Re: Valid Attribute and Unchecked Conversion

[dewar@merv.cs.nyu.edu (Robert Dewar)]

iNorman Cohen says

"But the work necessary to do this (mapping sparse encodings to a
contiguous set of values that can be used for array indexing) is already
necessary anyway to implement the 'Pos attribute."



My goodness, that's perhaps 10% of the effort involved, that's the easy
part. If you want to investigate this statement further, feel free to
examine the GNAT sources. Have a look at exp_pakd.adb where most, but 
certainly not all the work is done. Luckly in GNAT we could piggyback off
the packed array stuff, which was already done in the front end (it might
be better to do it in the backend, but that's for the future, and luckily
for this particular issue, was not done yet!)

Yes, conceptually, the only issue is the mapping, but the problem is that
you have a situation where the conceptual type and the implementation
type are different (the conceptual type is indexed by the enumeration type,
the actual type by the representation of the enumeration type, for us these
are two separate types). As always, it is the implementation details, not
the algorithms that are the hard work!