[Haskell-cafe] how can I get template haskell macro-expanded code from inferStartState? (repeated post, now with subject)

[Thomas Hartman]

(sorry, forgot the subject on my first post)

In the following code which uses template haskell, how can I get back the 
macro-expanded code generated from 

$(inferStartState ''MyState) 

I *can* recover the macro-expanded code for 

$(cnst 1 "x") 

using a debugging technique bulat describes on his tutorial at 
http://www.haskell.org/bz/th3.htm 

You can see what's going on in the function debugTemplates below. 

I'm trying to do this actually, to better understand how HAppS deals with 
state. It's a bit opaque now since the example on the tutorial uses TH. I 
think I would understand it better if I had code that didn't depend on TH. 


(MyState is from the happs tutorial at 
http://www.haskell.org/haskellwiki/HAppS_tutorial ) 

thanks! 

thomas. 

{-# OPTIONS -fglasgow-exts -fth  #-} 
module MyState where 
import HAppS 
import HAppS.Protocols.SimpleHTTP2 
  
import Data.Monoid 
import Data.Typeable 
import Control.Monad.State (get, put) 

import Language.Haskell.TH 
import Language.Haskell.TH.Syntax 
  
data MyState = MySt { appVal :: Int } deriving (Read, Show, Typeable) 
  
instance Serialize MyState where 
  encodeStringM = defaultEncodeStringM 
  decodeStringM = defaultDecodeStringM 
  
instance Monoid MyState where 
  mempty = MySt 0 
  mappend (MySt x) (MySt y) = MySt (x+y) 
  
-- in ghci...  -fth, :m + 

-- ghci... :t (inferStartState ''MyState) :: (Quasi m) => m [Dec] 
$(inferStartState ''MyState) -- boilerplate that will eventually be SYB 

-- ghci... :t cnst 1 "x" :: (Monad m) => m Exp 
cnst n s = return (LamE (replicate n WildP) (LitE (StringL s))) 

dumpSplice splice = runQ splice >>= putStrLn . pprint 
debugTemplates = do dumpSplice (cnst 1 "x") 
                    dumpSplice (inferStartState ''MyState) 

{- 
*MyState> debugTemplates 
\_ -> "x" 
Template Haskell error: Can't do `reify' in the IO monad 
*** Exception: user error (Template Haskell failure) 
-} 


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