[m-dev.] Ye Olde Subtyping Proposal

[Ralph Becket]

Fergus Henderson, Monday, 18 November 2002:
> 
> For example:
> 
> 	:- module fruit.
> 	:- interface.
> 
> 	:- type fruit ---> apple ; orange ; lemon.
> 	:- subtype citrus < fruit ---> orange ; lemon.
> 	:- subtype favourite_fruit < fruit.  % abstract
> 
> 	:- func my_favourite = favourite_fruit.
> 	:- func your_favourite = favourite_fruit.
> 
> Suppose this is initially implemented as
> 
> 	:- implementation.
> 
> 		% no-one likes apples.
> 	:- subtype favourite_fruit < fruit ---> orange ; lemon.
> 
> 	my_favourite = lemon.
> 	your_favourite = orange.
> 
> and some other module uses this
> 
> 	:- module fruit_comparison.
> 	:- interface.
> 	:- import_module io.
> 	:- pred main(io::di, io::uo) is det.
> 
> 	:- implementation.
> 	:- import_module fruit.
> 
> 	main --> print("My favour fruit is "),
> 		 print(comparison(my_favourite, your_favourite)),
> 		 print(" your favourite fruit."), nl.
> 
> 	:- func comparison(citrus, citrus) = string.
> 	comparison(orange, lemon) = "sweeter than".
> 	comparison(lemon, orange) = "yellower than".
> 	comparison(lemon, lemon) = "the same as".
> 	comparison(orange, orange) = "the same as".

This looks like it should be a type error to me: under the
abstract-types-obey-name-subtyping rule we do not have that
favourite_fruit < citrus.  It's just (un)happy coincidence
that this module compiles at all.

We would clearly need to add something to type checking to
handle these cases, but I don't see why that would be a
major effort.  Of course, I haven't mucked about in the
type checker yet...:-)

- Ralph
--------------------------------------------------------------------------
mercury-developers mailing list
Post messages to:       mercury-developers at cs.mu.oz.au
Administrative Queries: owner-mercury-developers at cs.mu.oz.au
Subscriptions:          mercury-developers-request at cs.mu.oz.au
--------------------------------------------------------------------------