[Haskell-beginners] Confused by type constraints

[Brent Yorgey]

On Fri, Jun 03, 2011 at 05:13:43PM +0300, Guy wrote:
> On 03/06/2011 16:55, Daniel Schoepe wrote:
> >On Fri, 03 Jun 2011 16:04:05 +0300, Guy<guytsalmaves-h at yahoo.com>  wrote:
> >>I have some code resembling
> >>
> >>class Foo f where
> >>	foo :: a ->  f a
> >>
> >>data Bar f a = Foo f =>  Bar {bar :: f a}
> >>
> >>instance Foo (Bar f) where
> >>	foo a = Bar $ foo a
> >>
> >>GHC insists on Foo f =>  in the instance declaration. However, the definition of Bar guarantees that this will always be
> >>the case. Why do I have to state this explicitly?
> >
> >The constraints in the data type declaration only affect the
> >constructors. It's a known issue, unfortunately there doesn't seem to be
> >a way around it.
> 
> It seems to work for simpler examples. For example, this is OK
> 
> data X x = Show x => X x
> 
> s :: X x -> String
> s (X x) = show x
> 
> I don't have to add a Show x => type constraint to s.

In this case, you are *destructing* an X value, and when you do, the
Show constraint on x becomes available, since the X constructor is
always paired with a Show constraint.

In your original example, you are *constructing* a Bar value.  You
have stated that the Bar constructor must be accompanied by a Foo
constraint, but there is no such constraint in sight!

So the difference is between *destructing* a value (when the
associated constraint becomes available) and *constructing* one (when
the required constraint must be provided).

I don't know if I've explained that well.  As others have mentioned,
the ability to specify constraints on constructors in this way is
probably going to be removed from the language anyway.

-Brent