What does a bang symbol do in a data type?

3
votes

I am new to Haskell and today I read though the OpenGL source code and I found this

data VertexArrayDescriptor a =
   VertexArrayDescriptor !NumComponents !DataType !Stride !(Ptr a)
   deriving ( Eq, Ord, Show )

http://hackage.haskell.org/package/OpenGL-2.9.1.0/docs/src/Graphics-Rendering-OpenGL-GL-VertexArrays.html#VertexArrayDescriptor

I tried to search for it and found http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/bang-patterns.html but it is probably something different.

What does it do? What is it for?

1
haskell

1 Answers

4
votes

It is to make it easier to write strict programs in Haskell and makes strictness more convenient for the developer to avoid being forced into being 'lazy' or non-strict just for convenience.

Take for example : 

> data PNat = PZero | PSuc !Nat deriving Show

The bang declares that PSuc is strict in its argument, i.e. PSuc bottom = bottom (where bottom is a non-terminating expression).

It is to indicate strictness in patterns:

f !x !y = x + y

Good Reference : https://ghc.haskell.org/trac/haskell-prime/wiki/BangPatterns