On Wed, Nov 23, 2022 at 12:28:46PM +1100, Clinton Mead wrote:

I have a class with a "method" which has a default definition, and that default definition has no arguments on the LHS, will a separate "instance" of that default definition be created for each instance of that class that inherits that default definition? The important consequence of that being that the default definition is only computed once per type.

Typically, an instance method will have the instance type variable present in either one of the parameter types or in the result type, making it possible to infer at call sites which instance to invoke. Such methods are polymorphic, and rarely admit a sensible default value. However, when a such a default value is possible, and if you disable any inlining that might trigger separate per call site evaluation, then indeed you can get a "once per-type" value. The below prints "Foo wuz here" only three times. Main.hs: module Main (main) where import Again import M main :: IO () main = do print $ one : foo print $ 'X' : foo print $ [one] : foo again where one = 1 :: Int Again.hs: module Again(again) where import M {-# NOINLINE again #-} again :: IO () again = do print $ one : foo print $ 'X' : foo print $ [one] : foo where one = 1 :: Int M.hs {-# LANGUAGE FlexibleInstances #-} module M (foo) where import Debug.Trace class M a where {-# NOINLINE foo #-} foo :: [a] foo = trace "Foo wuz here" $ [] instance M Int instance M Char instance M ([Int]) With `TypeApplications` and `AllowAmbiguousTypes`, you can define non-polymorphic instance methods that require an explicit type application at the call site. In that case, with inlining disabled and optimisation enabled, the various default `foo @sometype` calls can be collapsed to a single constant across multiple types. M.hs: {-# LANGUAGE AllowAmbiguousTypes, FlexibleInstances #-} {-# OPTIONS_GHC -O2 #-} module M (foo) where import Debug.Trace class M a where {-# NOINLINE foo #-} foo :: Int foo = trace "Foo wuz here" $ 42 instance M Int instance M Char instance M ([Int]) With the above class definition and instance definitions and the print statements necessarily written with type applications: print $ foo @Int print $ foo @Char print $ foo @[Int] the trace string is printed just once. -- Viktor.