mixing Fixed and Normed

[rapus95]

I recently had to add several differences of N0f8 values to finally take the abs of it. Thus I tried to do

#a,b of type N0f8
x = zero(Q7f8)
x += a - b

which probably would've failed due to the difference being negative and thus not representable.
Thus the next step for me would've been to convert a,b to Q7f8 which didn't work.

Thus my question: Is there any reason why we can't mix up these types?
btw promote(N0f8(.5), Q7f8(.5)) throws an error.

[johnnychen94]

An interesting issue related to this is:

julia> promote_type(UInt8, Int8)
UInt8

julia> UInt8(1) - Int8(2)
0xff

Update: detailed discussion goes in JuliaLang/julia#9292

[timholy]

We should define promotions that make sense. Care to give it a crack?

In practice, the two number types are slightly different: N1f7 is scaled by 1/127, but Q0f7 is scaled by 1/128. What this means is that arithmetic may be faster if you first convert everything to Float32, and then store the final result in whatever format you want.