Russ,
As an Oz novice, I too was puzzled by your examples, but remembering that
the variables introduced in patterns are fresh variables (see 5.6.1 and
5.6.2 of the OZ tutorial) for the scope of the then statement does I think,
clear up your questions.
From: Russ Abbott
In
local X in
case X of 3 then {Browse X} end
%%% 3 can't be made the same as the undefined X, so suspend until we know
more about X
case 3 of X then {Browse X} end
%%% The fresh undefined X can be made equal to 3 no problem
end
the first of these suspends while the second succeeds. And in
local A
proc {Pair X Y}
case X of Y then {Browse X#Y} end
%%% The Y in "of Y" is a fresh undefined var and not the procedure argument
Y
%%% so it can be made the 3 of {Pair 3 A} no problem
%%% and is just as undefined as the A of {Pair A 3}
end
in
{Pair 3 A}
{Pair A 3}
end
both succeed although they give different results: 3#3 and _#_ respectively!
(Why didn't the second call not suspend, and since it didn't suspend, why
didn't it bind Y to 3?)
%%%%
Re-writing as follows may make things clearer...
local X in
case X of 3 then {Browse X} end
case 3 of X1 then {Browse X1} end
% or perhaps
case 3 of !X then {Browse X} end
end
local A
proc {Pair X Y}
case X of Y1 then {Browse X#Y#Y1} end
end
in
{Pair 3 A}
{Pair A 3}
end
Cheers
Bryan
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