What does $! mean / do in Haskell? [duplicate]

Question

Context:

do state1 <- act state
   dispatch $! state1

What $! does ?

E.g. why it's not just dispatch state1 here?

Answer 1

$! is strict application, the difference from dispatch state1 is that state1 is guaranteed to be evaluated and not just kept as a lazy thunk. It's defined as

f $! x  = x `seq` f x

Forcing evaluation in this way can be important for efficiency issues, such as preventing memory leaks.