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

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

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