[Template-haskell] "contexts differ in length" fires when using splices in non-recursive bindings?

[Robert Greayer]

Not that this helps, but the following simplified version exhibits the same problem:

> {-# LANGUAGE TemplateHaskell #-}
> y :: Monad m => a -> m a
> y a = -- return a
>     $([| return a |])

> x :: (Monad m,Eq a) => m a
> x = -- return undefined
>   $([| return undefined |])

My understanding is that a splice of a quoted expression should be equivalent to the expression itself (with the caveat that different scoping rules for names apply)... e.g.
   $([| x |]) == x
more or less.  The error this produces seems to violate this principle.

This seems to traces back to something going on in TcBinds.generalise and related functions, but looking at this made me woozy, so I have given up for the moment...

--- On Sat, 7/12/08, Claus Reinke <claus.reinke at talk21.com> wrote:

> From: Claus Reinke <claus.reinke at talk21.com>
> Subject: [Template-haskell] "contexts differ in length" fires when using splices in non-recursive bindings?
> To: template-haskell at haskell.org
> Date: Saturday, July 12, 2008, 12:50 AM
> My TH is a bit rusty, so I'm probably doing something
> wrong here:
> 
>     {-# LANGUAGE TemplateHaskell #-}
>     module TH where
>     import Language.Haskell.TH
>     monad qe = qe >>= return . AppE (VarE
> 'return) 
> 
> The symptom appears in the following module, when I replace
> the native 'return's with the spliced ones:
> 
>     {-# LANGUAGE TemplateHaskell #-}
>     import TH
> 
>     y :: Monad m => a -> m a
>     y a = return a 
>       -- $(monad [| a |])
> 
>     x :: (Monad m,Eq a) => m a
>     x = return undefined 
>       -- $(monad [| undefined |]) 
> 
> There is no recursion, at least no intended one, but where
> the native
> version compiles just fine, the spliced version yields:
> 
>     THtest.hs:10:0:
>         Contexts differ in length
>           (Use -XRelaxedPolyRec to allow this)
>         When matching the contexts of the signatures for
>           y :: forall (m :: * -> *) a. (Monad m) => a
> -> m a
>           x :: forall (m :: * -> *) a. (Monad m, Eq a)
> => m a
>         The signature contexts in a mutually recursive
> group should all be identical
>         When generalising the type(s) for y, x
>     Failed, modules loaded: TH.
> 
> Could someone please tell me what is causing this? In the
> real code, the errors are even more confusing. Try removing
> the type signatures, for instance, or just remove the
> 'Eq a'
> constraint, then try evaluating '[y (),x]' in GHCi.
> 
> Claus
> 
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