question about GHC API on GHC plugin

Mike Izbicki mike at
Tue Aug 25 22:50:53 UTC 2015

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:

                (Var +::forall a. Num a => a -> a -> a)
                (Type a)
                (Var $p1Fractional::forall a. Fractional a => Num a)
                    (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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