What is the difference between $ (dollar) and $! (dollar exclamation point)

Question

Can anybody explain the difference in Haskell between the operators ($) and ($!) (dollar sign vs dollar sign exclamation point)?

I haven't seen the use of $! anywhere so far, but while browsing through the Haskell reference, I noticed its existence and that it has the exact same definition as $. When trying some simple statements in a Haskell interpreter (GHCi), I couldn't find any difference, nor could I find any reference to the operator in the top listed tutorials when searching for haskell tutorial.

So, just out of curiosity, what is the difference, if at all?

Answer 1

($!) is strict function application. That is, it evaluates the argument before evaluating the function.

This is contrary to normal lazy function application in Haskell, e.g. f x or f $ x, which first start to evaluate the function f, and only compute the argument x if it is needed.

For example succ (1 + 2) will delay the addition 1 + 2 by creating a thunk, and start to evaluate succ first. Only if the argument to succ is needed, will 1 + 2 be evaluated.

However, if you know for sure that the argument to a function will always be needed, you can use ($!), which will first evaluate the argument to weak head normal form, and then enter the function. This way, you don't create a whole big pile of thunks and this can be more efficient. In this example, succ $! 1 + 2 would first compute 3 and then enter the function succ.

Note that it is not always safe to just replace normal function application with strict function application. For example:

ghci> const 1 (error "noo!") 1 ghci> const 1 $! (error "noo!") *** Exception: noo! 

Answer 2

See the seq function, which forces the evaluation of a value. $! is defined in terms of seq.

This is a blog post which shows some nuances of its use.