Status of Haskell'?

Ross Paterson ross at
Mon Dec 3 09:54:14 CET 2012

On Sun, Dec 02, 2012 at 07:56:57PM +0000, Gábor Lehel wrote:
> Out of curiosity, to what degree does MultiParamTypeClasses have this
> issue? It seems to me like one of the few extensions which is
> straightforward, widely implemented, uncontroversial, and very useful.

There's some discussion of the linkages at

Without FlexibleInstances, each argument in an instance must be a
type constructor applied to type variables, with no repetition of type
variables across the head.  That would rule out instances like these
from the array packages:

instance IArray Array e
instance MArray (STArray s) e (ST s)
instance Storable e => MArray StorableArray e

Even without these, when reducing contexts you encounter situations
where one argument has a type constructor and another is a type variable.
So you have to change the notions of simple context and context reduction
errors (section 4.5.3).  GHC avoids that by deferring context reduction,
but that opens a whole other can of worms.

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