Dear GHC friends,
I would like to know whether the representation of FooD1:
data Foo = MkFoo A B
class FooD1 where
foo :: Foo
has a representation at least as efficient as FooD2:
class FooD2 where
fooA :: A
fooB :: B
in particular, that FooD1 doesn't occur an extra indirection. Now,
according to
https://gitlab.haskell.org/ghc/ghc/-/issues/20897
FooD1 used to be compiled as a newtype, so the answer was "yes". Then
something called "Plan B" (described in that ticket) was enacted, and
I believe the answer is therefore still "yes".
Can someone please confirm that?
For a teaser of why I want this: I find type classes very difficult to
evolve in a way that satisfies my stability needs. Part of the reason
for this is that type classes as typically used don't really permit
any form of data abstraction: you list out all the methods explicitly
in the class definition. There is no data hiding. To address this
issue I am considering defining classes abstractly, like
class MyClass where
myClassImpl :: MyClassD
for some abstract type MyClassD (i.e. whose implementation details are
not exposed to users) and I want to know if there would be a
performance cost.
Thanks,
Tom
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