[Haskell-cafe] problems with trace

[George Colpitts]

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 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
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