Am Montag 07 September 2009 23:54:17 schrieb Michael Mossey:
I'm trying to learn qtHaskell, an interface to the Qt GUI library. I am fairly new to Haskell, but have used Qt for a long time, so I thought I could probably reasonably attempt to grok qtHaskell at this point.
My main question is: anyone recommended a good explanation of the foreign function or foreign pointer interface?
My immediate question is that I was poking through the Qtc docs, and saw this:
Documentation
data Object a
Constructors
QObject !(ForeignPtr a)
I am not sure how Haddock works. Is this telling me that an Object constructor takes one argument, which is of type QObject, where QObject is a type constructor that takes !(ForeignPtr a)? Or is this saying it takes two arguments, a QObject and a !(ForeignPtr a). That latter makes more sense I guess. What does the ! mean?
It tells you that the (Object a) datatype has a data-constructor called QObject (I know you already figured that out) which takes one argument of type (ForeignPtr a). However, the constructor's argument type has a bang (!), which means that the constructor is strict in its argument. For nonstrict constructors (data Ob a = Con a), you can happily create a value Con _|_ which will be a nice and harmless citizen until you try to look at its contents. For example, let x = Con undefined in x `seq` 3 returns 3, only let x = Con undefined in case x of Con y -> y `seq` 3 will raise an exception. If you make your constructor strict (data Thing a = SCon !a), it doesn't accept a bottom, the strict constructor evaluates its argument to weak head normal form, if it encounters a bottom, an exception is raised. So, Con _|_ /= _|_ but SCon _|_ == _|_
Thanks, Mike