Robert Interesting example. Here's your code: createInstance' :: Q Type -> Q Dec createInstance' t = liftM head [d| instance Foo $t where type FooType $t = String |] When TH sees a quotation, it *typechecks* it. So it tries to typecheck instance Foo $t where type FooType $t = String During typechecking, it needs something to use as the type to which $t will expand. So it uses a fresh type variable -- but a different one each time. And this is what is biting you. In general that's right. You would not expect it to work if you said instance Foo $(t 19) where type FooType $(t 19) = String seeing that 't' is applied to the same argument each time. There no special case for plain variables. So it's hardly a feature, but I can't see how to do just what you want. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Robert Greayer | Sent: 25 January 2010 23:27 | To: glasgow-haskell-users@haskell.org | Subject: Splicing types.. should this work? | | Now that type-splicing works in TH, and TH has type-family support, I | was wondering if the following example should compile (with 6.12.1): | | > {-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, TypeFamilies, | > FlexibleInstances, OverlappingInstances #-} | | > module Sample where | | > import Control.Monad | > import Language.Haskell.TH | | > class Foo a where | > type FooType a | | > createInstance :: Q Type -> Q Dec | | > createInstance t = instanceD (return []) | > (conT ''Foo `appT` t) [ | > tySynInstD ''FooType [t] (conT ''String) | > ] | | > createInstance' :: Q Type -> Q Dec | > createInstance' t = liftM head [d| | > instance Foo $t where | > type FooType $t = String|] | | the function 'createInstance' compiles without a problem, but it's | (near) equivalent | written using TH quotations + splices fails with the error: | | Sample.lhs:22:10: | Type indexes must match class instance head | Found `t_aMn' but expected `t_aMl' | In the associated type instance for `FooType' | In the instance declaration for `Foo $t' | In the Template Haskell quotation | [d| | instance Foo $t where | type instance FooType $t = String |] | | The compiler seems to not be able to determine that the type spliced | in the class instance head will match the type spliced in the type | instance. | | The first version works fine for my purposes, but was curious whether | the failure of the 2nd was a bug or a feature. | | Thanks, | Rob | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users