Hello Shen, I’m not sure if I’m answering exactly what you’re asking, but perhaps this can move you in the right direction: - To apply an expression to some types, you can use mkTyApps, in the CoreSyn module. The type of that function (Expr b -> [Type] -> Expr b) should be self-explanatory. - To get the instance environments (where all class instances are stored), use tcGetInstEnvs, in the Inst module. - Then, to get the actual dictionary, you can do the lookup with lookupInstEnv, in the InstEnv module. Does this move you in the right direction? Richard On Apr 24, 2014, at 5:37 PM, Shen Chen Xu wrote:

Hello everyone,

I would like to write a GHC plugin which adds wrapper to functions. Say I have a function

addWrapper :: Ord a => (a -> b) -> a -> b

which is supposed to transform (assuming the argument is in the Ord class)

foo = \ x -> ...

into

foo = addWrapper (\ x -> ...)

This works fine if I'm only interested in wrapping functions of type Int -> Int, in which case I make addWrapper to also have type

addWrapper :: (Int -> Int) -> Int -> Int

and use mkCoreApp to apply addWrapper to the right hand side.

However, if I want it to be polymorphic as described above, the transformed GHC Core for an Int -> Int function should look like

foo = addHook @ GHC.Types.Int @ GHC.Types.Int GHC.Classes.$fOrdInt (\ x -> ...)

My question is, given the right hand side of foo, how do I construct the type/type class information inside Core shown above?

Cheers, Shen

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