INTEGER and FLOAT are mathematical concepts, but numbers mean things in the real world
Using pure numbers, we can write nonsense:
age := -20; height := age - class_size; shoe_size := 2 * no_on_bus;
Types help program values reflect the real world.
New data types can be derived from INTEGER:
type ages is new INTEGER range 0 .. 110;
age : ages;
voting_age : constant ages := 18;
type heights is range 0 .. 230;
height : heights;
min_enrolment : constant := 6;
max_enrolment : constant := 200;
type class_sizes is range 0..max_enrolment;
class_size : class_sizes;
These types are distinct from each other, and from INTEGER.
Types cannot be mixed in Ada, so nonsense can be avoided.
Must specify precision, may specify range
Normal FLOAT operations, attributes.
Again, cannot mix types.
Ada has strong typing: different types cannot be mixed
Explicit type conversion is permitted:
type length is digits 5 range 0.0 .. 1.0E10;
type area is digits 5 range 0.0 .. 1.0E20;
function area_rectangle (L,H : length) return area is
begin
return area(L) * area(H);
end;
Nonsense rejected by compiler
height := age - class_size;
"Out of range" rejected by compiler
age := -20;
"Out of range" run time error
class_size := class_size + 100;
Enforce distinct nature of different objects
Robust, elegant, effective programs