Re: [Haskell-beginners] About ! before a type signature

That declares those fields to be strict. See also http://www.haskell.org/haskellwiki/Performance/Data_types On Friday, June 6, 2014, Song Zhang wrote:

This is about syntax of Haskell. I am reading the source code of Yampa. I find that a definition like the following

data SF' a b where SFArr :: !(DTime -> a -> Transition a b) -> !(FunDesc a b) -> SF' a b

in Yampa.hs.