A piece of intertube about the Clojure programming language, algorithms and artificial intelligence.

Sunday, May 8, 2011

Dear language designers, do not forget Ada

These last days I took some time to refresh my old memories of the Ada language. While Ada, like all languages, has defaults on its own I'm still very impressed by some functionalities it offers and that current mainstream languages still don't have. Most of these functionalities are there since the first standardized version in 1983 (since then the language standard was extended in 1995, 2005 and the next version is expected for 2012).

The first impressive future is about primitive types. Here are some of the types defined in Ada and their C equivalent (taken from [1]):

Ada TypeDescriptionC Equivalent
CharacterA single characterchar
IntegerAn integer (32-bit) numberint
NaturalZero or positive integer-
PositiveA positive integer-
Long_IntegerA big integer (same as long in Gcc)long (same as int in Gcc)
Long_Long_IntegerA really big (64-bit) integerlong long
Short_IntegerA small (16-bit) integershort
Short_Short_IntegerA really small (8-bit) integerchar
FloatA real numberfloat
Long_FloatA big real numberdouble
Long_Long_FloatA really big real numberlong double
Short_FloatA smaller real number?
FixedA fixed-point real number-
StringAn Ada fixed-length stringchar array

Boolean types are also provided. New types are defined and type safety is guaranteed. Here we define a new type based on a Long_Float and a variable of this type:
type Speed is new Long_Float;

Speedy_Gonzales_Speed : Speed;

Since Speed is a new type, it is not a Long_Float and thus assignment from a Long_Float value to a Speed variable is forbidden:
X : Long_float := 300.0; 
Speedy_Gonzales_Speed := X;
The compiler effectively gives an error:

expected type "Speed" defined at line 6
found type "Standard.Long_Float"

When it makes sense, we can define a subtype of another primitive type. Assignments between types and their subtypes is allowed. Here we define the type Degree which is a subtype of Float:
subtype Degree is Long_Float;

Oven_Temperature : Degree;
Y : Long_Float := 255.0;

Oven_Temperature := Y;

Types can be constrained with a range definition (taken from [2]):
type Degrees is new Float range -273.15 .. Float'Last;

This allow a Degrees variable to range from -273.15 (absolute zero) to the last value allowed by a Float.

This strong typing provides safety and contrasts with what is seen in current mainstream programming languages. Languages like Java don't provide rich primitive types (there are for instance no unsigned integer in Java!) and require the programmer to define its own cumbersome classes if he wants new types (which poses readability problems if the types are new number types and the language does not support operator overloading). Some languages like C don't even bother so much about type conversion: a float can be assigned to an integer. This leads to solutions which are far from being satisfying (see for instance C FAQ - round).

A lot of other possibilities are offered to the Ada programmer, for instance decimal types can be defined with their precision. The precision is then guaranteed by the compiler:
type money is delta 0.01 digits 18;
Ada also supports multidimensional arrays, bit-level memory access, definition of memory pool, concurrency programming in a task-oriented way, object-oriented programming, generic packages etc.

At another level Scheme and Common Lisp supports a numeric towel. For example, 3 is an integer. Therefore 3 is also a rational, a real, and a complex (example from the Scheme standard).

;; in Scheme:
> (rational? (/ 1 5))
> (rational? 3)
> (real? 3)
While this discussion may not make so much sense for dynamic languages such as Clojure, I think new languages being designed, which are not dynamically typed, could really benefit of having such rich primitives types. A complex type systems like the one from ML or Haskell is not needed for this purpose and primitives types is the most basic and most used feature of a programming language, so dear languages designers, next time, have a small thought for Ada.

[1] http://www.pegasoft.ca/resources/boblap/book.html
[2] http://en.wikibooks.org/wiki/Ada_Programming