Missing class op rules when using API

[Erdi, Gergo]

PUBLIC

What I've found out is that this was because I was registering the ModDetails as it comes out of tidyProgram. If instead I recreate the ModDetails form the ModIface via initModDetails, the class op rules show up properly and everything works as expected.

Is this documented somewhere, that the ModDetails from tidyProgram is not good for all purposes? Should its type be different, then, from an all-purpose ModDetails?


From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Erdi, Gergo via ghc-devs
Sent: Monday, October 17, 2022 6:33 PM
To: ghc-devs at haskell.org
Subject: [External] Missing class op rules when using API

Hi,

I'm trying to compile the following two modules:

```
{-# LANGUAGE NoImplicitPrelude #-}
module MiniMonad where

class Functor f where
    fmap :: (a -> b) -> f a -> f b

class (Functor f) => Applicative f where
    pure :: a -> f a

class (Applicative m) => Monad m where
    return :: a -> m a
    return = pure

    (>>=)  :: m a -> (a -> m b) -> m b
    infixl 1 >>=
```

```
{-# LANGUAGE NoImplicitPrelude #-}
module SpecMain where

import MiniMonad

data Identity a = MkIdentity a

instance Functor Identity where
    fmap f (MkIdentity x) = MkIdentity (f x)

instance Applicative Identity where
    pure x = MkIdentity x

instance Monad Identity where

run :: Identity a -> a
run (MkIdentity x) = x

foo :: ()
foo = run (return ())
```

If I compile these from the CLI, I see three rules firing:

```
./_build/stage1/bin/ghc -debug -fforce-recomp -dno-typeable-binds input/SpecMain.hs -iinput -O0  -ddump-rule-rewrites -dsuppress-all
```

```
[1 of 2] Compiling MiniMonad        ( input/MiniMonad.hs, input/MiniMonad.o )
[2 of 2] Compiling SpecMain         ( input/SpecMain.hs, input/SpecMain.o )
tc_iface_decl/2 Monad

input/SpecMain.hs:14:10: warning: [-Wmissing-methods]
    * No explicit implementation for
        '>>='
    * In the instance declaration for 'Monad Identity'
   |
14 | instance Monad Identity where
   |          ^^^^^^^^^^^^^^
Rule fired
    Rule: Class op $p1Monad
    Module: (BUILTIN)
    Before: MiniMonad.$p1Monad
              TyArg SpecMain.Identity ValArg SpecMain.$fMonadIdentity
    After:  SpecMain.$fApplicativeIdentity
    Cont:   StrictArg MiniMonad.pure
            ApplyToTy a_avS
            Stop[RhsCtxt(Recursive)] a_avS -> SpecMain.Identity a_avS
Rule fired
    Rule: Class op pure
    Module: (BUILTIN)
    Before: MiniMonad.pure
              TyArg SpecMain.Identity ValArg SpecMain.$fApplicativeIdentity
    After:  $cpure_aw8
    Cont:   ApplyToTy a_avS
            Stop[RhsCtxt(Recursive)] a_avS -> SpecMain.Identity a_avS
Rule fired
    Rule: Class op return
    Module: (BUILTIN)
    Before: MiniMonad.return
              TyArg SpecMain.Identity ValArg SpecMain.$fMonadIdentity
    After:  $creturn_avP
    Cont:   ApplyToTy ()
            ApplyToVal nodup hole () -> SpecMain.Identity () GHC.Tuple.Prim.()
            Select nodup wild_X1
            Stop[RhsCtxt(NonRecursive)] ()
```

However, if I try this via the API (see attached `main.hs`), then only `Class op return` fires:


```
Processing module MiniMonad
Processing module SpecMain

input/SpecMain.hs:14:10: warning: [-Wmissing-methods]
    * No explicit implementation for
        '>>='
    * In the instance declaration for 'Monad Identity'
   |
14 | instance Monad Identity where
   |          ^^^^^^^^^^^^^^
Rule fired
    Rule: Class op return
    Module: (BUILTIN)
    Before: MiniMonad.return
              TyArg SpecMain.Identity ValArg SpecMain.$fMonadIdentity
    After:  $creturn_aqm
    Cont:   ApplyToTy ()
            ApplyToVal nodup hole () -> SpecMain.Identity () GHC.Tuple.Prim.()
            Select nodup wild_X1
            Stop[RhsCtxt(NonRecursive)] ()
All done.
```

This of course leads to much worse code further downstream (and is catastrophic for my use case where I rely on extensive specialisation).

Why is that? What is missing from my `main.hs` that only one of the three method rules is correctly used when compiling the second module?

Thanks,
Gergo


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