Undeliverable: Splicing types.. should this work?

[Simon Peyton-Jones]

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 at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Robert Greayer
| Sent: 25 January 2010 23:27
| To: glasgow-haskell-users at 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
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