[Haskell-cafe] What is the role of $!?
David Fox
ddssff at gmail.com
Mon Dec 10 01:16:26 EST 2007
Argh, that last sentence should read "the file is left alone"..
On Dec 9, 2007 10:15 PM, David Fox <ddssff at gmail.com> wrote:
> Here is a practical example I ran into a few days ago. With this
> expression:
>
> writeFile path (compute text)
>
> the file at path would be overwritten with an empty file if an error
> occurs while evaluating (compute text). With this one:
>
> writeFile path $! (compute text)
>
> the file alone when an error occurs.
>
>
> On Nov 17, 2007 8:04 PM, PR Stanley <prstanley at ntlworld.com> wrote:
>
> > Hi
> > okay, so $! is a bit like $ i.e. the equivalent of putting
> > parentheses around the righthand expression. I'm still not sure of
> > the difference between $ and $!. Maybe it's because I don't
> > understand the meaning of "strict application". While we're on the
> > subject, what's meant by Haskell being a non-strict language?
> > Cheers
> > Paul
> > At 01:50 15/11/2007, you wrote:
> > >On 14 Nov 2007, at 4:32 PM, Shachaf Ben-Kiki wrote:
> > >
> > >>On Nov 14, 2007 4:27 PM, Justin Bailey < jgbailey at gmail.com> wrote:
> > >>>It's:
> > >>>
> > >>> f $! x = x `seq` f x
> > >>>
> > >>>That is, the argument to the right of $! is forced to evaluate, and
> > >>>then that value is passed to the function on the left. The function
> > >>>itself is not strictly evaluated (i.e., f x) I don't believe.
> > >>
> > >>Unless you mean f -- which I still don't think would do much -- it
> > >>wouldn't make sense to evaluate (f x) strictly.
> > >
> > >Right. (f x) evaluates f and then applies it to x. (f $! x)
> > >evaluates x, evaluates f, and then applies f to x.
> > >
> > >jcc
> > >
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>
>
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