Re: global counters

[Jan de Wit]

 basically, i want a function getVar :: () - String which returns a new
 string every time.  i tried this:

  curVarId = newSTRef (0 :: Integer)
 
  {-# NO-INLINE newVar -}
 
  newVar = \_ - ('\0' : show x)
  where y = unsafePerformIO (stToIO curVarId)
x = unsafePerformIO $ stToIO $
do x - readSTRef y
   writeSTRef y (x + 1)
   return x

 and in ghci it works wonderfully, but when i actually compile, all i get
 is \NUL0.

 what's the proper way to write this?

I think (haven't tested this one) you should write (modulo layout):
| curVarIdRef :: IORef Integer
| curVarIdRef = unsafePerformIO $ newIORef (0 :: Integer)
|
| {-# NO-INLINE newVar -}
|
| newVar :: () - String
| newVar = \_ - ('\0' : show x) where
|   x = unsafePerformIO $
| do x - readIORef curVarIdRef
|   writeIORef curVarIdRef (x + 1)
|   return x

Because in your original version, curVarId allocates a new reference to 0
every time newVar is called.

Hope this works, Jan de Wit





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global counters

[Hal Daume III]

Please, no tirade about banishing unsafePerformIO.

I've seen this done before I just don't remember how.  I want to use a
state monad to count things, but don't want to monadify the thing I'm
using the counter in.

basically, i want a function getVar :: () - String which returns a new
string every time.  i tried this:

 curVarId = newSTRef (0 :: Integer)
 
 {-# NO-INLINE newVar -} 

 newVar = \_ - ('\0' : show x)
 where y = unsafePerformIO (stToIO curVarId)
   x = unsafePerformIO $ stToIO $
 do x - readSTRef y
writeSTRef y (x + 1)
return x

and in ghci it works wonderfully, but when i actually compile, all i get
is \NUL0.

what's the proper way to write this?

thanks

 - hal


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