[Haskell-cafe] Re: Data instance for a GADT

Simon Peyton-Jones simonpj at microsoft.com
Tue May 25 12:35:58 EDT 2010

There have been a couple of emails about Template Haskell support for

·         GADTs

·         View patterns

·         Reifying instances

There’s a ticket for this already http://hackage.haskell.org/trac/ghc/ticket/3497

It’s mainly a library design question, in this case the design of the Template Haskell data types in Language.Haskell.TH.Syntax.  It needs some motivated person to propose a design, get a consensus, and write some boilerplate code.   Just use the standard libraries process

An issue is the extent to which it’s ok to change the Template Haskell data types (and thereby break people’s code), but that’s something the interested parties can work out together.

If you evolve a consensus design I’m more than happy to make it part of GHC. I just don’t want to drive the consensus building!


From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Ozgur Akgun
Sent: 14 April 2010 10:53
To: Haskell cafe
Subject: [Haskell-cafe] Re: Data instance for a GADT

Seeing this old thread[1], I hope something happened towards enabling this.
Does anybody know the current status about using TH on GADTs?

[1] http://www.haskell.org/pipermail/template-haskell/2006-August/000567.html

On 14 April 2010 10:32, Ozgur Akgun <ozgurakgun at gmail.com<mailto:ozgurakgun at gmail.com>> wrote:
answering to myself: I guess this is related: http://hackage.haskell.org/trac/ghc/ticket/3497

On 14 April 2010 10:13, Ozgur Akgun <ozgurakgun at gmail.com<mailto:ozgurakgun at gmail.com>> wrote:

How can I provide a Data instance for a GADT? I am trying to TH on it, and Uniplate requires Data.
I tried StandaloneDeriving, but it seems not to work.

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