...

then the user can write hbar = 6.62606876e-34 `scalarm` (joule `mult` second)

That works, but IMO it's a bit unwieldy. It also doesn't help with actually defining a reasonable type signature for hbar which is the point of the exercise.

Well, how about this? hbar = (PhysicalUnit 6.62606876e-34)::t where x::t = (undefined::Energy) `mult` (undefined::Time) the use of bottom might be a bit bizarre -- OTH, a physicist of a kind that cares about Planck constant should get used to the infinities -- take QED for example. A physicist might even appreciate that equation. It's interesting that in every thread I have participated over the last couple of months I mentioned 'undefined'. It must be my favorite Haskell value.