[Haskell-cafe] Plug space leak with seq. How?

[Alexey Khudyakov]

Hello café!

This mail is literate haskell

I have some difficulties with understanding how bang patterns and seq
works.

 > {-# LANGUAGE BangPatterns #-}
 > import Control.Monad
 > import Control.Monad.Trans.State.Strict
 >
 > leak :: State Int ()
 > leak = do
 >   a <- get
 >   put (a+1)
 >   leak

This function have obvious space leak. It builds huge chain of thunks
so callling `runState leak 0' in ghci will eat all memory. Fix is 
trivial - add bang pattern. However I couldn't achieve same
effect with seq. How could it be done?

 > noLeak :: State Int ()
 > noLeak = do
 >   a <- get
 >   let !a' = (a + 1)
 >   put a'
 >   noLeak


Thanks.