Type classes and code generation

[Bayley, Alistair]

I had a discussion with someone over the type class mechanism and would like
to clarify something.

When I compile this trivial program:

> module Main where
> main = putStrLn (show (1 + 2))

with ghc -Wall, the compiler says:

Main.lhs:3:
    Warning: Defaulting the following constraint(s) to type `Integer'
	     `Num a' arising from the literal `2' at Main.lhs:3

This implies to me that the compiler is generating the code for (+) for the
particular instance, rather than using a run-time dispatch mechanism to
select the correct (+) function. Is this correct, or am I way off? Does the
compiler *always* know what the actual instances being used are? Is there
some way of preventing the type mechanism from generating code for the
instance type, as opposed to the class? 


If I am correct, does it work the same way across module boundaries? (I
would think so.) If a module exports a class but no instances for that
class, then a user of that class would have to install their own instances.
OTOH, if the class plus one or more instances were exported, then a user
could use the supplied instance types, and the compiler would still generate
code to use the specific instances.


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