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

Dean Herington heringtonlacey at mindspring.com
Mon Dec 10 21:43:10 EST 2007

Thanks, Tom, for a nice description of lazy evaluation.

Besides the minor things Derek pointed out, there's one more subtle 
but important thing to correct:

At 7:29 AM +0000 11/29/07, Thomas Davie wrote:
>$! is the special case, which means strictly apply.  It evaluates 
>its argument first, *then* does the application.  This may or may 
>not be faster (and usually isn't, due to evaluating more of the 
>f ($!) x = seq x (f x)
>seq is a special function that says "first fully evaluate my first 
>argument, then return my second argument", it breaks non-strict 

seq doesn't fully evaluate its first argument, rather only to what's 
called "weak head normal form".  Roughly, that means only enough to 
establish the top-level constructor (e.g., to distinguish [] from 


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