ISSUE: Limited collection fill

REVISION HISTORY: Rob MacLachlan, 18 February 94

STATUS: open

RELATED ISSUES: limited types design note

CATEGORY: change

PROBLEM DESCRIPTION:

It would be desirable if generic collection operations like map preserved
element type limitations when passed a limited collection.

One problem is that although limited collection types are instantiable (via
make), they are not classes, so presumably can't currently be returned by
class-for-copy.

Even if map can somehow get the limited collection type to instantiate, it
still can't call "make" on that type, since map can't get its hands on a legal
fill: value to allow creation of arrays whose element type doesn't include #f.

PROPOSAL:

Add a fill: keyword to limited (default #f):
    limited(<array>, of: <single-float>, fill: 0.0s0)

Change MAKE to use this fill value as a default when creating limited
collections.

Clarify that CLASS-FOR-COPY can return any instantiable type (not just
classes.)

AMENDMENT 1:

Rename CLASS-FOR-COPY to TYPE-FOR-COPY.

RATIONALE:

This allows generic collection operations to have the expected behavior on
limited collections.

EXAMPLES:

COST TO IMPLEMENTORS: 

COST TO USERS:

PERFORMANCE IMPACT:
    Allows efficient limited collection instances to be used as arguments to
    generic collection operations.  Doesn't allocate a worst-case result type.

BENEFITS: efficiency, less surprising behavior.

AESTHETICS:

FUTURE FEATURES: