On Mon, Nov 3, 2008 at 3:53 PM, Daniel Fischer <daniel.is.fischer@web.de> wrote:

Am Dienstag, 4. November 2008 00:26 schrieb Michael Snoyman:

Beware! Multiplication is usually not commutative, think about matrices. If
(a `mult` b) and (b `mult` a) are both defined (need not be if they have
different types), the products may have different types, so in general it is
not desirable to have both defined automatically in a way that doesn't force
you to supply the arguments in the correct order.

In your case, you could provide
instance Multiplicable MilesPerGallon Gallon Mile where
       mult = flip mult
-- or write the implementation out
and it should work whichever order the arguments are passed.