Instantiation of overloaded definition *in Core*

[Simon Peyton Jones]

You can look it up in the class instance environment, which the Simplifier does have access to it.  That's relatively easy when you have a simple dictionary like (Monad IO).  But if you want (Eq [Int]) you first of all have to look up the (Eq [a]) dictionary, then the Eq Int dictionary, and apply the former to the latter.  We don't (yet) have a simple API to do that, although it would not be hard to create one.

Simon

PS: I am leaving Microsoft at the end of November 2021, at which point simonpj at microsoft.com<mailto:simonpj at microsoft.com> will cease to work.  Use simon.peytonjones at gmail.com<mailto:simon.peytonjones at gmail.com> instead.  (For now, it just forwards to simonpj at microsoft.com.)

From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Erdi, Gergo via ghc-devs
Sent: 04 October 2021 10:30
To: 'GHC' <ghc-devs at haskell.org>
Cc: Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>
Subject: Instantiation of overloaded definition *in Core*


PUBLIC

Hi,

I'd like to instantiate Core definitions. For example, suppose I have the following Core definition:

foo :: forall m a b. Monad m => m a -> m b -> m b
foo = \ @m ($d :: Monad m) @a @b (ma :: m a) (mb :: m b) -> ...

Now let's say I'd like to instantiate it for m ~ IO. It is quite straightforward to go from the above to:

foo_IO_0 :: forall a b. Monad IO => IO a -> IO b -> IO b
foo_IO_0 = \ ($d :: Monad IO) @a @b (ma :: IO a) (mb :: IO b) -> ...

However, I would like to go all the way to:

foo_IO :: forall a b. IO a -> IO b -> IO b
foo_IO = \ @a @b (ma :: IO a) (mb :: IO b) -> ...

Because instances are coherent, it should be sound to replace all occurrences of $d with "the" dictionary for Monad IO. However, the places I've found for this kind of query seem to live in the typechecker. How do I access this information while working with Core?

Thanks,
            Gergo

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