[Template-haskell] splices in type signatures

[Bulat Ziganshin]

Hello Frederik,

Wednesday, June 14, 2006, 9:46:33 PM, you wrote:

> Is it possible to have a TH splice in a type signature? It seems to
> generate a parse error, but I thought there might be another syntax
> that I'm not aware of. Thanks,

the following straightforward code don't work

f :: $( ''Int )
f = 1

- i think because ghc still has serious limitations on the places
where splicing can occur

but you can emulate such behavior, it just needs more work. look at
the following program: 

{-# OPTIONS_GHC -cpp -fth -fglasgow-exts #-}
$([d|
 f :: Int
 f = 1
 |])
main = print f


it compiles and works ok. [d|...|] here generates list containing two
declaration statements, where first matches to the "SigD Name Type"
alternative in Dec type. as the result, you can substitute this Type
with what you want (Integer here):

{-# OPTIONS_GHC -cpp -fth -fglasgow-exts #-}
module Th1 where
import Language.Haskell.TH
generate = do
  [sig,def] <- [d|
                   f :: Int
                   f = 10
               |]
  let (SigD name _) = sig
  return [SigD name (ConT ''Integer), def]


and then use it:


{-# OPTIONS_GHC -cpp -fth -fglasgow-exts #-}
import Th1
$(generate)
main = print (f^40)


i've also attached example of using this technique to deriving Show
instances

-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com
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