Re: Superclass Equality constraints cp FunDeps

2017-04-30 Thread Richard Eisenberg 
Documentation is just about always suboptimal -- but the best people to suggest 
concrete improvements are those who were confused to begin with. So, by all 
means, submit patches!

Some relevant discussion on this point is on 
https://ghc.haskell.org/trac/ghc/ticket/10431

Currently, there is no extension for ~. It's enabled either by GADTs or 
TypeFamilies. I agree that this is not the best design, and the ticket above is 
a plan to fix it. Probably wouldn't take much more than someone with the will 
to make the changes!

Richard

> On Apr 30, 2017, at 3:35 PM, Brandon Allbery  wrote:
> 
> On Sun, Apr 30, 2017 at 3:31 PM, Richard Eisenberg  > wrote:
> > On Apr 30, 2017, at 6:37 AM, Anthony Clayden  > > wrote:
> > Is that behaviour officially documented somewhere?
> 
> Not that I can find. Documentation on functional dependencies is somewhat 
> lacking. This may be because fundeps has received little love of late.
> 
> Not just fundeps; the originally cited behavior of equality constraints could 
> also stand to be documented. In fact, I'd noticed a few shortcomings in 
> equality constraint documentation --- including the question of, are they 
> always enabled, or are there particular extensions that enable them? And if 
> so, should they also have their own extension that becomes implied by those 
> other extensions? So it's not even just documentation.
> 
> -- 
> brandon s allbery kf8nh   sine nomine associates
> allber...@gmail.com   
> ballb...@sinenomine.net 
> unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net 
> 
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Re: Superclass Equality constraints cp FunDeps

2017-04-30 Thread Brandon Allbery 
On Sun, Apr 30, 2017 at 3:31 PM, Richard Eisenberg 
wrote:
>
> > On Apr 30, 2017, at 6:37 AM, Anthony Clayden <
> anthony_clay...@clear.net.nz> wrote:
> > Is that behaviour officially documented somewhere?
>
> Not that I can find. Documentation on functional dependencies is somewhat
> lacking. This may be because fundeps has received little love of late.
>

Not just fundeps; the originally cited behavior of equality constraints
could also stand to be documented. In fact, I'd noticed a few shortcomings
in equality constraint documentation --- including the question of, are
they always enabled, or are there particular extensions that enable them?
And if so, should they also have their own extension that becomes implied
by those other extensions? So it's not even just documentation.

-- 
brandon s allbery kf8nh   sine nomine associates
allber...@gmail.com  ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net
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Re: Superclass Equality constraints cp FunDeps

2017-04-30 Thread Richard Eisenberg 

> On Apr 30, 2017, at 6:37 AM, Anthony Clayden  
> wrote:
> 
> Is that behaviour officially documented somewhere?

Not that I can find. Documentation on functional dependencies is somewhat 
lacking. This may be because fundeps has received little love of late.

Otherwise, I agree with your notes.

Documentation patches are warmly accepted!
Richard
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Re: Superclass Equality constraints cp FunDeps

2017-04-30 Thread Anthony Clayden 
> On at Apr 29 05:55:14 UTC 2017, Anthony Clayden wrote:  
> ...
> So should I reasonably have known that
> a superclass constraint 
> with FunDeps on the superclass
> induces FunDeps on the sub-class
> without explicitly needing to declare so?
> 
> (I'm not complaining, more surprised/impressed.)
> 

From experimenting, FunDeps seem to come through
from a superclass of a superclass of a superclass of the
class.

This from David Feuer says
http://stackoverflow.com/questions/35252562/find-an-element-in-an-hlist/35260970#35260970

"superclass context is critical 
 GHC always commits to an instance before checking the
instance constraints,
 but it doesn't even try to find an instance
 until after solving the superclass constraints.
 So we need to use the superclass constraint to fix ..."

Presumably that's the mechanism that induces FunDeps
from supeclass constraints.

Is that behaviour officially documented somewhere?

(I remember when Type Families and (~) Constraints
 were first released, they weren't available superclass.
 Then there were a couple of releases
 you could put superclass constraints,
 but they weren't effective.)


AntC
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