--  Comments are written with a double hyphen and exist until the end of
--  the line.

--  You do not need to call the entry point "Main" or "main," you should
--  name it based on what the program does.
procedure Empty is
   --  This is a declarative part.
begin
   --  Statements go here.
   null;  --  Do nothing here.
end Empty;

--  Ada compilers accept compilation units which can be library packages,
--  tasks, sub-programs, generics, etc.

--  This is where "context clauses" go, these can be pragmas or "with"
--  statements. "with" is equivalent to "include" or "import" in other
--  languages.
with Ada.Text_IO;  --  Get access to a library package.

procedure Hello is
begin
   Ada.Text_IO.Put_Line ("Hello, world");

   Ada.Text_IO.Put ("Hello again, world");
   Ada.Text_IO.New_Line;
end Hello;


--  Ada has a real module system. Modules are called packages and are split into
--  two component parts, the specification and a body.
--  It is important to introduce packages early, as you will be using them from
--  the start.
package Stuff is
   --  We could add the following line in order to tell the compiler that this
   --  package does not have to run any code before the "main" procedure starts.
   --  pragma Preelaborate;

   --  Packages can be nested within the same file or externally.
   --  Nested packages are accessed via dot notation, e.g. Stuff.Things.My.
   package Things is
      My : constant Integer := 100;
   end Things;

   --  If there are sub-programs declared within the specification, the body
   --  of the sub-program must be declared within the package body.
   procedure Do_Something;  --  If a subprogram takes no parameters, empty
                            --  parentheses are not required, unlike other
                            --  languages.

   --  We can also make generic sub-programs.
   generic
      type Element is (<>);  --  The "(<>)" notation specifies that only
                             --  discrete types can be passed into the generic.
   procedure Swap (Left, Right : in out Element);

   --  Sometimes we want to hide how a type is defined from the outside world
   --  so that nobody can mess with it directly. The full type must be defined
   --  within the private section below.
   type Blobs is private;

   --  We can also make types "limited" by putting this keyword after the "is"
   --  keyword, this means that the user cannot copy objects of that type
   --  around, like they normally could.
private
   type Blobs is new Integer range -25 .. 25;
end Stuff;


package body Stuff is
   --  Sub-program body.
   procedure Do_Something is
      --  We can nest sub-programs too.
      --  Parameters are defined with the direction of travel, in, in out, out.
      --  If the direction of travel is not specified, they are in by default.
      function Times_4 (Value : in Integer) return Integer is
      begin
         return Value * 4;
      end Times_4;

      I : Integer := 4;
   begin
      I := Times_4 (I);
   end Do_Something;


   --  Generic procedure body.
   procedure Swap (Left, Right : in out Element) is
      Temp : Element := Left;
   begin
      Left  := Right;
      Right := Temp;
   end Swap;
begin
   --  If we need to initialise something within the package, we can do it
   --  here.
   Do_Something;
end Stuff;


with Ada.Unchecked_Conversion;
with Ada.Text_IO;
with Stuff;

procedure LearnAdaInY is
   --  Indentation is 3 spaces.

   --  The most important feature in Ada is the type. Objects have types and an
   --  object of one type cannot be assigned to an object of another type.

   --  You can, and should, define your own types for the domain you are
   --  modelling. But you can use the standard types to start with and then
   --  replace them later with your own types, this could be called a form of
   --  gradual typing.

   --  The standard types would only really be a good starting point for binding
   --  to other languages, like C. Ada is the only language with a standardised
   --  way to bind with C, Fortran and COBOL! See the links in the References
   --  section with more information on binding to these languages.

   type Degrees is range 0 .. 360;  --  This is a type. Its underlying
                                    --  representation is an Integer.

   type Hues is (Red, Green, Blue, Purple, Yellow);  --  So is this. Here, we
                                                     --  are declaring an
                                                     --  Enumeration.

   --  This is a modular type. They behave like Integers that automatically
   --  wrap around. In this specific case, the range would be 0 .. 359.
   --  If we added 1 to a variable containing the value 359, we would receive
   --  back 0. They are very useful for arrays.
   type Degrees_Wrap is mod 360;

   --  You can restrict a type's range using a subtype, this makes them
   --  compatible with each other, i.e. the subtype can be assigned to an
   --  object of the type, as can be seen below.
   subtype Primaries is Hues range Red .. Blue;  --  This is a range.

