A question about run-time errors when class members are undefined

[Jurriaan Hage]

Hello,

We are adding classes and instances to Helium.

We wondered about the aspect that it is allowed to have a class instance
of which not all fields have a piece of code/value associated with them, and 
that as a result when you happen to call these, a run-time error results.
(see Sec. 4.3.2 of the Haskell 2010 report).

Does anyone know of a rationale for this choice, since it seems rather unhaskell-like.

best,
Jur