There was an interesting thread on haskell-prime [1], about the relationship
between functional dependencies and type families. This message is my attempt
to summarise the conclusions of that thread. I'm copying other interested
parties (eg Oleg, Dimitrios)
[1]
On Tue, Jun 14, 2011 at 4:40 PM, Dan Doel dan.d...@gmail.com wrote:
On Tue, Jun 14, 2011 at 5:36 AM, Simon Peyton-Jones
simo...@microsoft.com wrote:
There was an interesting thread on haskell-prime [1], about the relationship
between functional dependencies and type families. This message is
At Tue, 14 Jun 2011 09:36:41 +,
Simon Peyton-Jones wrote:
5. David wants a wiki page fixed. But which one? And how is it locked
down?
This page:
http://hackage.haskell.org/trac/haskell-prime/wiki/FunctionalDependencies
Currently under cons for FunctionalDependencies, it
On Tue, Jun 14, 2011 at 11:27 AM, Andrea Vezzosi sanzhi...@gmail.com wrote:
class C a b | a - b
instance C a R
instance C T U
Are you sure that worked before?
80%
The following still does anyhow:
data R
data T
data U
class C a b | a - b
instance TypeCast R b =
At Tue, 14 Jun 2011 10:40:48 -0400,
Dan Doel wrote:
1. As things stand in GHC you can do some things with functional
dependencies that you can't do with type families. The archetypical example
is type equality. We cannot write
type family Eq a b :: *
type instance Eq k
Sorry about the double send, David. I forgot to switch to reply-all in
the gmail interface.
On Tue, Jun 14, 2011 at 11:49 AM,
dm-list-haskell-pr...@scs.stanford.edu wrote:
You absolutely still can use FunctionalDependencies to determine type
equality in GHC 7. For example, I just verified the
At Tue, 14 Jun 2011 12:31:47 -0400,
Dan Doel wrote:
Sorry about the double send, David. I forgot to switch to reply-all in
the gmail interface.
Okay. I don't really write a lot of code like this, so maybe I missed
the quirks.
In that case, HList has been relying on broken behavior of
On Tue, Jun 14, 2011 at 1:19 PM,
dm-list-haskell-pr...@scs.stanford.edu wrote:
No, these are not equivalent. The first one TypeEq a b c is just
declaring an instance that works forall c. The second is declaring
multiple instances, which, if there were class methods, could have
different
|
http://hackage.haskell.org/trac/haskell-prime/wiki/FunctionalDependencies
|
| Currently under cons for FunctionalDependencies, it says:
|
| AssociatedTypes seem to be more promising.
|
| I proposed the following fix:
|
| AssociatedTypes seem to be more promising, but
