question about GHC API on GHC plugin

Ömer Sinan Ağacan omeragacan at
Tue Aug 25 02:10:51 UTC 2015

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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