Thanks Henning! I suspected some kind of optimization but I didn't think ghci would do any. I guess it is just a subtle consequence of eta reduction. It was very confusing to me. Cheers George On Fri, May 17, 2024 at 6:43 PM Henning Thielemann < lemming@henning-thielemann.de> wrote:
On Fri, 17 May 2024, George Colpitts wrote:
After I load the following into ghci and call main I expect to see "norm" in the output 3 times as I have called it 3 times but it only shows once. Why?
traceBug.hs --------------- {-# OPTIONS_GHC -Wall #-}
import Debug.Trace (trace)
norm :: [Int] -> [Int] norm = trace ("norm") id
main :: IO () main = do print $ norm [2] print $ norm [2] print $ norm [3] ---------------------
ghci session
% ghci traceBug.hs Loaded package environment from /Users/gcolpitts/.ghc/x86_64-darwin-9.8.2/environments/default GHCi, version 9.8.2: https://www.haskell.org/ghc/ :? for help [1 of 2] Compiling Main ( traceBug.hs, interpreted ) Ok, one module loaded. ghci> main norm [2] [2] [3]
I think this is correct. 'norm' is evaluated only once, because it is the same variable (for a function) in every call. It would be different, if you would define:
norm xs = trace "norm" xs