Makarius wrote:
On Wed, 16 Sep 2009, Makarius wrote:
On Wed, 16 Sep 2009, David Matthews wrote:
To be honest, when I think of an abstract type I think of the abstype
construction rather than opaque signature matching.
Interestingly, opaque signature matching also causes problems in
SML/NJ pretty printing. I consider to make another attempt at abstype
within a non-opaque structure, trying to avoid the nested structure
within the abstract type.
After looking at this again, I've found that all problems disappear if
SML97's opaque matching is eliminated. With plain old abstype and
SML90's signature matching, pretty printing works again as expected.
If you're only installing the pretty printer once it should be all right
if you install the pretty printer inside the structure and then apply an
opaque signature constraint to the result. The pretty printer will be
inherited by the type in the opaque signature. This was what was
intended by the section in the FAQ.
If you need to change the pretty printer it's more difficult with an
opaque signature. If there's the possibility of an internal pretty
printer leaking out through exceptions then you need to install the
pretty printer both for the internal type and for the external type.
As you say, these problems don't arise with abstypes and transparent
matching. I've updated the FAQ as a result of your comments.
David
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