Hello,
I'm working on coercion for interval fields (See Trac #24371) and there
is one issue that I don't know how to solve elegantly:
Sage has NamedConvertMap to convert via special methods like
_real_mpfi_(). When looking for a conversion map, it first tries to find
a coercion map and then it checks for a default conversion map, which
could be the NamedConvertMap. Now this seems to be the wrong order:
after all, if a conversion exists using the _real_mpfi_() map, there is
no reason to look further. Why try to find a coercion map first?
For coercion, it is slightly more complicated: ideally, one would want
to use the _real_mpfi_() map if it exists, but that is wrong since the
map might not define a coercion (only a conversion). So for coercion, it
seems that the logic should be:
1. Check for a coercion map.
2. If coercion is possible, check for a NamedConvertMap. If so, use that
instead.
3. If there is no NamedConvertMap, use the coercion map from step 1.
I feel like there should a mechanism to prefer a NamedConvertMap over
other maps. Has somebody encountered this problem before for some other
parent? I created Trac #24381 for this issue.
Jeroen.
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