[Haskell-beginners] truncate results depend on strict/lazy

[Bryan Vicknair]

On Mon, Sep 09, 2013 at 07:59:28PM +0200, Chaddaï Fouché wrote:
> > In the following expression, the result is: Success (Just 11).
> >
> > > Success $ Just $ truncate (f * 100)
> >
> > In the following expression, the result is: Success (Just 12)
> >
> > > let expanded =  f * 100
> > >     ans      = truncate expanded
> > > in trace (show expanded) $ Success $ Just $ ans
> >
> > That made me think that "f * 100" had to be strictly evaluated before
> > given to
> > truncate for some reason, so I tried using seq to get the same effect, but
> > that
> > didn't work.  Am I correct in assuming that laziness has something to do
> > with
> > this problem?
> >
> >
> It would be a serious bug if that was true since lazyness shouldn't change
> the semantic of a program, except sometimes by allowing the program to
> terminate where the strict version wouldn't.
> 
> On the other hand I can't reproduce your bug, couldn't you provide more
> details (including GHC version and parsing code)
> 
> -- 
> Jedaï

I put together a simple library and web app to demonstrate the behavior I'm
seeing: git clone git at bitbucket.org:bryanvick/truncate.git

The README has simple instructions to view the behavior in a repl or in a web
app.  The Lib.hs file is where the parsing code is.

I swear I saw different behaviour between separate cabal sandboxes while I was
testing this.  Sometimes the parsing would work as expected, sometimes it
wouldn't.  That made me think that maybe different versions of dependencies are
being installed for different runs.  I'll start paying attention to
"cabal sandbox hc-pkg" to see if this is the case.