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. ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************