[Haskell-cafe] Re: Trying to understand HList / hSequence now [why it works]

[Matthias Fischmann]

On Tue, Oct 10, 2006 at 07:45:28PM -0700, oleg at pobox.com wrote:
> To: fis at wiwi.hu-berlin.de
> Cc: haskell-cafe at haskell.org
> From: oleg at pobox.com
> Date: Tue, 10 Oct 2006 19:45:28 -0700 (PDT)
> Subject: Re: Trying to understand HList / hSequence now [why it works]
> 
> 
> Matthias Fischmann wrote:
> > instance (Monad m, HSequence m HNil HNil) => HSequence m HNil HNil 
> >     where hSequence _ = return HNil
> >
> > how can i use the goal of the declaration as one of the conditions
> > without causing some sort of black hole in the type inference
> > algorithm?
> 
> Very easily: the instance head is implicitly the part of its own
> context (so that a method can be recursive). A simple way to see that
> is the following deliberately erroneous class:

That explains why it is legal, but not why it can be necessary.  In my
code, I had to add the instance goal to the context in order for type
inference to quite complaining about incomplete context.

But never mind.  I should really do some reading.

Your hSequence implementation I like, yes.  I will try that instead of
mine, perhaps that'll soothen the type checker.

thanks!
matthias


> > class C a where mc :: a -> Bool
> > instance Eq a => C a where mc x = x > x
> 
> The error message says
> 
>     Could not deduce (Ord a) from the context (C a, Eq a)
>       arising from use of `>' at /tmp/f2.hs:30:36
> 
> It is revealing to observe the context that the typechecker thinks is
> available: it is (C a, Eq a). "Eq a" is there because we explicitly
> wrote it in the instance declaration. C a is there just by default. We
> could just as well written
> 
> > instance (Ord a, C a) => C a where mc x = x > x
> 
> Incidentally, the hSequence can be written as follows
> 
> > import TypeCastGeneric2
> > data ConsM
> >
> > instance (TypeCast (m1 l) (m l), Monad m) 
> >     => Apply ConsM (m a, m1 l) (m (HCons a l)) where
> >     apply _ (me,ml) = liftM2 HCons me (typeCast ml)
> >
> > hSequence l = hFoldr (undefined::ConsM) (return HNil) l
> 
> > hlist = HCons (Just 1) (HCons (Just 'c') HNil)
> > hlist2 = HCons ([1]) (HCons (['c']) HNil)
> > testHSequence = hSequence hlist
> > testHSequence2 = hSequence hlist2
> 
> *Foo> :t testHSequence
> testHSequence :: Maybe (HCons Integer (HCons Char HNil))
> *Foo> testHSequence
> Just (HCons 1 (HCons 'c' HNil))
> *Foo> testHSequence2
> [HCons 1 (HCons 'c' HNil)]
> 
> The typechecker will complain if we try to mix different monads within
> the same HList, and then sequence it.
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