>From the paper Fun with Type Funs, it's said:

> One compelling use of such type functions is to make type
> coercions implicit, especially in arithmetic. Suppose we want to be able to
> write add a b to add two numeric values a and b even if one is an Integer
> and the other is a Double (without writing fromIntegral explicitly).

And then an Add class is defined which can dispatch at the type-level
to appropriate functions which resolve two types into one, with a
catch-all case for Num.

Has anyone put this into a package, for all common arithmetic
operations? I would use it. Doing arithmetic stuff in Haskell always
feels labored because of having constantly convert between number
types.

I prefer the current way (which is interestingly what Go chose as well). With implicit casts it's easy to shoot yourself in the foot e.g. when doing bit-twiddling. These two are different

f :: Word8 -> Int -> Word32

f w8 n = fromIntegral (w8 `shiftL` n)

f' :: Word8 -> Int -> Word32

f' w8 n = (fromIntegral w8) `shiftL` n