RE: Arrowcase1

Ross

I think the safest thing to do is to write a new pat-binder-collector.
Here's the problem.  Consider

data T a where
   C :: Num a =3D> a -> Int -> T a

f ~(C (n+1) m) =3D (n,m)

Here, the pattern (C (n+1)) binds a hidden dictionary (d::Num a), and *also=
* uses that dictionary to match the (n+1) pattern.  Yet, the variables boun=
d by the lazy pattern are n,m, *not* the dictionary d.  So in mkSelectorBin=
ds in DsUtils, we want just m,n as the variables bound.


Perhaps when you do this, you could add this example, plus one about arrows=
, to explain why the two purposes need two different functions?

Thanks

Simon

| -----Original Message-----
| From: Ross Paterson [mailto:[email protected]]
| Sent: 14 January 2007 01:06
| To: Simon Peyton-Jones
| Cc: [email protected]
| Subject: Re: Arrowcase1
|
| On Mon, Jan 08, 2007 at 12:01:42PM +0000, Simon Peyton-Jones wrote:
| > | In the comment under collectPatsBinders, ignoring dictionary binders =
in
| > | ConPatOut is justified in terms of lazy patterns.  Presumably there i=
s
| > | more to this story, because if such dictionaries are guaranteed to be
| > | unused it would be safe for arrows too.
| >
| > They are only unused in lazy patterns, but not for strict ones.  Indeed=
,
| > they should be *empty* for lazy patterns; so I guess it'd be safe to
| > gather the dict bindings too, if that makes it easier for you.
|
| Changing the ConPatOut line to also collect the pat_binds fixes arrowcase=
1
| (and presumably anywhere else I handle patterns), but I don't know if
| it will break anything elsewhere.