   --  You can define variables or constants like this:
   --  Var_Name : Type := Value;

   --  10 is a universal integer. These universal numerics can be used with
   --  any type which matches the base type.
   Angle : Degrees := 10;
   Value : Integer := 20;
   --  New_Angle : Degrees := Value;  --  Incompatible types won't compile.
   --  New_Value : Integer := Angle;

   Blue_Hue   :          Primaries := Blue;  --  A variable.
   Red_Hue    : constant Primaries := Red;   --  A constant.
   Yellow_Hue : constant Hues      := Yellow;
   Colour_1   : constant Hues      := Red_Hue;
   --  Colour_2   : constant Primaries := Yellow_Hue;  --  uncomment to compile.

   --  You can force conversions, but then you are warned by the name of the
   --  package that you may be doing something unsafe.
   function Degrees_To_Int is new Ada.Unchecked_Conversion
     (Source => Degrees,   --  Line continuations are indented by 2 spaces.
      Target => Integer);

   New_Value_2 : Integer := Degrees_To_Int (Angle);  --  Note, space before (.

   --  GNAT is the GNU Ada Translator (compiler).
   --  Ada has a style guide and GNAT will warn you to adhere to it, and has
   --  option to check your style so that you can correct it so that all Ada
   --  source looks consistent. However, the style can be customized.

   --  Yes, you can even define your own floating and fixed point types, this
   --  is a very rare and unique ability. "digits" refers to the minimum
   --  digit precision that the type should support. "delta" is for fixed
   --  point types and refers to the smallest change that the type will support.
   type Real_Angles is digits 3 range 0.0 .. 360.0;
   type Fixed_Angles is delta 0.01 digits 5 range 0.0 .. 360.0;

   RA : constant Real_Angles  := 36.45;
   FA : constant Fixed_Angles := 360.0;  --  ".0" in order to make it a Float.

   --  You can have normal Latin 1 based strings by default.
   Str  : constant String    := "This is a constant string";
   --  When initialising from a string literal, the compiler knows the bounds,
   --  so we don't have to define them.

   --  Strings are arrays. Note how parentheses are used to access elements of
   --  an array? This is mathematical notation and was used because square
   --  brackets were not available on all keyboards at the time Ada was
   --  created. Also, because an array can be seen as a function from a
   --  mathematical perspective, so it made converting between arrays and
   --  functions easier.
   Char : constant Character := Str (Str'First);  --  "'First" is a type
                                                  --  attribute.

   --  Ada 2022 includes the use of [] for array initialisation when using
   --  containers, which were added in Ada 2012.

   --  Arrays are usually always defined as a type.
   --  They can be any dimension.
   type My_Array_1 is array (1 .. 4, 3 .. 7, -20 .. 20) of Integer;

   --  Yes, unlike other languages, you can index arrays with other discrete
   --  types such as enumerations and modular types or arbitrary ranges.
   type Axes is (X, Y, Z);

   --  You can define the array's range using the 'Range attribute from
   --  another type.
   type Vector is array (Axes'Range) of Float;

   V1 : constant Vector := (0.0, 0.0, 1.0);

   --  A record is the same as a structure in C, C++.
   type Entities is record
      Name     : String (1 .. 10);  --  Always starts at a positive value,
                                    --  inclusive range.
      Position : Vector;
   end record;

   --  In Ada, array bounds are immutable. You therefore have to provide a
   --  string literal with a value for every character.
   E1 : constant Entities := ("Blob      ", (0.0, 0.0, 0.0));

   --  An alternative is to use an array aggregate and assign a default value
   --  to every element that wasn't previously assigned in this aggregate.
   --  "others" is used to indicate anything else that has not been
   --  explicitly initialized.
   E2 : constant Entities := (('B', 'l', 'o', 'b', others => ' '),
                              (0.0, 0.0, 0.0));

   --  There are dynamic length strings (see references section) available in
   --  the standard library.

