Hi all,

I've been writing loops in a monad using `replicateM_`. However, recently I

discovered that it caused my program become extremely slow. For example,

> module Main where

> 

> import Control.Monad (replicateM_)

> 

> fib :: Int -> Int

> fib 0 = 0

> fib 1 = 1

> fib n = fib (n-1) + fib (n-2)

> 

> main = do

>   n <- readLn

>   let fn = fib n

>   replicateM_ 20 (print fn)

The program ran fine if no optimization is turned on. However, when I used

`-O` or `-O2` with GHC (7.4/7.6), the program became extremely slow, and if

I replaced `replicateM_` by `mapM_`, the problem disappeared.

I suspected that GHC inlined `fn`, causing `replicateM_` to recalculate the

value in every loop. Though the problem could be solved by either using `mapM_`,

lifting `print fn` to a global definition or manually demanding it be evaluated

strictly, I would expected `fn` not to be computed multiple times.

Any suggestions around this?

Thanks!

Sincerely,

suhorng