Missing class op rules when using API

Erdi, Gergo Gergo.Erdi at sc.com
Tue Oct 18 04:50:23 UTC 2022


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


This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations

Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm

Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer.

Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. 

Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same.

Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20221018/ba9b4a61/attachment.html>


More information about the ghc-devs mailing list