[m-dev.] subtypes discussion

[Mark Brown]

On 30-Dec-2002, Mark Brown <dougl at cs.mu.OZ.AU> wrote:
> Given this, I'm happy to agree to position (1).

Here's an outline of the planned implementation.

1) Parsing is straightforward. A 'subtype' operator will be added
(precedence 1199) to the operator table, and an extra arm added to the item
type to contain info about subtype declarations.  During parsing we will do
basic checks, such as checking that type variables appear in the head when
required.

2) We calculate the base type for each subtype by expanding out the
definitions in a way simlar to expanding equivalence types.  While doing
this we check for circularities in the subtype declarations (that is,
places where attempts to calculate the base type would loop) and report
them as errors.

3) At the same time as calculating the base type, we calculate the base
inst.  We check that the declared subtype represents a subset of the
supertype using inst_matches_initial, and report an error if it doesn't.

4) Propagation of a base inst into another inst will be done in a way
similar to abstractly_unify_inst and inst_merge.  This means we will add
a new inst_name, propagate_inst/2, which has the two insts in question
as arguments, and add a new field to the inst table to store the results.
The algorithm for propagation is almost identical to that for abstract
unification; the only significant difference is in how higher order terms
are handled.

Aside from feedback on the above, I need advice about where the
transformation should be done.  That is, at what point in compilation should
the types in pred/func declarations be replaced by their respective base
types, and the insts be replaced by those that result from propagating the
base inst?

Cheers,
Mark.

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