Re: [Haskell-cafe] Template Haskell: Generate annotated function of a typeclass

Hi Michael, I was able to do what I wanted using reify instead of reifyInstances, and also I used applyT instead of substT. Thanks 2012/4/13 Ismael Figueroa Palet

Thanks for your reply, in particular the reference to subsT! I will work more on this next monday, and report my progress

Cheers!

2012/4/13 Michael Sloan

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Hello!

It seems like you would want to use "reifyInstances" in order to get all of the instances associated with a class. Then, you can match up the variables in each instance with the variables in the class declaration, and create a mapping from the class variables to the instance parameters. Then, you can apply these mappings with substT:

http://hackage.haskell.org/packages/archive/haskell-src-meta/0.5.1.2/doc/htm...

The result would also need to have the context of the instance, perhaps reduced to just the constraints that mention the type variables used in the selected function.

-Michael Sloan

On Fri, Apr 13, 2012 at 11:37 AM, Ismael Figueroa Palet wrote:

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Hi all, I think this is the right place for the following questions and I thank beforehand for your answers :-)

I'm experimenting with typeclasses and TH, and I want to define a 'macro' that works more or less like this:

Given the name of a typeclass and a function, return the expressions corresponding to the type-annotated instances, for instance

$(foo Show show)

should translate to:

["(show :: Int -> String)", "(show :: Bool -> String)", ....]

for all instances currently in scope.

I'm currently playing with the isInstance function (I'm running GHC 7.4.1) and can get a list of instances, and check if a given type is part of a typeclass or not. But I don't know how to create the expression corresponding to "instantiated function", as above.

Thanks! -- Ismael

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-- Ismael

-- Ismael