFC yet, though.