Re: [Haskell-cafe] What is the role of $!?

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 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:

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On 14 Nov 2007, at 4:32 PM, Shachaf Ben-Kiki wrote:

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On Nov 14, 2007 4:27 PM, Justin Bailey wrote:

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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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