question about GHC API on GHC plugin

Ömer Sinan Ağacan omeragacan at
Wed Aug 26 02:17:36 UTC 2015

It seems like in your App syntax you're having a non-function in function
position. You can see this by looking at what failing function
(splitFunTy_maybe) is doing:

    splitFunTy_maybe :: Type -> Maybe (Type, Type)
    -- ^ Attempts to extract the argument and result types from a type
    ... (definition is not important) ...

Then it's used like this at the error site:

    (arg_ty, res_ty) = expectJust "cpeBody:collect_args" $
                       splitFunTy_maybe fun_ty

In your case this function is returning Nothing and then exceptJust is
signalling the panic.

Your code looked correct to me, I don't see any problems with that. Maybe you're
using something wrong as selectors. Could you paste CoreExpr representation of
your program?

It may also be the case that the panic is caused by something else, maybe your
syntax is invalidating some assumptions/invariants in GHC but it's not
immediately checked etc. Working at the Core level is frustrating at times.

Can I ask what kind of plugin are you working on?

(Btw, how did you generate this representation of AST? Did you write it
manually? If you have a pretty-printer, would you mind sharing it?)

2015-08-25 18:50 GMT-04:00 Mike Izbicki <mike at>:
> Thanks Ömer!
> I'm able to get dictionaries for the superclasses of a class now, but
> I get an error whenever I try to get a dictionary for a
> super-superclass.  Here's the Haskell expression I'm working with:
> test1 :: Floating a => a -> a
> test1 x1 = x1+x1
> The original core is:
> + @ a $dNum_aJu x1 x1
> But my plugin is replacing it with the core:
> + @ a ($p1Fractional ($p1Floating $dFloating_aJq)) x1 x1
> The only difference is the way I'm getting the Num dictionary.  The
> corresponding AST (annotated with variable names and types) is:
> App
>     (App
>         (App
>             (App
>                 (Var +::forall a. Num a => a -> a -> a)
>                 (Type a)
>             )
>             (App
>                 (Var $p1Fractional::forall a. Fractional a => Num a)
>                 (App
>                     (Var $p1Floating::forall a. Floating a => Fractional a)
>                     (Var $dFloating_aJq::Floating a)
>                 )
>             )
>         )
>         (Var x1::'a')
>     )
>     (Var x1::'a')
> When I insert, GHC gives the following error:
> ghc: panic! (the 'impossible' happened)
>   (GHC version 7.10.1 for x86_64-unknown-linux):
>         expectJust cpeBody:collect_args
> What am I doing wrong with extracting these super-superclass
> dictionaries?  I've looked up the code for cpeBody in GHC, but I can't
> figure out what it's trying to do, so I'm not sure why it's failing on
> my core.
> On Mon, Aug 24, 2015 at 7:10 PM, Ömer Sinan Ağacan <omeragacan at> wrote:
>> Mike, here's a piece of code that may be helpful to you:
>> Copy this module to your plugin, it doesn't have any dependencies other than
>> ghc itself. When your plugin is initialized, update `dynFlags_ref` with your
>> DynFlags as first thing to do. Then use Show instance to print AST directly.
>> Horrible hack, but very useful for learning purposes. In fact, I don't know how
>> else we can learn what Core is generated for a given code, and reverse-engineer
>> to figure out details.
>> Hope it helps.
>> 2015-08-24 21:59 GMT-04:00 Ömer Sinan Ağacan <omeragacan at>:
>>>> Lets say I'm running the plugin on a function with signature `Floating a => a
>>>> -> a`, then the plugin has access to the `Floating` dictionary for the type.
>>>> But if I want to add two numbers together, I need the `Num` dictionary.  I
>>>> know I should have access to `Num` since it's a superclass of `Floating`.
>>>> How can I get access to these superclass dictionaries?
>>> I don't have a working code for this but this should get you started:
>>>     let ord_dictionary :: Id     = ...
>>>         ord_class      :: Class  = ...
>>>      in
>>>         mkApps (Var (head (classSCSels ord_class))) [Var ord_dictionary]
>>> I don't know how to get Class for Ord. I do `head` here because in the case of
>>> Ord we only have one superclass so `classSCSels` should have one Id. Then I
>>> apply ord_dictionary to this selector and it should return dictionary for Eq.
>>> I assumed you already have ord_dictionary, it should be passed to your function
>>> already if you had `(Ord a) => ` in your function.
>>> Now I realized you asked for getting Num from Floating. I think you should
>>> follow a similar path except you need two applications, first to get Fractional
>>> from Floating and second to get Num from Fractional:
>>>     mkApps (Var (head (classSCSels fractional_class)))
>>>            [mkApps (Var (head (classSCSels floating_class)))
>>>                    [Var floating_dictionary]]
>>> Return value should be a Num dictionary.

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