Indeed, tyConDataCons does the trick. I don't know how to start with the Class (given a name string), but I'm able to use 'mkClsOcc . Unqual' with lookupRdrNameInModuleForPlugins and then lookupTyCon on the resulting name, followed by 'head . tyConDataCons' to get the constructor.

Thanks, Simon!

-- Conal

On Wed, Mar 23, 2016 at 1:32 AM, Simon Peyton Jones <simonpj@microsoft.com> wrote:

Start with the Class. Use classTyCon to get the TyCon. Use tyConDataCons to get the data constructor.

Simon

From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Conal Elliott
Sent: 23 March 2016 00:09
To: ghc-devs@haskell.org
Subject: lookupRdrNameInModuleForPlugins with constructors

I'm trying to construct a dictionary in a GHC plugin. I'm stuck on finding the constructor for the dictionary. When I use `-ddump-simpl` on the module that defines the class, I see "Circat.Rep.C:HasRep". To try finding that constructor, I say

> lookupRdrNameInModuleForPlugins hsc_env
> (mkModuleName "Circat.Rep") (mkVarUnqual "C:HasRep")

However, I keep getting `Nothing` as a result. (Same without the "C:".) I've also had this same difficulty when looking up constructors for algebraic data types and when looking up TyCons. For regular value Ids, lookup succeeds.

What am I missing?

Thanks, - Conal