That declares those fields to be strict. See also http://www.haskell.org/haskellwiki/Performance/Data_types

On Friday, June 6, 2014, Song Zhang <vxanica@gmail.com> 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.

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