[Haskell-cafe] Fwd: Increasing Haskell modularity

Gesh gesh at gesh.uni.cx
Wed Oct 1 16:33:55 UTC 2014

On 10/1/2014 6:02 PM, Carter Schonwald wrote:
> This proposal is missing a key ingredient, how to resolve import of two
> conflicting instances.  And I can't imagine any good selection strategy
> under this scoping scheme.
Why should two conflicting in-scope instances be a valid situation?
All I'm doing is making instance resolution more similar to other name
resolution. Creating a new instance in a scope shadows the instance
outside the scope, but multiple instances in the same scope aren't

> Additionally, how this would work in the case of fancier programs that
> use fundeps and multiparam type classes to construct type level programs
> is a bit confusing. Would this entail that those programs would have hot
> swappable parts? What does that mean?
You're right, I didn't pay much attention to the fancier stuff we can do.
I don't think this will cause trouble, but I'm not qualified to say that
with certainty.

> One hackage package that would suffer terribly would be the HList one I
> think.
A cursory glance at HList reveals several open type families.
I *think* that they can be rewritten to be closed families.

> Also I'm not sure what implications this would have for how type
> inference would work.  Having global uniqueness is a wonderful
> assumption to have, and losing it might result in losing huge swaths of
> type inference, because suddenly we have 'pick the instance in the
> nearest scope'..  So how would this notion interact with the instance
> resolution that happens for overlapping instances?
If there's an exact instance that matches the local instance, the local
instance shadows it, and overlapping instance inference proceeds as

Some examples:
 > module Foo where
 > import Bar (Baz) -- With an instance Show (Baz a)
 > instance Show (Baz a) where -- shadows the instance from Bar
 > foo = let instance Show (Baz a) where -- shadows the module instance
 >           instance Show (Baz Quz) where
 >           -- Does not shadow any instance, OverlappingInstances
 >           -- resolution proceeds with the two instances defined in
 >           -- the let block

Hope this clarifies things,


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