   --  We can make an object be initialised to its default values with the box
   --  notation, "<>". "others" is used to indicate anything else that has not
   --  been explicitly initialized.
   Null_Entity : constant Entities := (others => <>);

   --  Object-orientation is accomplished via an extension of record syntax,
   --  tagged records, see link above in first paragraph.

   --  We can rename objects (aliases) to make readability a bit better.
   package IO renames Ada.Text_IO;
begin
   --  We can output enumerations as names.
   IO.Put_Line ("Blue_Hue   = " &  --  & is the string concatenation operator.
                Blue'Image);       --  ' accesses attributes on objects.
                  --  The Image attribute converts a value to a string.
                  --  Ada 2022 has extended Image to custom types too.
                  --  Access this with -gnat2022 compiler flag.
   IO.Put_Line ("Yellow_Hue = " &
                --  We can use the type's attribute.
                Primaries'Image (Yellow_Hue));

   --  We can define local variables within a declare block, this can be made
   --  more readable by giving it a label.
   Enum_IO : declare
      package Hue_IO is new IO.Enumeration_IO (Hues);

      --  Using a package makes everything inside that package visible within
      --  this block, it is good practice to only do this locally and not on
      --  a whole package within the context clause.
      use Hue_IO;
   begin
      --  We can print out the enumeration values too.
      Put (Purple); --  Note we don't have to prefix the Put procedure with
                    --  Hue_IO.
      IO.New_Line;  --  We still need to prefix with IO here.
      Put (Red_Hue);
      IO.New_Line;
   end Enum_IO;

   --  Loops have a consistent form. "<form> loop ... end loop".
   --  Where "form" can be "while" or "for" or missing as below, if
   --  you place the "loop ... end loop;" construct on their own lines,
   --  you can comment out or experiment with different loop constructs more
   --  easily.
   declare
      Counter : Positive := Positive'First;  --  This is 1.
   begin
      --  We can label loops so we can exit from them more easily if we need to.
      Infinite :
      loop
         IO.Put_Line ("[Infinite loop] Counter = " & Counter'Image);

         Counter := Counter + 1;

         --  This next line implements a repeat ... until or do ... while loop construct.
         --  Comment it out for an infinite loop.
         exit Infinite when Counter = 5;  --  Equality tests use a single "=".
      end loop Infinite;  --  Useful when implementing state machines.
   end;

   declare  --  We don't have to have a label.
      Counter : Positive := Positive'First;  --  This is 1.
   begin
      while Counter < 10
      loop
         IO.Put_Line ("Counter = " & Counter'Image);

         Counter := Counter + 1;  --  There is no explicit inc/decrement.

         --  Ada 2022 introduced @ for LHS, so the above would be written as
         --  Counter := @ + 1;  --  Try it, -gnat2022.
      end loop;
   end;

   declare
      package Hue_IO is new IO.Enumeration_IO (Hues);

      --  We can have multiple packages on one line, but I tend to use one
      --  package per line for readability.
      use IO, Hue_IO;
   begin
      Put ("Hues : ");  --  Note, no prefix.

      --  Because we are using the 'Range attribute, the compiler knows it is
      --  safe and can omit run-time checks here.
      for Hue in Hues'Range
      loop
         Put (Hue);

         --  Types and objects know about their bounds, their First .. Last
         --  values. These can be specified as range types.
         if Hue /= Hues'Last then  --  The /= means "not equal to" like the
                                   --  maths symbol ≠.
            Put (", ");
         end if;
      end loop;

      IO.New_Line;
   end;

   --  All objects know their bounds, including strings.
   declare
      C : Character := Str (50);  --  Warning caused and exception raised at
                                  --  runtime.
      --  The exception raised above can only be handled by an outer scope,
      --  see wikibook link below.
   begin
      null;  --  We will never get to this point because of the above.
   end;
exception
   when Constraint_Error =>
      IO.Put_Line ("Caught the exception");
end LearnAdaInY;
$ gnatchop learn-ada-in-y.ada # This breaks the program into its specification ".ads" and body ".adb".
$ gnatmake empty.adb # gnatmake takes care of compilation of all units and linking.
$ gnatmake hello.adb
$ gnatmake learnadainy.adb
$ alr search learnadainy
$ alr get learnadainy
$ cd learnadainy
$ alr run empty
$ alr run hello
$ alr run learnadainy