[Haskell-cafe] problems with trace

[George Colpitts]

Agreed!

Thanks


On Sat, May 18, 2024 at 3:43 AM Tom Ellis <
tom-lists-haskell-cafe-2023 at jaguarpaw.co.uk> wrote:

> 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
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