Strange error in show for datatype
Wed, 03 Oct 2001 14:05:19 -0700
Simon Peyton-Jones wrote:
> msg :: forall a. Show a => String
>Urk! What "Show" dictionary should Hugs use when evaluating "msg"?
>You may say "it doesn't matter", but in general that's not the case.
>In the case of class Num, for example, we might have
> expr = 3+4
> msg = show expr
>and then the type involved really does matter.
>I wonder if the following is true. Given the ambiguous type
> forall a. Show a => T (where a does not appear in T)
>it's OK to pass the bottom dictionary. ...
Type systems with subtyping have what is needed to determine when the
choice of dictinionary "doesn't matter".
In type systems with subtyping, the most general type is the minimal
type. This means that type variables that occur only positively (in
covariant positions) can be minimized (and that negative variables can
be maximized). If there is an empty type, Empty say, which is a subtype
of all other types, then the type
forall a . Show a => T
is just as general as the type
Show Empty => T
since a occurs only positively in Show a => T, taking into account the
occurences of a in the definition of the Show class. The requirement
that a does not appear in T is overly restrictive. This simplification
can be made as long as all occurences of a in T are positive.
Assuming a language with subtyping that simplifes types in this way,
show (Left False)
would no longer be ambiguously overloaded. If the prelude provides
instance Show Empty -- no methods need to be defined
then types of the above expressions would all reduce to String, and they
would all compute to the expected results (i.e., the result you would
get by manually disambiguating the types, e.g., show (::[Int])).
The same trick applies to the Eq class, so that, e.g.,  ==  would be
unambiguous and compute to True.
So, obviously, the next version of Haskell should have a type system
with subtyping, don't you agree? :-)
PS In some previous version of Haskell (1.3?), the Prelude defined an
empty type called Void, but it has since been removed. Apparently,
people didn't see the potential of Void...
PPS For those who are afraid of subtypes :-), I think you can use the
information about variance of type variables used in subtype inference,
to determine when the choice of dictinonary "doesn't matter", without
introducing subtyping in the languange...