[Haskell-cafe] container-classes (was: Restricted type classes)

wren ng thornton wren at freegeek.org
Mon Sep 6 23:47:30 EDT 2010

On 9/6/10 12:53 PM, John Lato wrote:
> I'd like to make one more argument in favor of my preference for more
> splitting of type classes.

FWIW, I agree that more splitting is generally good. This is one of the 
problems I have with the various proposals for a ListLike class. They 
conflate the cons-list like, sequence like, string like, set like (i.e., 
Boolean algebra), and collection like (e.g., filterable) functions with 
nary a care. This in turn means that fewer types can implement the 
class, and moreover that we can state fewer properties about the 
instances which are there (e.g., what their running time complexity is 
or should be).

The countervailing force, for me, is the question of whether the smaller 
classes can still be well-defined--- as discussed separately in the 
"Pointed" thread fork. If MPTCs are involved, the other consideration is 
to make sure that type inference for common usage isn't borked by 
splitting things. Even though it's a common argument from others, I 
don't care about defining one instance with five methods vs five 
instances with one method each. This last argument seems to be one of 
the leading problems with redefining the numeric hierarchy.

Live well,

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