Re: [isabelle] map, difference, image, etc.



Hi John,

the type classes of Isabelle are restricted to a single type parameter and do not support constructor classes (i.e., type classes of higher kind).

The latter restriction is the reason why your proposal cannot be implemented using Isabelle's type classes.

If it is just about syntax, adhoc overloading would work. However, this can lead to many situations where explicit type constraints have to be added in order to avoid ambiguity. So I wouldn't go for that.

cheers

chris

On 04/22/2014 12:23 PM, John Wickerson wrote:
Hi,

I recently improved my life quite a lot by assigning an infix operator symbol to the ‘map’ function. Expressions like “map (f x) (g y)” could become expressions like “f x ``` g y” -- much more readable without all those parentheses. As you can see, I chose (```) as my infix operator.

But it got me thinking: why can’t I just use (`) instead? That is, the same symbol as is used for the image of a set. “Map” and “image” are pretty similar functions, after all:

map : (‘a => ‘b) => ‘a list => ‘b list
image: (‘a => ‘b) => ‘a set => ‘b set

There’s no problem with (+) being polymorphic over any sort of number, so why can’t (`) be polymorphic over any sort of “collection”? And while we’re at it, we could make the set-minus operator (-) polymorphic too, so that when X and Y are of type “list”, then “X-Y” is the list obtained by removing from X all elements in Y.

Would that work?

Thanks,
John






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