On 09/26/2017 03:48 AM, oldk1331 wrote:
> I tried to replace 'subtractIfCan' as a starter. It turns out more
> difficult than I thought.
>
> https://github.com/oldk1332/fricas/commit/36c1e89856fdd0b634993cc0d202f5fcfd2818ef
>
> The change itself is not difficult, but there are many compile errors.
> I'll try again later. Maybe because 'subtractIfCan' is at the heart
> of category tower.
Sorry, I'm a bit late with respect to the discussion about wrap and unwarp.
Maybe X is "X \cup \{ failed \}", i.e., it's just one element more, so
it's natural that there is an embedding from X to Maybe X. Embeddings in
FriCAS go by the name "coerce". The counterpart to coerce is
retract: Maybe(X) -> X
with the meaning that there is no error checking and if someone does
retract(failed), then anything can happen like crashing the computer.
Aldor uses [x] to embedd x into its Partial(X) domain. Also OK, but I
like coerce somewhat more, because then the FriCAS interpreter could do
this coercion if needed.
See also the thesis of Nicolas James Doye
http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=764504EFF2A238FDA098984AFC303ED7?doi=10.1.1.132.4943&rep=rep1&type=pdf
https://lists.gnu.org/archive/html/axiom-developer/2005-09/msg00130.html
https://dl.acm.org/citation.cfm?doid=309831.309944
Ralf
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