Proposal to solve Haskell's MPTC dilemma

Isaac Dupree ml at
Fri May 28 01:36:55 EDT 2010

On 05/27/10 17:42, Carlos Camarao wrote:
> On Thu, May 27, 2010 at 5:43 PM, David Menendez<dave at>  wrote:
>> On Thu, May 27, 2010 at 10:39 AM, Carlos Camarao
>> <carlos.camarao at>  wrote:
>>> Isaac Dupree:
>>>> Your proposal appears to allow /incoherent/ instance selection.
>>>> This means that an expression can be well-typed in one module, and
>>>> well-typed in another module, but have different semantics in the
>>>> two modules.  For example (drawing from above discussion) :
>>>> module C where
>>>> class F a b where f :: a ->  b
>>>> class O a where o :: a
>>>> module P where
>>>> import C
>>>> instance F Bool Bool where f = not
>>>> instance O Bool where o = True
>>>> k :: Bool
>>>> k = f o
>>>> module Q where
>>>> import C
>>>> instance F Int Bool where f = even
>>>> instance O Int where o = 0
>>>> k :: Bool
>>>> k = f o
>>>> module Main where
>>>> import P
>>>> import Q
>>>> -- (here, all four instances are in scope)
>>>> main = do { print P.k ; print Q.k }
>>>> -- should result, according to your proposal, in
>>>> -- False
>>>> -- True
>>>> -- , am I correct?
>>> If qualified importation of k from both P and from Q was specified, we
>>> would have two *distinct* terms, P.k and Q.k.
>> I think Isaac's point is that P.k and Q.k have the same definition (f
>> o). If they don't produce the same value, then referential
>> transparency is lost.
>> --
>> Dave Menendez<dave at>
>> <<>>
> The definitions of P.k and Q.k are textually the same but the contexts are
> different. "f" and "o" denote distinct values in P and Q. Thus, P.k and Q.k
> don't have the same definition.

Oh, I guess you are correct: it is like defaulting: it is a similar 
effect where the same expression means different things in two different 
modules as if you had default (Int) in one, and default (Bool) in the 
other.  Except: Defaulting according to the standard only works in 
combination with the 8 (or however many it is) standard classes; and 
defaulting in Haskell is already a bit poorly designed / frowned upon / 
annoying that it's specified per-module when nothing else in the 
language is*.(that's a rather surmountable argument)

It may be worth reading the GHC user's guide which attempts to explain 
the difference between incoherent and non-incoherent instance selection,
I didn't read both it and your paper closely enough that I'm sure 
anymore whether GHC devs would think your extension would require or 
imply -XIncoherentInstances ... my intuition was that 
IncoherentInstances would be implied...

*(it's nice when you can substitute any use of a variable, such as P.k, 
with the expression that it is defined as -- i.e. the expression written 
so that it refer to the same identifiers, not a purely textual 
substitution -- but in main above, you can't write [assuming you 
imported C] "print (f o)" because it will be rejected for ambiguity. 
(Now, there is already an instance-related situation like this where 
Main imports two different modules that define instances that overlap in 
an incompatible way, such as two different instances for Functor (Either 
e) -- not everyone is happy about how GHC handles this, but at least 
those overlaps are totally useless and could perhaps legitimately result 
in a compile error if they're even imported into the same module.))

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