On Wednesday, 19 February 2014 at 23:55:16 UTC, bearophile wrote:
Integral ranges and strong types:
type Age is range 0..125;
type Floor is range -5 .. 15;
Is it possible to implement this using the information that's
already available through Value Range Propagation? I think I
remember Andrei describing it in TDPL as numeric values carrying
around their ranges at compile time, but I don't remember exactly.
My_Age : Age;
My_Floor : Floor;
...
My_Age := 10; -- OK
My_Floor := 10; -- OK
My_Age := My_Floor; -- FORBIDDEN !
How close do you think std.typecons.Typedef is to supporting this
functionality? Is it a sufficient replacement?
Discriminated types:
ype Major is (Letters, Sciences, Technology);
type Grade is delta 0.1 range 0.0 .. 20.0;
type Student_Record (Name_Length : Positive;
With_Major : Major)
is record
Name : String(1 .. Name_Length); --Size depends on
discriminant
English : Grade;
Maths : Grade;
case With_Major is -- Variant part, according to
discriminant
when Letters =>
Latin : Grade;
when Sciences =>
Physics : Grade;
Chemistry : Grade;
when Technology =>
Drawing : Grade;
end case;
end record;
Ditto, except for with std.variant.Algebraic. It does need some
work, but I think the important part is whether it's
implementable in the library with a nice syntax. Then it only
requires somebody to do the necessary work.
Low level description of a record:
type BitArray is array (Natural range <>) of Boolean;
type Monitor_Info is
record
On : Boolean;
Count : Natural range 0..127;
Status : BitArray (0..7);
end record;
for Monitor_Info use
record
On at 0 range 0 .. 0;
Count at 0 range 1 .. 7;
Status at 0 range 8 .. 15;
end record;
Isn't this the same as a struct?
Static predicates (related to the "enum preconditions" I
suggested for D):
procedure Seasons is
type Months is (Jan, Feb, Mar, Apr, May, Jun,
Jul, Aug, Sep, Oct, Nov, Dec);
subtype Summer is Months
with Static_Predicate => Summer in Nov .. Dec |
Jan .. Apr;
A_Summer_Month : Summer;
begin
A_Summer_Month := Jul;
end Seasons;
The code gives:
warning: static expression fails static predicate check on
"Summer"
What are the limitations on what Static_Predicate can verify? It
seems like this could be quite powerful, but not as powerful as
runtime checks, since not everything can be checked at compile
time.
A kind of set syntax:
elsif First and then C in '+' | '0' .. ’9’ then
I think this would be quite easy to do using mixins. I've also
been thinking about list comprehensions a la Python, done with
mixins.
Loop variants and invariants:
procedure Loop_Var_Loop_Invar is
type Total is range 1 .. 100;
subtype T is Total range 1 .. 10;
I : T := 1;
R : Total := 100;
begin
while I < 10 loop
pragma Loop_Invariant (R >= 100 - 10 * I);
pragma Loop_Variant (Increases => I,
Decreases => R);
R := R - I;
I := I + 1;
end loop;
end Loop_Var_Loop_Invar;
This is a neat feature. I thought it was unique to Wiley, but I
guess the creator of Wiley must've got it from Ada. Does the loop
break if the invariant fails, or does it stop the program?
An example in Spark, to clarify access to global variables (for
D I suggested a simpler optional @outer() attribute:
https://d.puremagic.com/issues/show_bug.cgi?id=5007 ):
...
I don't think this is quite as important, seeing as D has the
pure keyword.
