[Haskell-cafe] problems with trace

[Tom Ellis]

I don't think optimization plays any role.  At the start of the
program 'norm' is a thunk which points to the code 'trace "norm" id'.
The first time it is evaluated the 'trace' prints "norm" and 'norm'
becomes bound to the value 'id'.

On Fri, May 17, 2024 at 08:26:44PM -0300, George Colpitts wrote:
> 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.
> 
> On Fri, May 17, 2024 at 6:43 PM Henning Thielemann <
> lemming at 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