i) Maybe is implemented in BOOT
ii) constructors like List are also coded in BOOT, and there is no
(know to me, you, any?)
way to do this in SPAD
At least for Aldor that is simple, but I believe for SPAD there is no
difference in this case.
http://svn.origo.ethz.ch/wsvn/algebraist/trunk/aldor/lib/aldor/src/base/sal_partial.as
http://svn.origo.ethz.ch/wsvn/algebraist/trunk/aldor/lib/aldor/src/datastruc/sal_list.as
Partial (which corresponds to Maybe) and List are completely
library-defined in libaldor.
And you probably see that the unit in Partial is a simple coerce
function. In List, it is not available, because it seems to be
uninteresting, to just produce a one-element list. But it can, of
course, easily be implemented via unit(t:T): % == cons(t,empty).
The join or mult operation is not available with a signature
M M A -> M A. The reason is simply that you cannot express this
signature when you are inside List or Partial. The closest one can get
would be
List(%) -> % or Partial(%) -> %.
That, however should work, even if one has to explicitly refer to the
name of the domain that one is going to define. It's just that you
cannot define it with an anonymous "add" structure.
But all that only implements the functionalities of monads. It doesn't
make the concept of a monad available in Aldor/SPAD, such that one could
inherit the signatures from that (coded) concept.
Ralf
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