Re: [Haskell-cafe] Overlapping instances with "instance F a => G a"

22 Jan 2015


      On Thu, 1 Jan 2015 17:15:31 -0500
Brandon Allbery  wrote:
...
On Thu, Jan 1, 2015 at 4:29 PM, Bryan Gardiner  wrote:
...
Doesn't "HostBinaryNum a => HostBinary a" create a HostBinary instance
for all instances of HostBinaryNum only?  So why would it cause
No, it creates an instance for all types, then checks for HostBinaryNum at
the point where it tries to use the instance.
Brandon, Tom, thanks for the reponses.  I can only claim rudimentary
typeclass knowledge but I find it odd for the instance statement to
create instances for all types if it's going to only be usable for
types with HostBinaryNum instances.  From my reading of the GHC
manual, it's generally only an error when a conflict actually occurs,
not if a conflict might occur.  Is there a benefit to having it this
way?

Many thanks,
Bryan