Building Blocks

Ada often uses different terminology than other languages. This overview avoids Ada jargon, only using "subprogram", which refers to functions and procedures, and "procedures" which in most C-like language would be a "function" which returns nothing (i.e. void).

Packages, the Building Blocks of Ada

Ada descends from Pascal, and yet uses many concepts already familiar to C or C++ programmers. If you're a C/C++ programmer, you'll find Ada leverages the concepts you're used to in a more formal and often compiler-checked way.

Subprograms (functions and procedures)

Ada draws a line between functions, which return values, and procedures which do not return a value. Collectively referred to as "subprograms", either of these may have input (in) and output (out) parameters, with parameters being allowed to also be both an input and output (in out). in is optional for functions. Note that parameters are separated by semicolons (;), not by commas (,).

procedure Rectangle_Area(Width : in Float; Height : in Float; Area : out Float) is
begin
    Area := Width * Height;
end Rectangle_Area;

function Rectangle_Area(Width : Float; Height : Float) return Float is
begin
    return Width * Height;
end Rectangle_Area;

Short functions may be written as expressions bounded by parentheses. in is also optional for functions, and parameters with the same type can be grouped.

function Add (L, R : Integer) return Float2 is (L + R);

Multiple subprograms can exist with the same name, so the one used is determined by the types of the parameters and also the returned type. There are no implicit conversions between floating point and integer types, which maximize clarity.

function Area (Width, Height : in Float) return Float is (Width * Height);
function Area (Width, Height : in Integer) return Integer is (Width * Height);

The basic operators can be overloaded as well. Assignment (:=) is not considered an operator, and therefore cannot be overloaded.

type Float2 is record
    X: Float
    Y: Float
end record;

function "+"(L, R : Float2) return Float2 is (L.X + R.X, L.Y + R.Y);

Packages

As the fundamental building block of Ada, "packages" compose Ada programs, namespaces which exhibit the logical and physical interfaces in a manner similar to C++ header and source files. Most languages don't have a concept of "physical interface"--it's things the compiler needs to know, but are not part of the logical interface of the program, such as size details for structs and classes.

Instead of a header files providing an informal spec and an associated source file being the translation unit, in Ada a package is roughly analogous to a "header file," and that package's body is the "source file", except as if everything in each file is in a namespace given by the file name.

Top-level package specifications, appear in *.ads (Ada specification) files, with their implementations ("bodies") in *.adb (Ada body) files, and only one top-level package specification or package body per file.

Ada packages can also be nested and support visibility rules for sharing details with child packages. Child packages are given by dotted names; A.B is a child of package A.

Packages can also contain initialization code for the package to run at program startup prior to entering the main procedure, with the ability to describe dependencies in package start up order. This solves a specific C++ issue in which static initialization order is not known, while also offering the ability to avoid deferred first-time usage costs, such as with singletons.

Ada uses "aspects" to denote additional properites of packages, subprograms, and types. Along with aspects, compiler pragmas allows description of initialization dependencies, as well as providing high level checks, such as pragma Preelaborate to ensure a package has no initialization, or the with Pure aspect to ensure that a package has no state and subprograms cannot have side effects.

Programs are readable from beginning to end

Program text must be clear without having to read ahead, referred to as: "Linear elaboration of declarations". A clear demarcation exists between declarations of things to exist and executable statements which use those things.

The simplicity backing this is the ability to make any declaration, including types, variables, functions/procedures, and packages in any declaration block. This means the basic rule of "declare, then use" repeats itself throughout the language, in package/package body, task/task body, subprograms, and executable blocks of code can have a declare ... begin ... end block.

package P is
    -- Not declaring Foo here is like making the subprogram `static` in C or C++ or
    -- putting it into an anonymous namespace.
    procedure Foo;
end P;

package body P is
    -- Declarations for the body of P go here.

    procedure Foo is
        -- Declarations for Foo can go here.

        -- Declare a subprogram, only visible to Foo, to be used to implement Foo.
        procedure Bar is
        begin
            null; -- "null statement" here since this subprogram actually does nothing.
                -- and one statement is required.
        end Bar;
    begin
        -- Executable statements go here.
        Ada.Text_IO.Put_Line("Hello, World!");

        declare
            -- Declare a package here, inside the subprogram, to show that you can.
            package Wat is
                -- Declare a new type, which has the same set of possible values
                -- as a Float, but is different than a Float.
                type Capricorn is new Float;
            end Wat;

            -- A constant created using the package defined inside of Foo.
            -- Temporary variables can be declared here too.
            Temp: constant Wat.Capricorn := 0.0;
        begin
            -- Print "0.0".
            -- "Image" is the Ada idiomatic equivalent of toString().
            -- ' is "tick" and is used to access compiler-defined properties of types.
            Ada.Text_IO.Put_Line(Wat.Capricorn'Image(Temp));

            -- Call the helper procedure defined in Foo.
            -- Procedures and functions without parameters are called
            -- without parentheses.
            Bar;
        end;
    end Foo;
begin
    -- Static initialization body of P.
end P;

This nesting of declarations very verbose. It does however makes it straightforward to refactor out behavior while you're working on a subprogram and then you can extract the newly created components into more appropriate places when you're done. The inability to use statements in declarations causes me to sometimes rewrite my declarations in sequential order of constant processing, and overall makes the declarations feel like a Haskell where clause.

function Normalize(F : Float2) return Float2 is
    L : constant F32 := Length(F);
begin
    return F / L;
end Normalize;

function Evaluate
    (Ctx : in out Context; Line : in Ada.Strings.Unbounded.Unbounded_String)
        return Evaluate_Result is
    use Ada.Containers;
    use type Ada.Strings.Unbounded.Unbounded_String;
    Whitespace     : constant Ada.Strings.Maps.Character_Set := Ada.Strings.Maps.To_Set (" ");
    Sanitized_Line : constant Ada.Strings.Unbounded.Unbounded_String :=
                        Ada.Strings.Unbounded.Trim (Line, Whitespace, Whitespace);
    Words          : String_Vectors.Vector := Split (Sanitized_Line);
    Command        : constant Ada.Strings.Unbounded.Unbounded_String := (if Words.Length > 0 then Words.First_Element else Ada.Strings.Unbounded.Null_Unbounded_String);
begin
    -- ...

Ada does not provide separate syntactical units for classes, structs and namespaces. Instead, packages contain types, constants and related subprograms. A lot of specialized syntax goes away due to this, for example there are no "member functions" and "class functions" and hence no specialized syntax for things like member function pointers or class function pointers exist. Namespacing and overloading on parameters and/or the returned type determine the subprogram called.

What would be "member functions" in C++ have the "controlling type(s)" as the first parameter(s). "const member functions" pass in the type as an in parameter, which are immutable. "non-const member functions (methods)" pass in the type as an in out parameter, allowing the parameter to be modified. This mirrors Rust's notation wherein it reflects C++-like const behavior of member functions with self, &self, and &mut self as a first parameter. These are referred to as "primitive operations."

-- Box "non-const function"
procedure Move(Box : in out AABB2; Direction : Float2) is
begin
    Box.Min := Box.Min + Direction;
    Box.Max := Box.Max + Direction;
end Move;

-- Box "const function"
function Midpoint(Box : in AABB2) return Float is
begin
    return (Box.Min + Box.Max) / 2.0;
end Midpoint;