Alasdair McAndrew wrote:
>
> I tried to define
>
> U==>Union(DoubleFloat, Integer)
>
> and then have the return list r to be of type U:
>
> r:List U:=...
>
> Then I thought r could happily include both DF's and integers. But it
> doesn't work in the way I hoped it would. Drat.
Well, at low level elements of Union consist of tag and value.
Tag if a small integer identifying which variant is at force.
More precisely, Spad union is interpreted as Lisp pair,
consisting of tag and value:
(9) -> PRETTYPRINT(2::U)$Lisp
(0 . 2.0)
(9) ()
Type: SExpression
The dot above indicates that this is not a list but
a general pair (pairs in lists have rest of list as second
element). For some reason interpreter decided to treat 2
as a double, so we got first variant (with tag 0).
Having list of such pairs is quite different than list
which mixes integers and doubles:
(10) -> PRETTYPRINT([2::U])$Lisp
((0 . 2.0))
(10) ()
Type: SExpression
(11) -> PRETTYPRINT([2])$Lisp
(2)
(11) ()
Type: SExpression
(12) -> PRETTYPRINT([2::DoubleFloat])$Lisp
(2.0)
(12) ()
Type: SExpression
--
Waldek Hebisch
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