An introduction to Ada’s simple numeric types

TL;DR: This article introduces the numeric types available in Ada 2012

Ada expects the programmer to not use the built-in number types directly, but create new number types to match the application’s specific needs. By giving pointers to the compiler about the required numeric characteristics, Ada implementations can pick the best hardware number type for the job, providing efficiency without compromising portability.

The built-in number types (in the Standard package) consist of Integer, Positive (an integer type that starts from 1), Natural (an integer type that starts from 0), and Float. Ada implementations may provide other types, but its usually recommended to avoid them for portability reasons.

All the number types support the typical arithmetic operations, plus other operators such as mod (modulo), rem (remainder), abs (absolute), and more advanced ones defined in the Ada.Numerics package. For the programmer’s convenience, Ada supports underscores as delimiters for the sole purpose of making number literals easier to read. The programmer can also express number literals in bases other than 10 by writing the base, and then the number between # symbols. Thus, the number 3 is written as 2#011#.

Ada allows the programmer to define custom integer and real number types.

Integer Types

Integers are discrete types, which means they have the property of unique predecessors and successors. Ada allows the programmer to define both signed and modular integer types.

Signed Integers

Signed integers are the simplest number type. The System.Min_Int and System.Max_Int constants define the minimum and maximum signed integer values that the given system can represent.

The programmer can define a signed integer type like this:

type My_Integer_Type is range Start .. End;

Where Start and End are expressions that define the type’s lower and upper bounds.

The programmer can perform text IO on signed integer types by instantiating the Ada.Text_IO.Integer_IO generic package:

package My_Integer_Type_IO is
  new package Ada.Text_IO.Integer_IO(My_Integer_Type);

Modular Integers

Modular integers are unsigned integer types that use modular arithmetic. The Ada programmer can define these types of integers like this:

type My_Modular_Type is mod N;

Where the modulus, N needs to be a static expression. Our My_Modular_Type type can hold values from 0 (because it is an unsigned integer) to N - 1.

Modular types also implement the and, or, xor, and not operators, which treat the number as a bit pattern.

The programmer can use the Mod attribute of a modular type to convert a signed integer into such type. For example:

My_Modular_Type'Mod(15);

If the attribute argument is not within the modular type’s bounds, then modular arithmetic will make it fit.

Notice that modular arithmetic applies if an expression results in a value greater than the modulus, but the programmer is not allowed to set an out of bounds value directly. For example, given M is larger than the modulus, X: My_Modular_Type := M will raise a constraint error.

The programmer can instantiate the generic text IO library for a modular type by using the Ada.Text_IO.Modular_IO package:

package My_Modular_Type_IO is
  new Ada.Text_IO.Modular_IO(My_Modular_Type);

Real Types

Ada supports floating-point and fixed-point number types. Floating-point values have a relative error while fixed-point values have an absolute error.

Since there are infinite values between any two real numbers, computers have precise representations only for a set of those values, which are called model numbers. The computer approximates the remaining values using the closest model number.

Floating-Point Types

Floating-point types are internally represented as a kind of scientific notation, usually IEEE 754. In Ada, the programmer determines the minimum amount of digits required for the significand and the implementation will pick the hardware type that better suits that constraint.

The programmer can define a floating-point type like this:

type My_Floating_Point_Type is digits N range Start .. End;

The static value N represents the minimum amount of significant digits, which should be no greater than System.Max_Base_Digits.

The programmer may provide an optional range. If a range is not provided, Ada will create a floating-point type with the widest possible range. The number of significant digits for floating-point types without a range is determined by the System.Max_Digits constant.

The programmer can consult the real amount of digits provided by the hardware for a floating-point type at runtime using the Digits attribute of the base type:

My_Floating_Point_Type'Base'Digits;

Take the following floating-point type as an example:

type My_Floating_Point_Type is digits 4 range 0.0 .. 100.0;

Since the number of significant digits is 4, we can use this type to represent numbers such as 99.86, or 5.456. If a particular number has more digits than what the significand supports, like 99.456, then Ada will round the number to the closest representable number, which in the case of 99.456 would be 99.46.

The programmer can instantiate the text IO package for a floating-point type by instantiating the Ada.Text_IO.Float_IO generic package:

package My_Floating_Point_Type_IO is
  new Ada.Text_IO.Float_IO(My_Floating_Point_Type);

Keep in mind that the representational error for a floating-point number type gets larger as the number gets larger. The reason for this is that as the number gets larger, the integral part occupies more significant digits, leaving fewer digits to the fractional part.

Ordinary Fixed-Point Types

Floating-point number types allow the radix to “float” through the significant digits. Thus, the amount of digits available for the integral and fractional parts is variable. Fixed-point types, on the other hand, have a fixed amount of digits for both parts.

This type of number representation has certain advantages:

The programmer can define these real number types like this:

type My_Fixed_Point_Type is delta N range Start .. End;

Where the delta static expression defines the maximum distance between model numbers that the programmer is willing to tolerate. The maximum representational error is half of this distance.

Notice that ordinary fixed-point types make use of a scaling factor that is a power of two, and therefore the actual delta will be the largest power of two that is less or equal to the given value. If the defined delta is 1/3, then Ada will use 1/4 (which equals 2^-2, since 1/3 is not a power of 2.

The programmer can perform text IO on decimal fixed-point types by instantiating the Ada.Text_IO.Fixed_IO generic package:

package My_Fixed_Point_Type_IO is
  new Ada.Text_IO.Fixed_IO(My_Fixed_Point_Type);

Decimal Fixed-Point Types

This type is a fixed-point type with a scaling factor that is a power of ten.

The Ada programmer can define a decimal fixed-point type as an ordinary fixed point type that includes a minimum number of significant digits, just like with floating-point types:

type My_Decimal_Fixed_Point_Type is delta 0.01 digits 10;

Where the delta expression must be a static power of ten (otherwise the compiler raises an error).

The defined minimum number of significant digits covers the number of fractional digits required by the delta expression, so in the above example we can represent numbers using 8 digits for the integral part, and 2 digits for the fractional part.

Notice that assigning a number literal with more integral or fractional digits than specified will result in an error, instead of resulting in a rounded number like in the case of floating-point types.

The programmer can perform text IO on decimal fixed-point types by instantiating the Ada.Text_IO.Decimal_IO generic package:

package My_Decimal_Fixed_Point_Type_IO is
  new Ada.Text_IO.Decimal_IO(My_Decimal_Fixed_Point_Type);

The Base Type

Ada’s approach to number types is to let the programmer describe the required constraints, and then let the implementation choose the best hardware-specific type for the job.

This distinction introduces what we call the “base” type. The base type of a number type refers to the underlying hardware type that represents it and is accessible through the Base attribute of any number type.

Notice that base number operations are defined in terms of the base type, so numbers may be out of bounds during intermediate computations. This means that even if carefully define our number types using ranges, the program might still not be fully portable due to intermediate computations that might overflow on certain hardware.

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