[Haskell-cafe] Problem trying to get class Bounded to work

Brian Hulley brianh at metamilk.com
Tue May 23 12:37:10 EDT 2006

Jacques Carette wrote:
> I guess I prefer a type annotation over a dummy function that is
> there just to force the type checker to believe me.  If one has to
> force the type checker, may as well do it with a type, not code!

I agree totally.
Also, I think there is a more general issue regarding the relative 
importance of type inference when programming, since it seems to me that a 
lot of awkwardness is introduced into Haskell by the insistence that the 
type inference algorithm should be able to infer everything by itself to 
surprise the programmer with knowledge that the programmer should already 
know in the first place and could quite easily have written down directly.

An example is the fact that constructors and record field names currently 
have to be unique within a whole module. If instead of this, constructors 
and field names were always in seperate namespaces indexed by type, instead 
of them determining the type, programming would be tremendously easier 
because the same names could be reused without worrying about conflicts.

eg compare

           data Either a b = Left a | Right b   -- Prelude got there first 
as usual
           data Location = Location_Left | Location_Right

           foo (Location_Left, Location_Left, Location_Left) = Left 5
           foo (Location_Left, Location_Left, Location_Right) = Right 
           -- 6 more wordy patterns


           data Either a b = Left a | Right b
           data Location = Left | Right   -- all names local to type 

           foo :: (Location, Location, Location) -> Either Int String
           foo (Left, Left, Left) = Left 5
           foo (Left, Left, Right) = Right "something"
           -- 6 more simple patterns

Regards, Brian. 

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